Peripheral Nerve Flashcards
Which of the following nerves most commonly arises from the roots of the brachial plexus?
Answers:
A. Radial Nerve
B. Thoracodorsal Nerve
C. Dorsal Scapular Nerve
D. Suprascapular Nerve
E. Lower Subscapular Nerve
Dorsal Scapular Nerve
The trunks of the brachial plexus are formed through the confluence or continuation of the C5
through T1 roots. The brachial plexus can be divided into five segments: roots, trunks, divisions,
cords, and branches. The roots and trunks can be found in the supraclavicular space. The
divisions typically are retroclavicular. The cords and branches are infraclavicular. The branches
typically arise at the level of the axilla. The upper trunk is formed from the union of the C5 and C6
roots, the middle trunk is the continuation of the C7 root, and the lower trunk is the union of the C8
and T1 roots. Occasionally, the C4 root may contribute to the upper trunk or the T2 root may
contribute to the lower trunk, with these variations termed a pre-fixed (C4 contribution) or postfixed
(T2 contribution) brachial plexus.
The rhomboid muscles (rhomboid major and minor) are innervated by the dorsal scapular nerve,
which arises proximally from the C5 nerve root. In addition to the rhomboid major and minor, the
dorsal scapular nerve also innervates the levator scapulae. The dorsal scapular nerve arises at the
level of the nerve roots from C5, passing through the substance of the middle scalene muscle. The
C5 root also gives contributions to the phrenic nerve and then the long thoracic nerve, also formed
within the substance of the middle scalene. Distal to these branches, C5 and C6 join to form the
upper trunk of the brachial plexus. The upper trunk then trifurcates into the suprascapular nerve,
posterior division, and anterior division of the upper trunk, though this can be a bifurcation with the
suprascapular nerve arising from the posterior division.
Due to the very proximal location of the dorsal scapular nerve origin, in the setting of a brachial
plexus injury and absent rhomboid function, one should suspect a very proximal injury, possibly
pre-ganglionic. The rhomboids begin on the lower cervical and upper thoracic spinous processes
and insert onto the medial border of the scapula. They function to stabilize the scapula during arm
movement. The branches that form the phrenic nerve from C3, C4, and C5 and the branches that
form the long thoracic nerve from C5, C6, and C7 also occur proximally. Thus, loss of function of
the phrenic and long thoracic nerves also suggests a proximal injury.
The suprascapular nerve arises from the upper trunk of the brachial plexus and innervates the
supraspinatus and infraspinatus muscles. The radial nerve is one of the terminal branches of the
posterior cord of the brachial plexus. The lower subscapular nerve branches from the posterior
cord of the brachial plexus and innervates the subscapularis and teres major muscles. The
thoracodorsal nerve also branches from the posterior cord and innervates the latissimus dorsi
muscle.
Foot dorsiflexion weakness with preservation of foot inversion is most characteristic of which of the
following?
Answers:
A. L5 Radiculopathy
B. Tibial Neuropathy
C. Superficial Peroneal Neuropathy
D. S1 Radiculopathy
E. Common Peroneal Neuropathy
Common Peroneal Neuropathy
A patient presenting with dorsiflexion weakness and preserved inversion is characteristic of a
common peroneal neuropathy.
The common peroneal nerve is one of the two divisions of the sciatic nerve and is responsible for
toe extension, ankle dorsiflexion, and eversion. The tibial nerve is the other division of the sciatic
nerve and is responsible for toe flexion, ankle plantar flexion, and inversion.
At the nerve root level, dorsiflexion is primarily mediated by the L5 nerve, whereas plantar flexion
is primarily mediated by the S1 nerve. Inversion is also primarily mediated by the L5 nerve,
whereas eversion is primarily mediated by the L5 and S1 nerves.
When a patient presents with a foot drop (dorsiflexion weakness), the main differential diagnosis is
a common peroneal neuropathy versus an L5 radiculopathy. Dorsiflexion plus eversion weakness
would be typical of a common peroneal neuropathy. Inversion is spared in a common peroneal
neuropathy. Conversely, dorsiflexion weakness plus inversion weakness would be typical of an L5
radiculopathy. In this way, inversion and eversion can be utilized to help differentiate a common
peroneal neuropathy from an L5 radiculopathy.
Electrodiagnostics and imaging can then be used to support the suspected diagnosis. For a
common peroneal neuropathy, the typical compression point is at the fibular tunnel around the
fibular neck, where the nerve can be compressed by the deep fascia of the peroneus longus
muscle.
Lateral interbody fusion at which of the following levels poses the highest risk for postoperative
lumbar plexopathy?
Answers:
A. L1-2
B. L4-5
C. L5-S1
D. L2-3
E. L3-4
L4-5
Lateral lumbar interbody fusion has become a more commonly used technique in recent years to
achieve anterior column interbody fusion. The psoas musculature is either retracted posteriorly or
split to achieve this goal. If this approach is pursued, knowledge of the anatomical location of the
lumbar plexus, femoral nerve, and the vasculature in this region is of paramount importance.
Typically, the lumbar plexus and femoral nerve are located in the posterior aspect of the psoas
musculature. Thus, by either retracting the entire muscle posteriorly or splitting the muscle and
retracting the posterior portion of the muscle posteriorly, the plexus remains protected. However,
variant anatomy needs to be considered, such as the lumbar plexus coursing anteriorly in the
psoas or the psoas being located anteriorly on the disc space, as this could put these nerves at
more risk. In addition, vascular structures should also be considered. During transpsoas
approaches, these are typically not visualized and not at risk. However, the venous structures are
located more posteriorly on the right side, and this should be taken into account prior to
proceeding with surgery.
Lateral approaches to L4-5 present the greatest risk for lumbar plexus injury, as the plexus runs
most anteriorly within the psoas at this level. Thus, splitting the psoas muscle at this level
potentially puts the lumbar plexus at higher risk of injury. In addition, as stated above, right-sided
approaches at every lumbar level carry increased risk of vascular injury compared to left-sided
approaches. This is felt to be secondary to the more posterior location of the vasculature on the
right side.
The median nerve innervates which of the following sets of muscles?
Answers:
A. Lumbricals 3/4, Dorsal Interossei, Palmar Interossei, Adductor Pollicis
B. Brachioradialis, Extensor Carpi Radialis Brevis, Extensor Carpi Radialis Longus
C. Extensor Digitorum Communis, Extensor Carpi Ulnaris, Abductor Pollicis Longus
D. Lumbricals 1/2, Opponens Pollicis, Abductor Pollicis Brevis, Flexor Pollicis Brevis
E. Biceps Brachii, Brachialis, Coracobrachialis
Lumbricals 1/2, Opponens Pollicis, Abductor Pollicis Brevis, Flexor Pollicis Brevis
The median nerve arises from contributions from the lateral cord and medial cord of the brachial
plexus, with the lateral cord contributing most of the sensory fibers and the medial cord
contributing most of the motor fibers. The median nerve carries fibers arising from C6, C7, C8,
and T1. The median nerve runs through the arm in close proximity to the brachial artery and does
not have any sensory or motor function in the arm. It enters the forearm traversing the cubital
fossa next to the biceps brachii tendon. The nerve passes deep to the lacertus fibrosus, a
potential point of compression and then courses through the forearm between the heads of the
pronator teres muscle to run under the sublimus arch of the flexor digitorum superficialis, additional
points of potential compression. In the forearm, the median nerve innervates the pronator teres,
palmaris longus (if present), flexor carpi radialis, flexor digitorum superficialis, and through the
anterior interosseous nerve innervates the lateral half of the flexor digitorum profundus, flexor
pollicis longus, and pronator quadratus. There is no sensory function of the median nerve in the
forearm. The median nerve then passes deep to the transverse carpal ligament through the carpal
tunnel to enter the hand. The median nerve innervates lumbricals 1/2, opponens pollicis, abductor
pollicis brevis, and flexor pollicis brevis (LOAF muscles) in the hand. The median nerve provides
sensory innervation to the palmar surface of the radial 3.5 digits and to the corresponding palm.
The palm is innervated through the palmar cutaneous branch which arises in the forearm and does
not traverse the carpal tunnel. Thus, the palmar distribution of the median nerve is spared in
carpal tunnel syndrome.
Lumbricals 3/4, the dorsal interossei, palmar interossei, and adductor pollicis are innervated by the
ulnar nerve. The biceps brachii, brachialis, and coracobrachialis are innervated by the
musculocutaneous nerve. The extensor digitorum communis, extensor carpi ulnaris, and abductor
pollicis longus are innervated by the posterior interosseous nerve (radial nerve). The
brachioradialis, extensor carpi radialis brevis, and extensor carpi radialis longus are innervated by
the radial nerve.
Median nerve function in the hand can be tested by:
Answers:
A. Extension of the distal interphalangeal joint of digit 2
B. Thumb adduction
C. Abduction of digits 2-5
D. Extension of the distal interphalangeal joint of digit 5
E. Adduction of digits 2-5
Extension of the distal interphalangeal joint of digit 2
The median nerve arises as a terminal branch of the brachial plexus formed as a confluence of
contributions from the medial and lateral cords. The fibers of the median nerve originate from the
C6-T1 nerves. The median nerve descends from its origin in the axilla to run a course down the
entire upper limb, crossing both the elbow and wrist joints, terminating as motor and sensory
branches to the hand and digits.
The median nerve in the forearm innervates the pronator teres, flexor carpi radialis, and flexor
digitorum superficialis and gives off the anterior interosseous nerve as a branch that innervates the
flexor digitorum profundus, flexor pollicis longus, and pronator quadratus. In the forearm, the
median nerve is susceptible to compression by the lacertus fibrosus, between the two heads of the
pronator teres, and at the sublimus arch of the flexor digitorum superficialis. The nerve then
continues through the carpal tunnel to enter the hand. In the hand, the median nerve provides
sensation to the palmar aspect of the radial 3.5 digits and innervates the LOAF muscles
—lumbricals 1 and 2, opponens pollicis, abductor pollicis brevis, and flexor pollicis brevis. All of
the hand intrinsic muscles are innervated by the ulnar nerve, except for the LOAF muscles which
are median-innervated. The palmar cutaneous branch arises in the distal forearm prior to the
median nerve entering the carpal tunnel to provide sensation to the palm. Thus, the palm is spared
in carpal tunnel syndrome.
Which of the following muscles are innervated solely by the radial nerve?
A. Deltoid
B. Brachioradialis
C. Pronator Teres
D. Brachialis
E. Flexor Carpi Radialis
Brachioradialis
The posterior cord of the brachial plexus terminates by dividing into the axillary nerve and the
radial nerve. The radial nerve travels along the posterior wall of the axilla supplying motor
branches to the three heads of the triceps and sensation to the posterior aspect of the arm. The
nerve continues through the triangular interval between the long head of the triceps and the
humerus. The radial nerve then passes down the spiral groove and wraps around the humerus to
pass between the brachioradialis muscle and brachialis muscle to enter the forearm anterior to the
lateral epicondyle. In the proximal forearm, the radial nerve terminates by dividing into the
superficial radial (sensory) and the deep radial/posterior interosseous nerve (PIN).
The radial nerve innervates the triceps brachii, brachioradialis, extensor carpi radialis longus, and
extensor carpi radialis brevis. The posterior cutaneous nerve of the arm and posterior cutaneous
nerve of the forearm are sensory branches of the radial nerve that supply sensation to the
posterior arm and forearm. The superficial radial nerve is a terminal branch of the radial nerve that
is a pure sensory nerve, supplying sensation to the dorsum of the radial 3.5 digits. The other
terminal branch of the radial nerve is the deep radial nerve/posterior interosseous nerve. This
nerve supplies the supinator, extensor carpi ulnaris, extensor digitorum communis, extensor digiti
minimi, abductor pollicis longus, extensor pollicis longus, extensor pollicis brevis, and extensor
indicis.
Potential compression points include the radial nerve at the spiral groove by the lateral
intermuscular septum, the posterior interosseous nerve at the supinator/Arcade of Frohse, and the
superficial radial nerve between the brachioradialis and extensor carpi radialis longus tendons.
Due to the course of the radial nerve in close association with the humerus, the radial nerve is
particularly prone to injury in association with mid-shaft humeral fractures.
Clinically, proximal radial nerve injuries are characterized by weakness or loss of elbow extension,
wrist extension, and finger extension. The triceps branches arise very proximally such that elbow
extension is preserved with injuries to the radial nerve in the mid-arm. A posterior interosseous
palsy is characterized by loss of finger extension, with preserved wrist extension but radial
deviation during wrist extension. This occurs due to the preservation of function in the extensor
carpi radialis longus and brevis (innervated by the radial nerve), with loss of function in the
extensor carpi ulnaris (innervated by the posterior interosseous nerve).
The brachialis is innervated by the musculocutaneous nerve. The deltoid is innervated by the
axillary nerve. The pronator teres and flexor carpi radialis are innervated by the median nerve.
During electromyographic examination of a patient, an initial action potential evoked in a rested
muscle by a single maximal nerve stimulus is found to be greatly reduced in amplitude. Exercise
produces a brief but marked facilitation of the action potential. Which of the following is the most
likely diagnosis?
Answers:
A. Lambert-Eaton Syndrome
B. Myasthenia Gravis
C. Polymyalgia Rheumatica
D. Charcot-Marie-Tooth Disease
E. Amyotrophic Lateral Sclerosis
Lambert-Eaton Syndrome
Lambert-Eaton myasthenic syndrome (LEMS) is an autoimmune condition in which antibodies are
directed against presynaptic calcium channels at the neuromuscular junction. It is often a
paraneoplastic syndrome, with small cell lung carcinoma being a commonly associated
malignancy. These antibodies prevent the influx of calcium at the presynaptic terminal upon
depolarization by an action potential arriving at the neuromuscular junction that then results in less
acetylcholine neurotransmitter release. As a result, less acetylcholine is available in the
neuromuscular junction to bind to post-synaptic acetylcholine muscle receptors required for muscle
cell depolarization and contraction. On electromyography, what is recorded as a result of this is a
motor unit potential amplitude that is smaller than normal. However, with repetitive stimulation or
activity (action potential depolarization impulses arriving at the presynaptic neuromuscular
junction) there is enough calcium influx to overcome the effect of the LEMS antibodies, which
triggers more acetylcholine release and a return to normal (or increase) in motor unit amplitude (a
phenomenon known as facilitation). In LEMS, the presynaptic production and storage of
acetylcholine is normal, as is the postsynaptic response to the acetylcholine. Myasthenia gravis is
another neuromuscular junction autoimmune disorder; however, unlike LEMS, myasthenia gravis
autoantibodies are directed against acetylcholine postsynaptic neuromuscular junction receptors.
As a result, although acetylcholine is released into the synaptic cleft of the neuromuscular junction,
it is unable to bind to its postsynaptic receptor and depolarize the muscle cell. This results in early
fatigability and clinical muscle weakness after repetitive activation (opposite phenomenon
compared to LEMS).
In contrast, the other conditions listed are not considered neuromuscular junction disorders.
Amyotrophic lateral sclerosis is a degenerative motor neuron disease affecting motor neurons
primarily in the primary motor cortex and brain stem (upper motor neurons) and spinal cord (lower
motor neurons) that results in progressive weakness and loss of function. Polymyalgia rheumatica
is an inflammatory condition affecting the musculoskeletal system that results in pain, stiffness and
weakness. Charcot-Marie Tooth disease is a hereditary motor and sensory neuropathy of the
peripheral nervous system due to genetic mutations affecting the neuronal axon and its myelin
sheath.
Tarsal tunnel syndrome is caused by compression of which of the following nerves?
Answers:
A. Sural Nerve
B. Tibial Nerve
C. Saphenous Nerve
D. Common Peroneal Nerve
E. Superficial Peroneal Nerve
Tibial Nerve
The tibial nerve is the larger and more medial division of the sciatic nerve, the other being the
common peroneal nerve. The tibial nerve provides sensation to the lateral aspect of the foot
through the medial sural nerve, which arises from the tibial nerve around the popliteal fossa, and to
the plantar surface of the foot (including the heel) through the medial plantar, lateral plantar, and
calcaneal nerves. It provides motor innervation to the gastrocnemius, soleus, plantaris, tibialis
posterior, flexor digitorum longus, and flexor hallucis longus. Taken together, these muscles plantar
flex the foot and toes and invert the ankle. The tibial nerve also innervates all of the foot intrinsic
muscles, except for the extensor hallucis brevis and extensor digitorum brevis (which are
innervated by the deep peroneal nerve).
Tarsal tunnel syndrome, as it is commonly referred to, should more appropriately be called
posterior tarsal tunnel syndrome, since there is also an anterior tarsal tunnel. Posterior tarsal
tunnel syndrome is a clinical syndrome characterized by paresthesias, burning pain, or
dysesthesias in the medial ankle and plantar foot. Pain is typically worse with activity or long
periods of standing. (Posterior) Tarsal tunnel syndrome occurs secondary to compression of the
tibial nerve or its terminal branches, including the medial plantar, lateral plantar, and calcaneal
nerves.
The proximal floor of the posterior tarsal tunnel is comprised of the talar bone and posterior part of
the tibia, while the distal portion is comprised of the calcaneus. The roof is formed by the flexor
retinaculum. Multiple important structures course through the tarsal tunnel, including, from anterior
to posterior, the tibialis posterior tendon, flexor digitorum longus tendon, posterior tibial artery and
veins, the tibial nerve, and the flexor hallucis longus tendon. Within the tarsal tunnel, the tibial
nerve may be compressed due to a ganglion cyst, lipoma, bony abnormalities, or tenosynovitis of
the associated tendons. Other risk factors include systemic processes such as diabetes,
hypothyroidism, or inflammatory arthropathies. Lifestyle factors may also play a role, with tarsal
tunnel syndrome reported at an increased frequency in patients who stand for long periods of time
for work or who are particularly active. Trauma, including surgery around the foot/ankle, is also a
risk factor. The terminal branches of the tibial nerve—the medial plantar nerve, lateral plantar
nerve, and calcaneal nerve—all individually course through fibrous tunnels and may be
compressed within their respective tunnels. Thus, the tibial nerve may be compressed or the
compression may be limited to one or more of the terminal branches.
Diagnosis remains a challenge, as many diagnostic studies, including electromyography/nerve
conduction studies, may be normal. Provocative maneuvers in the clinic, such as ankle eversion
and dorsiflexion, may exacerbate symptoms. The foot and ankle region should be evaluated for a
hindfoot varus or valgus deformity. The area should be palpated for masses. A Tinel sign may be
elicited in the region of the tarsal tunnel and may be more pronounced when assessed with the
foot maximally dorsiflexed and everted and the toes extended. Management of tarsal tunnel
syndrome includes a trial of lifestyle modifications, oral medications, and injections to the affected
area. For patients who have failed non-surgical management, surgery may be considered.
The common peroneal nerve is susceptible to compression at the fibular tunnel by the deep fascia
of the peroneus longus muscle and leads to a syndrome referred to as fibular tunnel syndrome.
The superficial peroneal nerve is susceptible to compression as it pierces the fascia to enter the
subcutaneous tissue. This typically occurs approximately 2/3 of the distance from the fibular head
to the lateral malleolus. The sural nerve does not have a typical compression point. The
saphenous nerve is susceptible to compression in the thigh, within the adductor (Hunter’s) canal.
The nerve can be compressed by the vastoadductor membrane.
For each movement of the foot, select the nerve (A-E) most likely to be involved.
Inversion of the foot:
Answers:
A. Tibial Nerve
B. Deep Peroneal Nerve
C. Saphenous Nerve
D. Superficial Peroneal Nerve
E. Sural Nerve
Tibial Nerve
The tibial nerve is a branch of the sciatic nerve which passes through the popliteal fossa to reach
its muscles of innervation. It is composed of fibers originating from the L4, L5, S1, S2 and S3
nerve roots. As it passes through the popliteal fossa, it lies superficial to the popliteal vessels.
Ultimately, the nerve terminates by dividing into the medial and lateral plantar branches. Potential
compression points along the course of the tibial nerve include the soleal sling in the calf and at
the tarsal tunnel in the ankle, where compression occurs related to the flexor retinaculum. The
calcaneal branch, medial plantar nerve, and lateral plantar nerve can also be compressed as they
enter the foot in their own respective fibrous tunnels.
The tibial nerve has important motor and sensory innervations. The motor innervations include the
gastrocnemius, soleus, plantaris, tibialis posterior, flexor digitorum longus, flexor hallucis longus,
abductor hallucis, flexor digitorum brevis, flexor hallucis brevis, flexor digiti minimi, adductor
hallucis, abductor digiti minimi, interossei, and lumbricals. With regard to sensation, at or around
the popliteal fossa, the medial sural cutaneous nerve branches off of the tibial nerve to supply the
lateral aspect of the lower leg and the lateral aspect of the foot. The calcaneal nerve branch arises
around the ankle and supplies sensation to the posterior aspect of the heel. Finally, the medial and
lateral plantar branches supply the sole of the foot.
Inversion is mediated primarily by the tibialis posterior, which is innervated by the tibial nerve.
Eversion is mediated primarily by the peroneus longus and peroneus brevis, both of which are
supplied by the common peroneal nerve via the superficial peroneal nerve. Dorsiflexion is
mediated primarily by the tibialis anterior, which is supplied by the common peroneal nerve via the
deep peroneal nerve. Plantar flexion is mediated primarily by the gastrocnemius and soleus, both
of which are supplied by the tibial nerve.
The saphenous nerve has no motor function. It is a pure sensory nerve that provides sensation to
the medial aspect of the leg, the infrapatellar region, and the medial ankle. The sural nerve is also
a pure sensory nerve which provides sensation to the lateral aspect of the calf and the lateral
aspect of the foot. The superficial and deep peroneal nerves arise from the common peroneal
nerve. The deep peroneal nerve supplies motor branches to the tibialis anterior, extensor hallucis
longus, extensor digitorum longus, peroneus tertius, extensor digitorum brevis and extensor
hallucis brevis. In addition, the deep peroneal nerve provides sensory fibers to the web space
between the first and second digits. The superficial peroneal nerve provides motor innervation to
the peroneus longus and brevis muscles. It also provides sensory fibers to the anterolateral aspect
of the leg and most of the dorsum of the foot, save the webspace between the first and second
digits.
Which of the following structures is divided to accomplish ulnar nerve decompression in the Guyon
canal?
Answers:
A. Pisohamate Ligament
B. Hypothenar Arcus Tendineus
C. Flexor Retinaculum
D. Scaphocapitate Ligament
E. Extensor Retinaculum
Hypothenar Arcus Tendineus
The ulnar nerve arises from the medial cord, carrying fibers predominantly from the C8 and T1
roots, though there can be fibers from C7. The ulnar nerve provides motor innervation to the flexor
carpi ulnaris and the ulnar half of the flexor digitorum profundus in the forearm and then innervates
all of the hand intrinsic muscles except for the LOAF muscles—lumbricals 1 and 2, opponens
pollicis, abductor pollicis brevis, and flexor pollicis brevis—which are innervated by the median
nerve. The ulnar nerve provides sensation to the ulnar 1.5 digits through the terminal superficial
branch of the ulnar nerve, to the dorsal ulnar hand through the dorsal cutaneous branch, and to
the palmar ulnar hand through the palmar cutaneous branch, both of which arise in the forearm,
proximal to Guyon’s canal.
Ulnar nerve entrapment occurs primarily in two places: 1) at or around the elbow, referred to as
cubital tunnel syndrome, and 2) in the palm, referred to as Guyon’s canal syndrome. Entrapment
of the ulnar nerve at the elbow is much more common than at the palm and represents the second
most common entrapment neuropathy in the upper extremity, following carpal tunnel syndrome.
Understanding the common points of compression for the ulnar nerve is important in surgical
treatment of ulnar entrapment neuropathy. Commonly described points of compression around the
elbow from proximal to distal include:
1. Arcade of Struthers – Musculoaponeurotic band in the upper arm extending from the medial
head of triceps to the medial intermuscular septum. This is not to be confused with the
Struthers ligament which is a fibrous band extending from a supracondylar process to the
medial epicondyle and associated with entrapment of the median nerve.
2. Medial intermuscular septum
3. Medial epicondyle
4. Osborne’s ligament – fibrous band extending from medial epicondyle to the olecranon
5. Osborne’s fascia – fascia bridging the two heads of the flexor carpi ulnaris muscle
Entrapment at the elbow can be differentiated from entrapment at Guyon’s canal by examining the
dorsal and palmar hand. The branches that innervate the dorsal hand and the palm originate
distal to the cubital tunnel in the forearm and do not traverse Guyon’s canal. Thus, reduced
sensation on the dorsal hand and the palm are associated with cubital tunnel but not Guyon’s
canal syndrome, whereas both syndromes will have reduced sensation in the ulnar 1.5 digits.
Ulnar neuropathy should also be differentiated from more proximal etiologies, including thoracic outlet syndrome and C8 radiculopathy. Ulnar neuropathy will present with sensory loss that does
not extend proximal to the wrist and that reliably splits the fourth digit, whereas C8 radiculopathy
will extend proximal to the wrist and will not split the fourth digit. With regard to motor function,
hand weakness related to an ulnar neuropathy will spare the LOAF muscles, whereas a C8
radiculopathy will involve all of the hand intrinsic muscles (both the ulnar- and median-innervated
muscles).
Guyon’s canal is a fibro-osseous tunnel located in the medial aspect of the wrist. The floor is the
transverse carpal ligament proximally and the pisohamate and pisometacarpal ligaments distally.
The lateral wall is the hook of the hamate and transverse carpal ligament. The medial wall is the
pisiform bone and the flexor carpi ulnaris tendon. The roof is formed by the palmaris brevis
muscle, palmar carpal ligament, and the hypothenar arcus tendineus. All of the structures forming
the roof of the canal plus the distal antebrachial fascia are divided to fully decompress Guyon’s
canal. In Guyon’s canal, the ulnar nerve divides into a deep and superficial branch. The two
branches are separated by the hypothenar arcus tendineus, which forms the roof of the canal
through which the deep branch passes.
There are three potential zones of compression within Guyon’s canal. Zone I compression
happens prior to the bifurcation of the superficial and deep branches. Patients may present with
both weakness and numbness/paresthesias with zone I compression. In Zone II compression,
only the deep motor branch is affected. Thus, patients will present with weakness only, without
sensory loss. Zone III compressions are the least common and affect only the superficial branch.
These patients present with sensory loss and weakness of the palmaris brevis, with sparing of the
rest of the ulnar-innervated muscles.
Which of the following muscles may be weak with C5 nerve root dysfunction, but not with an injury
to the rotator cuff?
Answers:
A. Triceps Brachii
B. Teres Minor
C. Deltoid
D. Infraspinatus
E. Supraspinatus
Deltoid
The rotator cuff comprises four muscles around the shoulder: the supraspinatus, infraspinatus,
teres minor, and subscapularis. The supraspinatus is innervated by C5 (predominantly) and C6
via the suprascapular nerve. The infraspinatus is innervated by C5 (predominantly) and C6 via the
suprascapular nerve. The teres minor is innervated by C5 and C6 via the axillary nerve. The
subscapularis is innervated by C5 and C6 via the upper and lower subscapular nerves. With a
rotator cuff injury, the injury would be limited to any or all of these four muscles.
Other important muscles that are innervated by the C5 nerve root that are not part of the rotator
cuff include the rhomboids (dorsal scapular nerve), teres major (lower subscapular nerve), deltoid
(axillary nerve), biceps brachii (musculocutaneous nerve), brachialis (musculocutaneous nerve),
brachioradialis (radial nerve), and supinator (posterior interosseous nerve). With an injury to the
C5 nerve root, all of the rotator cuff muscles plus these muscles would be expected to be affected.
The correct answer to this question is the deltoid. The deltoid receives its innervation
predominantly from C5 via the axillary nerve but is not part of the rotator cuff.
A C5 nerve injury can be differentiated from an axillary neuropathy based on the pattern of
weakness. With an axillary neuropathy, the weakness would be isolated to the deltoid and the
teres minor, whereas in a C5 nerve root injury both of these muscles would be weak plus the other
C5 innervated muscles listed above.
Among the answer choices, the triceps brachii is incorrect because it is neither part of the rotator
cuff nor innervated by C5. The triceps brachii is innervated by C6, C7, and C8 via the radial nerve.
Proximal lesions of the peroneal component of the sciatic nerve are distinguished on
electromyography (EMG) from distal peroneal lesions across the fibular neck by demonstration of
abnormalities on EMG in which of the following muscles?
Answers:
A. Long Head of the Biceps Femoris
B. Peroneus Longus
C. Tibialis Anterior
D. Extensor Hallucis Longus
E. Short Head of the Biceps Femoris
Short Head of the Biceps Femoris
The sciatic nerve arises via the lumbosacral plexus from the L4, L5, S1, S2, and S3 spinal nerve
roots. The sciatic nerve is two nerves joined together that share the same epineurium – the
common peroneal (posterior divisions of L4-S2) and tibial nerve (anterior divisions of L4-S3). It
exits the pelvis through the greater sciatic foramen, most commonly coursing under the piriformis
muscle, and runs deep to the gluteus maximus. The sciatic nerve then travels in the middle of the
posterior thigh between the hamstrings. Approximately two-thirds of the way down the thigh, the
sciatic nerve divides into the tibial and common peroneal nerves. The tibial nerve supplies muscles
that mediate plantar flexion, foot inversion, and toe flexion. Muscles responsible for dorsiflexion,
eversion, and toe extension are supplied by the common peroneal nerve.
In the thigh, the short head of the biceps femoris is the only peroneal-innervated muscle above the
fibular head. The remainder of the hamstring muscles are innervated by the tibial division of the
sciatic nerve. Distal to the fibular neck, the common peroneal nerve branches into the superficial
and deep peroneal nerves. The first branch of the deep peroneal nerve is to the tibialis anterior.
The first branch of the superficial peroneal nerve is to the peroneus longus.
Proximal lesions of the peroneal division of the sciatic nerve are distinguished from distal common
peroneal nerve lesions at or below the fibular neck based on EMG abnormalities and evidence of
denervation in the short head of the biceps femoris. For instance, classic compression of the
common peroneal nerve at the fibular tunnel would not demonstrate any EMG abnormalities
proximal to the tibialis anterior. Thus, there would be no EMG abnormalities in the short head of
the biceps femoris.
The extensor hallucis longus, peroneus longus, and tibialis anterior are all innervated by branches
of the common peroneal nerve distal to the fibular neck. The long head of the biceps femoris is
innervated by the tibial division of the sciatic nerve.
A 30-year-old man has numbness and point tenderness of the right gluteal region two months after
undergoing pedicle screw fixation and posterolateral fusion at L5-S1 with harvest of the right
posterior iliac crest. Which of the following is the most likely cause of the patient’s symptoms?
Answers:
A. Inferior Cluneal Nerve Injury
B. Middle Cluneal Nerve Injury
C. Superior Gluteal Nerve Injury
D. Superior Cluneal Nerve Injury
E. Inferior Gluteal Nerve Injury
Superior Cluneal Nerve Injury
The cluneal nerves are pure sensory nerves that provide innervation to the low back, posterior iliac
crest, and gluteal region. The superior cluneal nerves originate most commonly from the dorsal
rami of the L1, L2, and L3 nerve roots and travel infero-lateral towards the iliac crest. The nerves
pierce the latissimus dorsi and thoraco-lumbar fascia to run over the posterior iliac crest. The area
where the superior cluneal nerves cross the posterior iliac crest is approximately 7-8 cm from the
midline along the crest. In some instances, the superior cluneal nerves run in a fibro-osseous
tunnel across the posterior iliac crest. These nerves are susceptible to compression in these fibroosseous
tunnels, when present, and at the point where they pierce the thoraco-lumbar fascia. The
course across the posterior iliac crest makes the superior cluneal nerves susceptible to injury
during iliac crest harvest for spine surgery.
The superior cluneal nerves provide sensory innervation to the low back and central to lateral
buttock. On this basis, superior cluneal neuropathy or nerve injury typically presents with low back
and lateral buttock pain. There can be a referred component of the pain down the leg. A Tinel sign
may be present where the nerves penetrate the fascia superior to the iliac crest or where they
cross the iliac crest. Diagnostic blocks of the nerves can aid in differentiating superior cluneal
nerve injury or entrapment from other causes of low back pain.
The middle cluneal nerves provide sensory innervation to the medial buttock. The inferior cluneal
nerves provide sensory innervation to the area around the gluteal fold. The superior gluteal nerve
exits the pelvis through the greater sciatic foramen, passing over the piriformis muscle to provide
motor innervation to the gluteus medius, gluteus minimus, and tensor fascia lata muscles. The
inferior gluteal nerve exits the pelvis through the greater sciatic foramen, passing under the
piriformis muscle to provide innervation to the gluteus maximus. The inferior gluteal nerve is
susceptible to injury during total hip arthroplasty using a posterior approach.
The C8 nerve root exits the spinal canal at which of the following locations?
A. C7-C8 Neuroforamen
B. C7-T1 Neuroforamen
C. C7-T1 Foramen Transversarium
D. C8-T1 Neuroforamen
E. C8-T1 Foramen Transversarium
C7-T1 Neuroforamen
The C8 nerve root exits the spinal canal at the C7-T1 neuroforamen.
At each cervical level, the dorsal rootlets carrying sensory afferent fibers coalesce into the dorsal
root, while the ventral rootlets carrying motor axons coalesce into the ventral root. The dorsal and
ventral roots merge to form each cervical nerve. Each cervical nerve exits the neural foramen.
The boundaries of each neural foramen are defined superiorly by the pedicle above, inferiorly by
the pedicle below, anteriorly by the uncinate process, vertebral body, and intervertebral disc, and
posteriorly by the superior articulating facet. There are 7 cervical vertebrae and 8 cervical spinal
nerves. The cervical 1 spinal nerve (C1) through cervical 7 (C7) spinal nerve exit above their
named cervical vertebrae (e.g., C4 exits above C4 through the C3-4 neural foramen). The cervical
8 (C8) nerve exits above the T1 vertebrae, through the C7-T1 foramen, while the thoracic 1 (T1)
nerve exits through the T1-2 foramen. Thus, for a patient presenting with a C8 radiculopathy, the
C7-T1 foramen should be examined closely for pathology. Due to the anatomy of the cervical
spine, cervical disc herniations typically affect the exiting nerve root at that level (e.g., a C5-6 disc
herniation affects the exiting C6 nerve).
The neuroforamina transmit the nerves. The foramen transversarium is the bony opening through
which the vertebral artery passes.
A gunshot wound to the axilla that severs only the proximal medial cord of the brachial plexus
would interrupt axons traveling to which of the following peripheral nerves?
Answers:
A. Ulnar Nerve
B. Axillary Nerve
C. Suprascapular Nerve
D. Musculocutaneous Nerve
E. Radial Nerve
Ulnar Nerve
The brachial plexus can be divided into five segments: roots, trunks, divisions, cords, and
branches. The roots and trunks can be found in the supraclavicular space. The divisions typically
are retroclavicular. The cords and branches are infraclavicular. The branches typically arise at the
level of the axilla.
The medial cord is the continuation of the anterior division of the lower trunk, carrying fibers from
the C8 and T1 roots. The medial cord gives rise to the medial pectoral nerve, medial brachial
cutaneous nerve, and medial antebrachial cutaneous nerve before terminating as a bifurcation into
the medial cord contribution to the median nerve and ulnar nerve. Of note, the medial cord
contribution to the median nerve joins the lateral cord contribution to form the median nerve. The
medial cord contribution carries primarily motor fibers, whereas the lateral cord contribution carries
primarily sensory fibers.
The musculocutaneous nerve arises from the lateral cord. The axillary nerve and radial nerve
arise from the posterior cord. The suprascapular nerve arises from the upper trunk.
The rule of 3s can be used as a rough guide for the management of traumatic nerve injuries.
Open, sharp nerve injuries should be repaired within 3 days of the injury. Open, ragged nerve
injuries are typically repaired 3 weeks after the injury, allowing time for the zone of injury to fully
demarcate. In many instances, a more acute exploration is undertaken to tag the injured nerves to
be repaired later. Management of closed nerve injuries is typically delayed for 3 months, with
surgical management reserved for those patients not demonstrating clinical or electrodiagnostic
evidence of recovery. The management of closed injuries is the most nuanced. Gunshot wounds
rarely sever the nerve, and instead typically cause a concussive injury from the velocity of the
round or a heat injury related to the round traversing the tissue. Correspondingly, nerve injuries
related to gunshot wounds are typically managed similarly to closed injuries.
The superficial branch of the ulnar nerve innervates which of the following muscle groups?
Answers:
A. Adductor Pollicis
B. Abductor Digiti Minimi
C. Palmaris Brevis
D. First Dorsal Interosseous
E. Palmaris Longus
Palmaris Brevis
The ulnar nerve arises from the medial cord, carrying fibers predominantly from the C8 and T1
roots, though there can be fibers from C7. The ulnar nerve provides motor innervation to the flexor
carpi ulnaris and the ulnar half of the flexor digitorum profundus in the forearm and then innervates
all of the hand intrinsic muscles except for the LOAF muscles—lumbricals 1 and 2, opponens
pollicis, abductor pollicis brevis, and flexor pollicis brevis—which are innervated by the median
nerve. The ulnar-innervated hand intrinsic muscles include the flexor digiti minimi brevis, abductor
digiti minimi, opponens digiti minimi, lumbricals 3 and 4, adductor pollicis, dorsal interossei, and
palmar interossei via the deep branch and the palmaris brevis via the superficial branch. The ulnar
nerve provides sensation to the ulnar 1.5 digits through the terminal superficial branch of the ulnar
nerve, to the dorsal ulnar hand through the dorsal cutaneous branch, and to the palmar ulnar hand
through the palmar cutaneous branch, both of which arise in the forearm, proximal to Guyon’s
canal.
Ulnar nerve entrapment occurs primarily in two places: 1) at or around the elbow, referred to as
cubital tunnel syndrome, and 2) in the palm, referred to as Guyon’s canal syndrome. Entrapment
of the ulnar nerve at the elbow is much more common than at the palm and represents the second
most common entrapment neuropathy in the upper extremity, following carpal tunnel syndrome.
Understanding the common points of compression for the ulnar nerve is important in surgical
treatment of ulnar entrapment neuropathy. Commonly described points of compression around the
elbow from proximal to distal include:
1. Arcade of Struthers – Musculoaponeurotic band in the upper arm extending from the medial
head of triceps to the medial intermuscular septum. This is not to be confused with the
Struthers ligament which is a fibrous band extending from a supracondylar process to the
medial epicondyle and associated with entrapment of the median nerve.
2. Medial intermuscular septum
3. Medial epicondyle
4. Osborne’s ligament – fibrous band extending from medial epicondyle to the olecranon
5. Osborne’s fascia – fascia bridging the two heads of the flexor carpi ulnaris muscle
6. Entrapment at the elbow can be differentiated from entrapment at Guyon’s canal by examining the
dorsal and palmar hand. The branches that innervate the dorsal hand and the palm originate
distal to the cubital tunnel in the forearm and do not traverse Guyon’s canal. Thus, reduced
sensation on the dorsal hand and the palm are associated with cubital tunnel but not Guyon’s
canal syndrome, whereas both syndromes will have reduced sensation in the ulnar 1.5 digits.
Ulnar neuropathy should also be differentiated from more proximal etiologies, including thoracic
outlet syndrome and C8 radiculopathy. Ulnar neuropathy will present with sensory loss that does
not extend proximal to the wrist and that reliably splits the fourth digit, whereas C8 radiculopathy
will extend proximal to the wrist and will not split the fourth digit. With regard to motor function,
hand weakness related to an ulnar neuropathy will spare the LOAF muscles, whereas a C8
radiculopathy will involve all of the hand intrinsic muscles (both the ulnar- and median-innervated
muscles).
The palmaris longus is innervated by the median nerve. The adductor pollicis, abductor digiti
minimi, and first dorsal interosseus muscle are all innervated by the ulnar nerve via the deep
branch.
Which of the following nerve fibers carries pain and temperature sensations?
Answers:
A. A Omega Fibers
B. A Alpha Fibers
C. A Beta Fibers
D. A Delta Fibers
E. A Gamma Fibers
Sensory nerve fibers can be classified into several different types depending on the type of
sensory receptors activated and stimulus information they convey. A alpha fibers (type 1a and 1b
fibers) convey sensory information from proprioceptors regarding muscle and joint position, speed,
and force of contraction from muscle spindles and Golgi tendon organs. A beta fibers (type 2
fibers) convey information from mechanoreceptors regarding touch and pressure sensation. A
delta fibers (type 3 fibers) convey information from mechanoreceptors, nociceptors, and
thermoreceptors regarding light touch, well localized/sharp pain, and temperature sensation. C
fibers (type 4 fibers) convey information from nociceptors and thermoreceptors regarding pain and
temperature sensation that is poorly localized, dull/aching pain. A alpha and A beta fibers are large
diameter, myelinated fibers with fast conduction velocity. A delta fibers are smaller diameter with
thinly myelinated axons and slower conduction velocities. C fibers are small, unmyelinated axons
and have the slowest conduction velocity. Upon stimulation beyond their depolarization and action
potential threshold, the A delta and C fibers will convey a pain stimulus impulse from the periphery
to their primary sensory cell body in the dorsal root ganglion. From there, they synapse with
second order neurons in the Rexed laminae of the spinal cord. Projections from the second order
neurons then decussate to the contralateral side of the spinal cord within a few segments through
the anterior white commissure and ascend in the lateral spinothalamic tract to the ventral
posterolateral (VPL) nucleus of the thalamus (note that some fibers convey these pain impulses
through the ventral spinothalamic tract and to various areas of the brain stem, the details of which
are beyond the scope of this description). The cell bodies of the third order neurons are within the
VPL of the thalamus. These third order neurons project via the posterior limb of the internal
capsule to the primary somatosensory cortex in the post-central gyrus.
Motor nerve fibers include A alpha fibers (alpha motor neurons) that innervate extrafusal muscle
fibers and A gamma fibers (gamma motor neurons) that innervate intrafusal muscle fibers.
In response to tissue damage, a variety of chemical substances are released including bradykinin,
substance P, serotonin, prostaglandin, and histamine. These trigger a chemical cascade within the
nerve fibers initiating an action potential.
Which of the following structures is the medial border of the Kambin triangular working zone, which
defines safe access to the transforaminal region?
Answers:
A. Exiting Nerve Root
B. Superior Articulating Facet
C. Midline
D. Superior Border of the Caudal Vertebra
E. Inferior Border of the Rostral Vertebra
Superior Articulating Facet
Kambin’s triangle is formed by three structures. The inferior aspect of the triangle is formed by the
superior endplate of the caudal vertebral body. The height of the triangle and medial border is
formed by the superior articulating facet. Finally, the hypotenuse of the triangle is formed by the
exiting nerve root at this level.
This anatomical zone is important for neurosurgeons, as it represents a safe area to access the
intervertebral disc, most commonly for interbody fusion techniques. Within this space, no neural or
vascular structures of importance are present
A stab wound that severs the lower trunk of the brachial plexus interrupts axons traveling to which
of the following peripheral nerves?
Answers:
A. Musculocutaneous Nerve
B. Long Thoracic Nerve
C. Ulnar Nerve
D. Suprascapular Nerve
E. Axillary Nerve
Ulnar Nerve
The brachial plexus can be divided into five segments: roots, trunks, divisions, cords, and
branches. The roots and trunks can be found in the supraclavicular space. The divisions typically
are retroclavicular. The cords and branches are infraclavicular. The branches typically arise at the
level of the axilla.
The C5 and C6 nerves merge to form the upper trunk. The C7 nerve continues independently as
the middle trunk. The C8 and T1 nerves merge to form the lower trunk. The anterior division of
the lower trunk continues as the medial cord, while the posterior division joints the posterior
divisions of the upper and middle trunk to form the posterior cord. The medial cord gives rise to
the medial pectoral nerve, medial brachial cutaneous nerve, and medial antebrachial cutaneous
nerve before terminating as a bifurcation into the medial cord contribution to the median nerve and
ulnar nerve. Of note, the medial cord contribution to the median nerve joins the lateral cord
contribution to form the median nerve. The medial cord contribution carries primarily motor fibers,
whereas the lateral cord contribution carries primarily sensory fibers.
The musculocutaneous nerve arises from the lateral cord of the brachial plexus, carrying fibers
originating predominantly from the upper trunk, though there may also be fibers from the middle
trunk. The suprascapular nerve arises from the upper trunk or from the posterior division of the
upper trunk, carrying fibers from C5 and C6. The axillary nerve is one of the terminal branches of
the posterior cord, carrying fibers originating in the upper trunk. The long thoracic nerve forms
from the merger of branches from C5, C6, and C7.
The rule of 3s can be used as a rough guide for the management of traumatic nerve injuries.
Open, sharp nerve injuries should be repaired within 3 days of the injury. Open, ragged nerve
injuries are typically repaired 3 weeks after the injury, allowing time for the zone of injury to fully
demarcate. In many instances, a more acute exploration is undertaken to tag the injured nerves to
be repaired later. Management of closed nerve injuries is typically delayed for 3 months, with
surgical management reserved for those patients not demonstrating clinical or electrodiagnostic
evidence of recovery. The management of closed injuries is the most nuanced. An open, sharp
injury, such as the one described in this question, should be explored and repaired within 3 days of
the injury.
Axonal regeneration after injury is a slow process, often quoted to proceed at an average rate of 1
millimeter per day or approximately 1 inch per month.
A 50-year-old man is being evaluated after sustaining injuries to his back and left knee in a motor
vehicle collision. To distinguish L5 radiculopathy from peroneal nerve injury in this patient, which of
the following is the most appropriate muscle to test?
Answers:
A. Tibialis Anterior Muscle
B. Soleus Muscle
C. Gastrocnemius Muscle
D. Extensor Hallucis Longus Muscle
E. Tibialis Posterior Muscle
Tibialis Posterior Muscle
The sciatic nerve has nerve root contributions originating from L4-S3. It comprises the tibial
(medial) and common peroneal (lateral) divisions. After exiting from the pelvis through the greater
sciatic foramen, the sciatic nerve divides in the posterior thigh a variable distance from the
popliteal fossa into the tibial nerve and common peroneal nerve. The common peroneal nerve
(CPN) then continues deep and medial to the biceps femoris muscle and tendon as it proceeds
distally into the leg. It courses around the neck of the fibula where it is usually quite superficial and,
as a result, susceptible to injury. The CPN trifurcates into the superficial peroneal nerve, deep
peroneal nerve, and the articular branch to the superior tibiofibular joint around the fibular tunnel
inlet. At the fibular tunnel inlet, the common peroneal nerve can be compressed by the deep
fascia of the peroneus longus. The superficial peroneal nerve continues in the lateral
compartment of the leg (most commonly) between the peroneus longus and brevis, supplying
sensory innervation to the anterolateral leg and dorsal aspect of the foot (with the exception of a
small area in the first web space between digits 1 and 2), as well as motor innervation to the
peroneus longus and brevis for foot eversion. The deep peroneal nerve pierces the peroneus
longus muscle and courses under the extensor digitorum longus to run between the tibialis anterior
and extensor digitorum longus in the superior portion of the leg and between the tibialis anterior
and extensor hallucis longus in the inferior leg. As it moves distally through the anterior
compartment of the leg on the anterior surface of the interosseous membrane, it travels adjacent
to the anterior tibial artery to the dorsal foot. Along this route, it supplies motor innervation to the
tibialis anterior, extensor digitorum longus, extensor hallucis longus, extensor digitorum brevis,
extensor hallucis brevis, and peroneus tertius to enable ankle dorsiflexion and toe extension. It
also supplies cutaneous sensory innervation to a small area of skin in the first webspace.
The major differential diagnosis for a patient presenting with a foot drop is an L5 radiculopathy
versus a common peroneal neuropathy. Common peroneal neuropathy will present with weakness
of dorsiflexion and eversion. Inversion will not be weak in a patient with common peroneal
neuropathy. In an L5 radiculopathy, the patient will typically present with weakness of dorsiflexion
and inversion, though eversion may also be weak. Thus, inversion and eversion can be used on
physical examination to help differentiate an L5 radiculopathy from a common peroneal
neuropathy. Inversion is mediated by the tibialis posterior muscle, innervated by the tibial nerve.
Eversion is mediated by the peroneus longus and brevis, innervated by the common peroneal
nerve via the superficial peroneal nerve.
Two main peripheral nerve injury classification systems exist to describe the severity of nerve
injury and are based on the severity of injury to the axons and the connective tissue layers
surrounding them. The first classification system was proposed by Seddon and consists of 3
grades: neurapraxia, axonotmesis, and neurotmesis. Neurapraxia is the mildest from of peripheral
nerve injury and is strictly an injury to the myelin sheath, with preservation of the axon,
endoneurium, perineurium, and epineurium. Axonotmesis is the next most severe grade of injury
and results in demyelination, as well as injury to the axon. Axonotmesis has preservation of the
epineurium, with variable degrees of injury to the endoneurium and perineurium. In contrast,
neurotmesis is the most severe type of nerve injury in this schema and results in injury to both the
axon and its surrounding connective tissue, including the epineurium, perineurium, and
endoneurium. Subsequent to the Seddon classification being created, Sunderland then elaborated
on it by expanding and describing the various degrees of injury that can occur to the nerve
connective tissue. The resultant Sunderland classification system consisted of 5 grades. Grade 1
injuries correspond to neurapraxic injuries, grade 5 to neurotmetic injuries, and grades 2-4 to
axonotmetic injuries in the Seddon classification. Grades 2-4 are differentiated depending on the
degree of connective tissue injury: Grade 2: axonal injury only, Grade 3: axonal injury with
endoneurial injury, Grade 4: axonal injury with endoneurial and perineurial injury.
A lesion of the common peroneal (anterior tibial) portion of the sciatic nerve in the buttocks is most
likely to result in weakness of which of the following muscles?
Answers:
A. Gastrocnemius
B. Short head of the biceps femoris
C. Soleus
D. Long head of the biceps femoris
E. Flexor digitorum longus
Short head of the biceps femoris
The sciatic nerve is formed via contributions from the L4-S3 roots and is composed of distinct tibial
and peroneal components (divisions). The sciatic nerve descends through the posterior pelvis
toward the lower limb and exits the pelvis into the gluteal region via the greater sciatic foramen.
After exiting the sciatic foramen, the sciatic nerve typically courses underneath the piriformis
muscle before entering the posterior compartment of the thigh. As the nerve courses through the
posterior thigh, it provides branches innervating the hamstring muscles that are responsible for
knee flexion (semitendinosus, biceps femoris, semimembranosus, hamstring portion of adductor
magnus). The tibial division of the sciatic nerve innervates the semitendinosus,
semimembranosus, hamstring portion of the adductor magnus, and long head of the biceps
femoris, while the peroneal division innervates the short head of the biceps femoris. This is the
only muscle innervated by the peroneal division/common peroneal nerve above the knee. The
sciatic nerve courses down the posterior thigh and divides into the common peroneal nerve
(lateral) and tibial nerve (medial) just proximal to the popliteal fossa. The common peroneal nerve
courses laterally through the popliteal fossa, around the fibular neck. At the inlet to the fibular
tunnel, the common peroneal nerve trifurcates into the superficial peroneal nerve, deep peroneal
nerve, and the articular branch to the superior tibiofibular joint. The superficial peroneal nerve
continues in the lateral compartment of the leg (most commonly) between the peroneus longus
and brevis, supplying sensory innervation to the anterolateral leg and dorsal aspect of the foot
(with the exception of a small area in the first web space between digits 1 and 2), as well as motor
innervation to the peroneus longus and brevis for foot eversion. The deep peroneal nerve pierces
the peroneus longus muscle and courses under the extensor digitorum longus to run between the
tibialis anterior and extensor digitorum longus in the proximal portion of the leg and between the
tibialis anterior and extensor hallucis longus in the distal leg. As it moves distally through the
anterior compartment of the leg on the anterior surface of the interosseous membrane, it travels
adjacent the anterior tibial artery to the dorsal foot. Along this route, it supplies motor innervation to
the tibialis anterior, extensor digitorum longus, extensor hallucis longus, extensor hallucis brevis,
extensor digitorum brevis, and peroneus tertius to enable ankle dorsiflexion and toe extension. It
also supplies cutaneous sensory innervation to a small area of skin in the first webspace between
digits 1 and 2. The tibial nerve after the sciatic bifurcation continues into the deep posterior
compartment of the leg. It innervates the gastrocnemius and soleus for plantar flexion, the tibialis
posterior for inversion, the flexor hallucis longus and flexor digitorum longus for toe flexion, as well
as all of the foot intrinsic muscles except the extensor hallucis brevis and extensor digitorum
brevis. The tibial nerve also provides cutaneous sensation to the plantar foot via the medial
plantar nerve, lateral plantar nerve, and calcaneal nerve.
Beaton and Anson described variations of sciatic nerve anatomy around the piriformis muscle.
The piriformis is a potential point of compression and understanding these variations has surgical
relevance when treating sciatic nerve pathology around the piriformis. Type I is the most common,
where both the peroneal and tibial divisions course under the piriformis muscle. In Types II-VI, one
or both of the divisions course over or through the piriformis muscle.
Proximal lesions of the peroneal division of the sciatic nerve are distinguished from distal common
peroneal nerve lesions at or below the fibular neck based on EMG abnormalities and evidence of
denervation in the short head of the biceps femoris. For instance, classic compression of the
common peroneal nerve at the fibular tunnel would not demonstrate any EMG abnormalities
proximal to the tibialis anterior. Thus, there would be no EMG abnormalities in the short head of
the biceps femoris. Additionally, when tracking recovery of a proximal injury to the peroneal
division of the sciatic nerve, the short head of the biceps femoris would be the first muscle
expected to potentially show reinnervation, making the short head of the biceps femoris critical to
examine on EMG in these circumstances.
The innervation of the rhomboid muscles is derived from which of the following nerves?
Answers:
A. Upper Subscapular Nerve
B. Suprascapular Nerve
C. Long Thoracic Nerve
D. Dorsal Scapular Nerve
E. Lower Subscapular Nerve
Dorsal Scapular Nerve
The rhomboid muscles (rhomboid major and minor) are innervated by the dorsal scapular nerve,
which arises proximally from the C5 nerve root. In addition to the rhomboid major and minor, the
dorsal scapular nerve also innervates the levator scapulae. The dorsal scapular nerve arises at the
level of the nerve roots from C5, passing through the substance of the middle scalene muscle. The
C5 root also gives contributions to the phrenic nerve and then the long thoracic nerve, also formed
within the substance of the middle scalene. Distal to these branches, C5 and C6 join to form the
upper trunk of the brachial plexus. The upper trunk then trifurcates into the suprascapular nerve,
posterior division, and anterior division of the upper trunk, though this can be a bifurcation with the
suprascapular nerve arising from the posterior division.
Due to the very proximal location of the dorsal scapular nerve origin, in the setting of a brachial
plexus injury and absent rhomboid function, one should suspect a very proximal injury, possibly
pre-ganglionic. The rhomboids begin on the lower cervical and upper thoracic spinous processes
and insert onto the medial border of the scapula. They function to stabilize the scapula during arm
movement.
The long thoracic nerve (C5-7) innervates one muscle, the serratus anterior. The suprascapular
nerve (C5-6) arises from the upper trunk and innervates both the supraspinatus and infraspinatus.
The upper subscapular nerve (C5) arises from the posterior cord and innervates the upper portion
of the subscapularis muscle. The lower subscapular nerve (C5-6) arises from the posterior cord
and innervates the lower portion of the subscapularis and teres major muscles.
Atrophy of the first dorsal interosseous muscle is usually associated with a lesion of the:
Answers:
A. Radial Nerve
B. Median Nerve
C. Posterior Interosseous Nerve
D. Anterior Interosseous Nerve
E. Ulnar Nerve
Ulnar Nerve
The first dorsal interosseous muscle is innervated by the ulnar nerve, so a lesion of the ulnar nerve
is the most likely to be associated with atrophy of the first dorsal interosseus muscle.
The ulnar nerve arises from the medial cord, carrying fibers predominantly from the C8 and T1
roots, though there can be fibers from C7. The ulnar nerve provides motor innervation to the flexor
carpi ulnaris and the ulnar half of the flexor digitorum profundus in the forearm and then innervates
all of the hand intrinsic muscles except for the LOAF muscles—lumbricals 1 and 2, opponens
pollicis, abductor pollicis brevis, and flexor pollicis brevis—which are innervated by the median
nerve. The ulnar nerve provides sensation to the dorsal ulnar hand through the dorsal cutaneous
branch and to the palmar ulnar hand through the palmar cutaneous branch, both of which arise in
the forearm, proximal to Guyon’s canal, as well as to the ulnar 1.5 digits through the terminal
superficial branch of the ulnar nerve.
Ulnar nerve entrapment occurs primarily in two places: 1) at or around the elbow, referred to as
cubital tunnel syndrome, and 2) in the palm, referred to as Guyon’s canal syndrome. Entrapment
of the ulnar nerve at the elbow is much more common than at the palm, and represents the second
most common entrapment neuropathy in the upper extremity, following carpal tunnel syndrome.
Understanding the common points of compression for the ulnar nerve is important in surgical
treatment of ulnar entrapment neuropathy. Commonly described points of compression around the
elbow from proximal to distal include:
1. Arcade of Struthers – Musculoaponeurotic band in the upper arm extending from the medial
head of triceps to the medial intermuscular septum. This is not to be confused with the
Struthers ligament which is a fibrous band extending from a supracondylar process to the
medial epicondyle and associated with entrapment of the median nerve.
2. Medial intermuscular septum
3. Medial epicondyle
4. Osborne’s ligament – fibrous band extending from medial epicondyle to the olecranon
5. Osborne’s fascia – fascia bridging the two heads of the flexor carpi ulnaris muscle.
Entrapment at the elbow can be differentiated from entrapment at Guyon’s canal by examining the
dorsal and palmar hand. The branches that innervate the dorsal hand and the palm originate
distal to the cubital tunnel in the forearm and do not traverse Guyon’s canal. Thus, reduced
sensation on the dorsal hand and the palm are associated with cubital tunnel but not Guyon’s
canal syndrome, whereas both syndromes will have reduced sensation in the ulnar 1.5 digits.
Ulnar neuropathy should also be differentiated from more proximal etiologies, including thoracic
outlet syndrome and C8 radiculopathy. Ulnar neuropathy will present with sensory loss that does
not extend proximal to the wrist and that reliably splits the fourth digit, whereas C8 radiculopathy
will extend proximal to the wrist and will not split the fourth digit. With regard to motor function,
hand weakness related to an ulnar neuropathy will spare the LOAF muscles, whereas a C8
radiculopathy will involve all of the hand intrinsic muscles (both the ulnar and median nerve
innervated muscles).
As it travels distal to the ulnar groove, the ulnar nerve enters the:
Answers:
A. Spiral Groove
B. Radial Tunnel
C. Cubital Tunnel
D. Guyon’s Canal
E. Carpal Tunnel
Cubital Tunnel
The ulnar nerve arises from the medial cord, carrying fibers predominantly from the C8 and T1
roots, though there can be fibers from C7. The ulnar nerve provides motor innervation to the flexor
carpi ulnaris and the ulnar half of the flexor digitorum profundus in the forearm and then innervates
all of the hand intrinsic muscles except for the LOAF muscles—lumbricals 1 and 2, opponens
pollicis, abductor pollicis brevis, and flexor pollicis brevis—which are innervated by the median
nerve. The ulnar nerve provides sensation to the ulnar 1.5 digits through the terminal superficial
branch of the ulnar nerve. Additionally, the ulnar nerve provides sensation to the dorsal ulnar hand
through the dorsal cutaneous branch and to the palmar ulnar hand through the palmar cutaneous
branch, both of which arise in the forearm, proximal to Guyon’s canal.
After arising from the medial cord, the ulnar nerve then descends with the brachial artery up to the
insertion point of the coracobrachialis muscle. It subsequently pierces the medial intermuscular
septum and enters the posterior compartment of the arm, where it then travels posteromedial to
the humerus and behind the medial epicondyle in the post-condylar/ulnar groove to enter the
cubital tunnel. It then enters the anterior compartment of the forearm between the two heads of the
flexor carpi ulnaris. Once it reaches the wrist, it enters the hand superficial to the flexor retinaculum
and lateral to the pisiform bone traversing Guyon’s canal. It then terminally divides into superficial
and deep branches. Two major sensory branches arise in the forearm, distal to the cubital tunnel
but proximal to Guyon’s canal: the palmar cutaneous branch and the dorsal cutaneous branch.
The two major zones of compression for the ulnar nerve are around the elbow and around Guyon’s
canal. Compression around the elbow, known as cubital tunnel syndrome, can occur related to the
medial intermuscular septum, the Arcade of Struthers, bony abnormalities of the medial
epicondyle, Osborne’s ligament, or Osborne’s fascia. The major sensory branches in the forearm,
the dorsal cutaneous and palmar cutaneous branches, can be used to help differentiate cubital
tunnel syndrome from Guyon’s canal syndrome. Sensory loss in the distribution of these nerves
suggests cubital tunnel syndrome, since these branches arise proximal to Guyon’s canal and are
spared in Guyon’s canal syndrome. In Guyon’s canal, compression can often occur related to the
presence of a ganglion cyst or related to external compression such as can occur from a bicycle
handlebar.
The radial nerve runs along the humerus in the spiral groove. The median nerve passes through
the carpal tunnel. The radial nerve/posterior interosseous nerve passes through the radial tunnel
in the forearm.
In a patient undergoing evaluation for carpal tunnel syndrome, individual muscle testing is most
likely to disclose weakness of which of the following muscles?
Answers:
A. Flexor Pollicis Longus
B. Adductor Pollicis
C. Pronator Teres
D. Abductor Digiti Minimi
E. Abductor Pollicis Brevis
Abductor Pollicis Brevis
The median nerve traverses the carpal tunnel, along with four tendons of the flexor digitorum
superficialis, four tendons of the flexor digitorum profundus, and the tendon of the flexor pollicis
longus. The roof of the carpal tunnel is the transverse carpal ligament, which attaches to the
scaphoid and trapezium on the radial side and the pisiform and hamate on the ulnar side.
Compression of the median nerve can occur due to thickening of the transverse carpal ligament,
tenosynovitis, or related to a space occupying lesion in the carpal tunnel.
The ulnar nerve innervates all of the hand intrinsic muscles, except for the LOAF muscles
—lumbricals 1/2, opponens pollicis, abductor pollicis brevis, and flexor pollicis brevis—which are
innervated by the median nerve. The median nerve sensory distribution is the palmar aspect of
the radial 3.5 digits and the corresponding palm. The palm is innervated by the palmar cutaneous
branch, which arises proximal to the carpal tunnel and does not pass through the carpal tunnel.
Carpal tunnel syndrome presents with numbness in the radial 3.5 digits (sparing the palm),
paresthesias, and/or weakness of the LOAF muscles. Pain and paresthesias often have nocturnal
worsening and patients frequently describe shaking out their hand to relieve the symptoms. The
median-innervated forearm muscles include the pronator teres, flexor carpi radialis, flexor
digitorum superficialis, and the anterior interosseous-innervated muscles, including the flexor
digitorum profundus, flexor pollicis longus, and pronator quadratus. The median-innervated
forearm muscles are spared in carpal tunnel syndrome. On examination, provocative maneuvers
that may prove useful in diagnosing carpal tunnel syndrome include the Tinel sign, Durkan
compression test, Phalen maneuver, and reverse Phalen maneuver. Ancillary studies including
electrodiagnostics and ultrasound can aid in diagnosis.
Treatment options typically include bracing, carpal tunnel steroid injections, and surgical carpal
tunnel release.
The abductor digiti minimi and adductor pollicis are innervated by the ulnar nerve, so are not
involved in carpal tunnel syndrome. The pronator teres and flexor pollicis longus are innervated by
the median nerve, but proximal to the carpal tunnel, so they are not involved in carpal tunnel
syndrome.
Which of the following muscles is innervated by the dorsal scapular nerve?
Answers:
A. Infraspinatus
B. Subscapularis
C. Rhomboids
D. Serratus Posterior
E. Serratus Anterior
Rhomboids
The rhomboid muscles (rhomboid major and minor) are innervated by the dorsal scapular nerve,
which arises proximally from the C5 nerve root. In addition to the rhomboid major and minor, the
dorsal scapular nerve also innervates the levator scapulae. The dorsal scapular nerve arises at the
level of the nerve roots from C5, passing through the substance of the middle scalene muscle. The
C5 root also gives contributions to the phrenic nerve and then the long thoracic nerve, also formed
within the substance of the middle scalene. Distal to these branches, C5 and C6 join to form the
upper trunk of the brachial plexus. The upper trunk then trifurcates into the suprascapular nerve,
posterior division, and anterior division of the upper trunk, though this can be a bifurcation with the
suprascapular nerve arising from the posterior division.
Due to the very proximal location of the dorsal scapular nerve origin, in the setting of a brachial
plexus injury and absent rhomboid function, one should suspect a very proximal injury, possibly
pre-ganglionic. The rhomboids begin on the lower cervical and upper thoracic spinous processes
and insert onto the medial border of the scapula. They function to stabilize the scapula during arm
movement.
The differential diagnosis for a winged scapula includes a dorsal scapular neuropathy with
weakness of the rhomboid muscles, a spinal accessory neuropathy with weakness of the
trapezius, and a long thoracic nerve palsy with weakness of the serratus anterior. A long thoracic
nerve palsy causes medial winging, whereas dorsal scapular neuropathy and spinal accessory
neuropathy cause lateral winging. The winging associated with a dorsal scapular neuropathy is
typically very subtle.
The serratus anterior muscle is innervated by the long thoracic nerve (C5-7). The infraspinatus
muscle is innervated by the suprascapular nerve (C5-6). The subscapularis muscle is innervated
by the upper and lower subscapular nerves (C5-6). The serratus posterior muscle is innervated by
intercostal nerves (T2-5 for serratus posterior superior and T9-12 for serratus posterior inferior).
The patient with the injury shown in the x-ray film is most likely to sustain a nerve injury that results
in impairment of which of the following movements?
Answers:
A. Finger Flexion
B. Wrist Flexion
C. Finger Adduction
D. Elbow Extension
E. Wrist Extension
Wrist Extension
Radial nerve injury associated with a humeral shaft fracture is a characteristic injury pattern and
would cause weakness in wrist and finger extension. The radial nerve is formed from the posterior
divisions of the brachial plexus (C5-T1) and is one of two terminal branches of the posterior cord,
the other being the axillary nerve. The radial nerve runs behind the brachial artery and along the
posterior border of the axilla. In the arm, it passes between the long head of the triceps and the
shaft of the humerus, beneath the teres major muscle (triangular interval). At the mid-humeral
level, the nerve crosses the posterior aspect of the humerus moving from medial to lateral, lying in
the spiral groove and traveling with the profunda brachii artery. The radial nerve innervates the
triceps brachii very proximally and then distal to the spiral groove innervates the brachioradialis,
extensor carpi radialis longus, and extensor carpi radialis brevis. In the proximal forearm, the nerve
bifurcates into the superficial radial nerve (sensory) and the deep radial/posterior interosseous
nerve (PIN). The posterior interosseous nerve innervates the supinator, extensor carpi ulnaris,
extensor digitorum communis, extensor digiti minimi, abductor pollicis longus, extensor pollicis
longus, extensor pollicis brevis, and extensor indicis.
Potential compression points include the radial nerve at the spiral groove by the lateral
intermuscular septum, the posterior interosseous nerve at the supinator/Arcade of Frohse, and the
superficial radial nerve between the brachioradialis and extensor carpi radialis longus tendons.
Due to the course of the radial nerve in close association with the humerus, the radial nerve is
particularly prone to injury in association with mid-shaft humeral fractures.
Clinically, proximal radial nerve injuries are characterized by weakness or loss of elbow extension,
wrist extension, and finger extension. The triceps branches arise very proximally such that elbow
extension is preserved with injuries to the radial nerve in the mid-arm. A posterior interosseous
palsy is characterized by loss of finger extension, with preserved wrist extension but radial
deviation during wrist extension. This occurs due to the preservation of function in the extensor
carpi radialis longus and brevis (innervated by the radial nerve), with loss of function in the
extensor carpi ulnaris (innervated by the posterior interosseous nerve).
While humeral shaft fractures account for only approximately 3% of all fractures, these have a high
association with radial nerve palsies. This incidence has been noted to be anywhere between 7
and 17% of all humeral shaft fractures, which makes this the most common nerve injury
associated with long bone fractures.
There is debate regarding timing of surgical exploration. Traditionally, radial nerve injuries
associated with closed humeral fractures have been treated expectantly, with ~70% spontaneous
recovery. When combined with those patients who underwent delayed surgical exploration due to
lack of recovery, the overall recovery rate rose to ~88%. In this series, the recovery rate when
early exploration was utilized was ~88%, suggesting that early exploration and delayed exploration
for those that do not spontaneously recover are equivalent. However, in some more updated
series, there has been some suggestion that early exploration may be better. In a larger, more
recent series, expectant management was noted to yield a 77% recovery rate, while early
explorations (<3 weeks from injury) yielded a 90% recovery rate. Thus, more recent data have
suggested that early exploration may provide more benefit than expectant management in radial
nerve injury associated with humerus fractures. Regardless, a good outcome can be expected in
the majority of patients with radial nerve injuries associated with humeral fractures. When
examining for spontaneous recovery, the branch to the brachioradialis is the first branch distal to
the spiral groove. Thus, the brachioradialis is the muscle that is expected to show recovery first
and is critical to examine.
Finger and wrist flexion weakness occur primarily with median injuries, though the ulnar nerve
contributes to these movements as well. Finger abduction and adduction are innervated by the
ulnar nerve. Elbow extension is mediated by the triceps brachii muscle, which is innervated by the
radial nerve. However, the branches to the triceps brachii occur proximal to the spiral groove, so
the triceps brachii is typically spared with injury to the radial nerve associated with mid-shaft
humeral fractures.
Two days after peripheral nerve transection, electromyography and nerve conduction velocity
studies distal to the lesion are most likely to show:
Answers:
A. Absent Sensory Nerve Action Potential, Normal Motor Nerve Action Potential, Normal
Voluntary Motor Unit Action Potentials, Absent Fibrillation Potentials
B. Normal Sensory Nerve Action Potential, Normal Motor Nerve Action Potential, Absent
Voluntary Motor Unit Action Potentials, Absent Fibrillation Potentials
C. Normal Sensory Nerve Action Potential, Normal Motor Nerve Action Potential, Normal
Voluntary Motor Unit Action Potentials, Fibrillation Potentials Present
D. Normal Sensory Nerve Action Potential, Normal Motor Nerve Action Potential, Normal
Voluntary Motor Unit Action Potentials, Absent Fibrillation Potentials
E. Normal Sensory Nerve Action Potential, Absent Motor Nerve Action Potential, Normal
Voluntary Motor Unit Action Potentials, Absent Fibrillation Potentials
Normal Sensory Nerve Action Potential, Normal Motor Nerve Action Potential, Absent
Voluntary Motor Unit Action Potentials, Absent Fibrillation Potentials
When axons are disrupted, the distal axon degenerates through a process called Wallerian
degeneration. This degenerative process occurs in an anterograde fashion distal to the site of
injury. Within hours of the injury, axonal and myelin fragmentation occurs with breakdown of its
cytoskeletal components and subsequent loss of conductivity by 2-4 days postinjury, and the
terminal, injured, axonal end sealing itself off. Due to this process taking several days, immediately
after the injury action potentials can continue to be transmitted in the injured axon resulting in no
detectable electrophysiological abnormality. For mixed nerves, this means that a motor action
potential and sensory action potential can be detected and will be normal for several days after a
nerve transection, assuming the stimulating and recording electrodes are both placed proximal to
the injury or distal to the injury and not across the injury. Despite the motor action potential being
normal, the action potential cannot cross the site of transection, leading to loss of voluntary motor
unit action potentials, which occurs immediately at the time of the injury. Schwann cells respond to
the injury immediately by proliferating and upregulating gene expression essential to their function
in Wallerian degeneration. Macrophages, having migrated into the zone of injury from nearby blood
vessels, phagocytose the axonal and myelin debris preparing it for axonal regeneration. With the
macrophages, other inflammatory cells also gain entry into the site and begin phagocytosing the
debris, which can take several weeks to several months postinjury depending on the severity of
the injury. Wallerian degeneration of the distal process continues over the subsequent weeks to
months. When injuries are more severe, the inflammatory response is typically more vigorous with
subsequent necrosis, cellular infiltration, and proliferation that correlates with the severity of the
initial injury. Endoneurial vascular disruption also occurs resulting in hemorrhage, edema, and
influx of neutrophils and fibroblasts. The latter ultimately results in intraneural scarring with
collagen deposition. Positive sharp waves and fibrillation potentials typically take 10-14 days to
develop, as the muscle membrane becomes unstable due to denervation when Wallerian
degeneration reaches the neuromuscular junction.
As mentioned above, the process of Wallerian degeneration typically takes weeks to months.
Because of this, in the event of an intraoperative injury, stimulating the nerve distal to the site of
injury will still result in the detection and recording of action potentials due to the axons still being
in continuity and conducting the action potentials toward the recording electrode. In the same vein,
performing nerve conduction studies distal to the site of injury, including compound motor action
potentials (CMAPs) and sensory nerve action potentials (SNAPs), immediately after injury may not
detect any abnormality due to the fact that the axons conducting these action potentials have not
yet degenerated through Wallerian degeneration, assuming both the stimulating electrode and
recording electrode are distal to the site of injury. On the other hand, if the stimulating electrode
and the recording electrode are on opposite sides of the transected nerve, then no action
potentials will be recorded. In order for both SNAPs and CMAPs to be recorded, integrity of the
lower motor neuron, specifically the distal axonal segment, must be preserved. As such, both
SNAP and CMAP recordings will be affected in the event of a post-ganglionic injury. However,
SNAPs and CMAPs differ in the setting of a pre-ganglionic injury in that CMAPs will be affected,
but SNAPs will be preserved due to the injury occurring proximal to the cell body and thus the
integrity of the distal axonal segment being preserved.
The deep branch of the peroneal nerve supplies motor function to which of the following muscles?
Answers:
A. Short Head of the Biceps Femoris
B. Tibialis Posterior
C. Peroneus Longus
D. Tibialis Anterior
E. Soleus
Tibialis Anterior
The deep peroneal nerve is a terminal branch of the common peroneal nerve. After the sciatic
bifurcation, the common peroneal nerve (CPN) continues deep and medial to the biceps femoris
muscle and tendon as it proceeds distally into the leg. It courses around the neck of the fibula
where it is usually quite superficial and as a result, susceptible to injury. The common peroneal
nerve trifurcates into the superficial peroneal nerve, deep peroneal nerve, and the articular branch
to the superior tibiofibular joint around the fibular tunnel inlet at the fibular neck. At the fibular
tunnel inlet, the common peroneal nerve can be compressed by the deep fascia of the peroneus
longus. The superficial peroneal nerve continues in the lateral compartment of the leg (most
commonly) between the peroneus longus and brevis, supplying sensory innervation to the
anterolateral leg and dorsal aspect of the foot (with the exception of a small area in the first web
space between digits 1 and 2), as well as motor innervation to the peroneus longus and brevis for
foot eversion. The deep peroneal nerve pierces the peroneus longus muscle and courses under
the extensor digitorum longus to run between the tibialis anterior and extensor digitorum longus in
the superior portion of the leg and between the tibialis anterior and extensor hallucis longus in the
inferior leg. As it moves distally through the anterior compartment of the leg on the anterior surface
of the interosseous membrane, it travels adjacent to the anterior tibial artery to the dorsal foot.
Along this route, it supplies motor innervation to the tibialis anterior, extensor digitorum longus,
extensor hallucis longus, extensor digitorum brevis, extensor hallucis brevis, and peroneus tertius
to enable ankle dorsiflexion and toe extension. It also supplies cutaneous sensory innervation to a
small area of skin in the first webspace (between digits 1 and 2).
The common peroneal nerve is susceptible to compression at the fibular tunnel inlet by the deep
fascia of the peroneus longus. The superficial peroneal nerve can be compressed in the distal leg
where it pierces the fascia to leave the lateral compartment (typically) and enter the subcutaneous
tissue. The deep peroneal nerve is susceptible to compression in the anterior tarsal tunnel by the
inferior extensor retinaculum or in the mid-foot by the tendon of the extensor hallucis brevis. This
should not be confused with what is commonly referred to as tarsal tunnel syndrome. Tarsal
tunnel syndrome should be more appropriately referred to as posterior tarsal tunnel syndrome.
Posterior tarsal tunnel syndrome involves compression of the tibial nerve by the flexor
retinaculum. The common peroneal nerve is the nerve most susceptible to formation of an
intraneural ganglion cyst. Synovial fluid from a degenerative superior tibiofibular joint travels along
the articular branch into the common peroneal nerve to form the cyst. Due to the anatomic
relationship at the trifurcation of the common peroneal nerve, peroneal intraneural ganglion cysts
often preferentially affect the deep peroneal nerve over the superficial peroneal nerve. The deep
peroneal nerve is anatomically adjacent to the articular branch to the superior tibiofibular joint,
which is the conduit for formation of peroneal intraneural ganglion cysts.
The soleus and tibialis posterior muscles are innervated by the tibial nerve. The short head of the
biceps femoris is innervated by the peroneal division of the sciatic nerve in the thigh.
Which of the following structures surrounds the individual fascicles within a nerve?
Answers:
A. Exoneurium
B. Myelin Sheath
C. Perineurium
D. Epineurium
E. Endoneurium
Perineurium
Axons are bundled together into groups called fascicles. Fascicles are bundled together to form
nerves. Axons can be myelinated or unmyelinated. Endoneurium is the intrafascicular connective
tissue surrounding the axons. Perineurium is the connective tissue surrounding each fascicle.
Epineurium surrounds the entire nerve.
Two main peripheral nerve injury classification systems exist to describe the severity of nerve
injury and are based on the severity of injury occurring to the axons and the connective tissue
layers surrounding them. The first classification system was proposed by Seddon and consists of 3
grades: neurapraxia, axonotmesis, and neurotmesis. Neurapraxia is the mildest from of peripheral
nerve injury and is strictly an injury to the myelin sheath, with preservation of the axon,
endoneurium, perineurium, and epineurium. Axonotmesis is the next most severe grade of injury
and results in demyelination, as well as injury to the axon. Axonotmesis has preservation of the
epineurium, with variable degrees of injury to the endoneurium and perineurium. In contrast,
neurotmesis is the most severe type of nerve injury in this schema and results in injury to both the
axon and its surrounding connective tissue, including the epineurium, perineurium, and
endoneurium. Subsequent to the Seddon classification being created, Sunderland elaborated on it
by expanding and describing the various degrees of injury that can occur to the nerve connective
tissue. The resultant Sunderland classification system consisted of 5 grades. Grade 1 injuries
correspond to neurapraxic injuries, grade 5 to neurotmetic injuries, and grades 2-4 to axonotmetic
injuries in the Seddon classification. Grades 2-4 are differentiated depending on the degree of
connective tissue injury: Grade 2: axonal injury only, Grade 3: axonal injury with endoneurial injury,
Grade 4: axonal injury with endoneurial and perineurial injury.
During an open carpal tunnel release, transecting a small median nerve branch is most likely to
result in weakness of which of the following muscles?
Answers:
A. Flexor Pollicis Longus
B. Adductor Pollicis
C. Extensor Pollicis Longus
D. Opponens Pollicis
E. Abductor Pollicis Longus
Opponens Pollicis
In the upper extremity, as the median nerve travels from the anterior compartment of the forearm
to the palmar hand, it passes through the carpal tunnel. The roof of the carpal tunnel is formed by
the flexor retinaculum or transverse carpal ligament (fibrous ligament attaching from the pisiform
and hamate medially to the trapezium and scaphoid laterally). The floor is made up of the
scaphoid, lunate, and triquetral carpal bones. Along with the median nerve, there are 9 tendons
that traverse the carpal tunnel, including the flexor digitorum superficialis and profundus tendons
and the flexor pollicis longus tendon. As the median nerve approaches the carpal tunnel in the
distal forearm, a palmar cutaneous branch emerges from it, piercing the deep antebrachial fascia
to course superficial to the flexor retinaculum and supply cutaneous sensory innervation to the
base and palmar aspect of the thenar eminence. Because this branch does not traverse the
carpal tunnel, it is spared in carpal tunnel syndrome. The recurrent thenar branch of the median
nerve arises under or just distal to the flexor retinaculum and winds around the distal border of the
retinaculum to reach the thenar muscles and the lateral two lumbricals (lumbricals 1 and 2). All of
the hand intrinsic muscles minus the LOAF muscles—lumbricals 1 and 2, opponens pollicis,
abductor pollicis brevis, and flexor pollicis brevis—are innervated by the ulnar nerve, while the
LOAF muscles are innervated by the median nerve. Thus, the LOAF muscles would be expected
to be weak in carpal tunnel syndrome or with injury to the median nerve or recurrent thenar branch
around the carpal tunnel. Once past the carpal tunnel, in addition to the aforementioned motor
innervation, the median nerve supplies sensory innervation to the palmar aspect of digits 1-3 and
lateral half of digit 4 by dividing into common palmar digital nerves and subsequently proper
palmar digital nerves.
The abductor pollicis longus is innervated by the radial nerve via the posterior interosseous nerve.
The adductor pollicis is innervated by the ulnar nerve, specifically the deep branch of the ulnar
nerve. The extensor pollicis longus is innervated by the radial nerve via the posterior interosseous
nerve. The flexor pollicis longus is innervated by the median nerve via the anterior interosseous
nerve in the forearm.
A 17-year-old female gymnast has atrophy of the supraspinatus muscle and difficulty abducting the
arm. She reports a recent three-week history of shoulder pain that has since abated. The most
likely cause of her symptoms is:
Answers:
A. Polymyalgia Rheumatica
B. Charcot-Marie-Tooth Disease
C. Parsonage-Turner Syndrome
D. Hereditary Neuropathy with Liability to Pressure Palsies
E. Amyotrophic Lateral Sclerosis
Parsonage-Turner Syndrome
Parsonage-Turner syndrome, also referred to as idiopathic brachial plexopathy or neuralgic
amyotrophy, classically presents with sudden onset of severe shoulder or periscapular pain that
lasts from hours to a few weeks. Following resolution (or significant improvement) of the pain,
weakness then begins and progresses over a few days to weeks. While Parsonage-Turner
syndrome is poorly understood, it is thought to be an autoimmune inflammatory neuropathy. Often
a potential precipitator can be identified in the patient history, including surgery, trauma, viral
infection, vaccination, pregnancy, or significant stress.
For unclear reasons, Parsonage-Turner syndrome does have a predilection for certain nerves.
The most commonly involved nerves include the long thoracic nerve, suprascapular nerve, axillary
nerve, musculocutaneous nerve, posterior interosseous nerve, and anterior interosseous nerve.
The nerves are often affected in a patchy manner, displaying various degrees of involvement and
sparing of nerves from the same root or plexus level. Electromyography can help identify
muscles/nerves with subclinical involvement, which can help support the diagnosis.
Oral steroids or immunotherapy have been recommended as a potential treatment early in the
course of the disease (typically within the first week after onset) but remain controversial. Physical
therapy remains the mainstay of treatment. The prognosis is good following a diagnosis of
Parsonage-Turner syndrome, with most patients demonstrating functional recovery within 2-3
years.
The differential diagnosis in this case would include suprascapular nerve entrapment at the
suprascapular notch. Entrapment of the suprascapular nerve can present with weakness of
shoulder abduction and external rotation, as well as pain, typically around the posterolateral
shoulder. Athletes and laborers that engage in frequent overhead activity, such as volleyball,
tennis, and swimming, are at risk for suprascapular nerve entrapment. Ganglion cysts, both
extraneural and intraneural, can also cause suprascapular neuropathy.
The typical presentation of polymyalgia rheumatica is widespread muscle aching and stiffness in
the shoulders and hips, most often in the morning. Amyotrophic lateral sclerosis presents with
both upper and lower motor neuron dysfunction including progressive asymmetric muscle
weakness, fasciculations, increased reflexes, difficulty speaking, and swallowing difficulty.
Hereditary Neuropathy with Liability to Pressure Palsies (HNPP) presents with recurrent episodes
of mononeuropathies often with minor trauma or pressure mostly affecting the peroneal, ulnar, and
median nerves. Charcot-Marie-Tooth disease is a hereditary polyneuropathy that often presents
with weakness of the lower extremities, foot deformities, and progressive gait difficulties and, later
in the course, can involve weakness of the upper extremities.
A 30-year-old man comes for evaluation two weeks after sustaining a penetrating injury to the left
popliteal fossa. An MR scan of the left knee shows a 5-cm abscess with compression of the tibial
nerve. This patient is most likely to exhibit weakness with which of the following maneuvers?
Answers:
A. Dorsiflexion
B. Plantar flexion
C. Knee Extension
D. Knee Flexion
E. Eversion
Plantar flexion
The sciatic nerve has nerve root contributions originating from L4-S3. It comprises tibial (medial)
and common peroneal (lateral) divisions. After exiting from the pelvis through the greater sciatic
foramen, the sciatic nerve divides in the posterior thigh a variable distance from the popliteal fossa
into the tibial nerve and common peroneal nerve. The tibial division of the sciatic nerve innervates
the majority of the hamstring muscles (semitendinosus, semimembranosus, and long head of the
biceps femoris) in the posterior thigh, with the exception of the short head of the biceps femoris
(supplied by the peroneal division of the sciatic nerve). After the sciatic bifurcation, the tibial nerve
then continues inferiorly through the middle of the popliteal fossa, coursing deep to the two heads
of the gastrocnemius muscle. It then passes under the fibrous/tendinous arch of the soleus
muscle, known as the soleal sling. The soleal sling is a potential point of compression/entrapment
for the tibial nerve. As the tibial nerve courses distally, it supplies motor innervation to the
gastrocnemius muscle, soleus muscle, and plantaris muscle in the proximal leg. Typically, just
proximal to the popliteal fossa, the tibial nerve gives of the medial sural cutaneous nerve. The
common peroneal nerve gives off the lateral sural cutaneous nerve. Though there is significant
sural nerve variability, the most common configuration is for the lateral sural cutaneous nerve to
give off a sural communicating branch that joins the medial sural cutaneous nerve in leg to form
the sural nerve proper. Once deep to the soleus, the tibial nerve continues posterior to the tibia
between the flexor digitorum longus (medially) and flexor hallucis longus (laterally), both of which it
supplies in addition to the tibialis posterior muscle. At the level of the ankle, the tibial nerve passes
between the medial malleolus and the Achilles tendon through the posterior tarsal tunnel. The
posterior tarsal tunnel is covered superficially (roof) by the flexor retinaculum which attaches to the
medial malleolus and calcaneal tuberosity. In analogous fashion to the carpal tunnel being a site of
entrapment for the median nerve in the upper extremity, the posterior tarsal tunnel with its
overlying flexor retinaculum can also serve as a site of entrapment for the tibial nerve in the lower
extremity. In proximity to or within this tunnel, the tibial nerve terminates into the medial plantar,
lateral plantar, and calcaneal nerves, which supply cutaneous innervation to the medial, lateral,
and calcaneal portion of the plantar foot surface, respectively. Motor innervation to the foot intrinsic
muscles also arises from the medial (abductor hallucis, flexor digitorum brevis, and flexor hallucis
brevis) and lateral (adductor hallucis, flexor digiti minimi, abductor digiti minimi, interossei) plantar
nerves. The lateral plantar, medial plantar, and calcaneal nerves also individually run through
fibrous tunnels to reach the plantar surface of the foot. Each of these nerves is subject to possible
compression within its individual fibrous tunnel.
Compression of the tibial nerve or injury to the tibial nerve at the level of the popliteal fossa would
result in weakness of plantar flexion, inversion, toe flexion, toe abduction, and toe adduction.
Knee flexion is mediated by the hamstring muscles, which are innervated by the tibial division of
the sciatic nerve predominantly (the short head of the biceps is innervated by the peroneal division
of the sciatic), with the motor branches arising in the thigh proximal to the popliteal fossa. Knee
extension is mediated by the quadriceps muscles, which are innervated by the femoral nerve.
Dorsiflexion is mediated by the tibialis anterior, which is innervated by the common peroneal nerve
via the deep peroneal nerve. Eversion is mediated by the peroneus longus and brevis, which are
innervated by the common peroneal nerve via the superficial peroneal nerve.
Pain impulses are conducted to the central nervous system via which of the following nerve fiber
groups?
A. A beta and C fibers
B. A alpha and A delta fibers
C. A beta and A delta fibers
D. A alpha and A beta fibers
E. A delta and C fibers
A delta and C fibers
Sensory nerve fibers can be classified into several different types depending on the type of
sensory receptors activated and stimulus information they convey. A alpha fibers (type 1a and 1b
fibers) convey sensory information from proprioceptors regarding muscle and joint position, speed,
and force of contraction from muscle spindles and Golgi tendon organs. A beta fibers (type 2
fibers) convey information from mechanoreceptors regarding touch and pressure sensation. A
delta fibers (type 3 fibers) convey information from mechanoreceptors, nociceptors, and
thermoreceptors regarding light touch, well localized/sharp pain, and temperature sensation. C
fibers (type 4 fibers) convey information from nociceptors and thermoreceptors regarding pain and
temperature sensation that is poorly localized, and dull/aching pain. A alpha and A beta fibers are
large diameter, myelinated fibers with fast conduction velocity. A delta fibers are smaller diameter,
with thinly myelinated axons and slower conduction velocities. C fibers are small, unmyelinated
axons and have the slowest conduction velocity. Upon stimulation beyond their depolarization and
action potential threshold, the A delta and C fibers will convey a pain stimulus impulse from the
periphery to their primary sensory cell body in the dorsal root ganglion. From there, they synapse
with second order neurons in the Rexed laminae of the spinal cord. Projections from the second
order neurons then decussate to the contralateral side of the spinal cord within a few segments
through the anterior white commissure and ascend in the lateral spinothalamic tract to the ventral
posterolateral (VPL) nucleus of the thalamus (note that some fibers convey these pain impulses
through the ventral spinothalamic tract and to various areas of the brain stem, the details of which
are beyond the scope of this description). The cell bodies of the third order neurons are within the
VPL of the thalamus. These third order neurons project via the posterior limb of the internal
capsule to the primary somatosensory cortex in the post-central gyrus.
An L3-4 disc herniation in which of the following zones is most likely to cause L3 radiculopathy?
Answers:
A. Inferior
B. Lateral
C. Posterior Paracentral
D. Anterior
E. Posterior Central
Lateral
Disc herniations occur as a result of disc degeneration and desiccation, which may lead to
herniation of the nucleus pulposus. The pattern of symptoms depends on the anatomic location
within the spinal column, but also where the disc herniation is located relative to the spinal canal.
In the lumbar spine, the exiting nerve roots are named with the corresponding vertebral level. For
example, the L3 nerve root passes under the L3 pedicle through the L3-4 neuroforamen. The
thoracic nerve roots are named similarly. This is in contrast to the cervical spine, where the exiting
nerve root is named for the level below. For example, the C5 nerve root passes under the C4
pedicle through the C4-5 neuroforamen. The cervical spine has 8 pairs of cervical nerve roots but
only 7 cervical vertebrae. C1 passes between the occiput and C1, while C8 traverses the C7-T1
neuroforamen. The thoracic spine (most commonly) has 12 pairs of thoracic nerves and 12
vertebrae. The lumbar spine (most commonly) has 5 pairs of lumbar nerves and 5 vertebrae.
There are typically 5 pairs of sacral nerves and 1 pair of coccygeal nerves.
Disc herniations may occur in a variety of anatomic locations such as posterior central, posterior
paracentral, inferior, or lateral. These are named in relation to the vertebral body. Anterior (ventral)
herniations do not cause radicular symptoms by themselves. The pattern of symptoms varies with
the location of the disc herniation. A lumbar paracentral disc herniation is more likely to affect the
traversing nerve root rather than the exiting nerve root. Thus, a paracentral disc herniation at the
L3-4 level is more likely to affect the L4 nerve root. A lumbar lateral disc herniation is more likely to
affect the exiting nerve root rather than the traversing nerve root. Thus, a lateral disc herniation at
the L3-4 level is more likely to affect the L3 nerve root.
Surgery for lateral disc herniations uses the “Wiltse approach,” or posterolateral approach, to the
spine. This may be done open or minimally invasively. The fascial plane between the medial
multifidus and lateral longissimus is developed until the level of the appropriate transverse process
or facet joint is identified. The discectomy can then be safely performed once the exiting nerve root
is identified and protected.
In addition to the median nerve, which of the following structures is found in the carpal tunnel?
Answers:
A. Dorsal Cutaneous Branch of the Ulnar Nerve
B. Flexor Digitorum Superficialis Tendons
C. Palmar Cutaneous Branch of the Median Nerve
D. Radial Artery
E. Ulnar Artery
Flexor Digitorum Superficialis Tendons
The median nerve traverses the carpal tunnel, along with four tendons of the flexor digitorum
superficialis, four tendons of the flexor digitorum profundus, and the tendon of the flexor pollicis
longus. The roof of the carpal tunnel is the transverse carpal ligament, which attaches to the
scaphoid and trapezium on the radial side and the pisiform and hamate on the ulnar side.
Compression of the median nerve can occur due to thickening of the transverse carpal ligament,
tenosynovitis, or a space occupying lesion in the carpal tunnel.
The ulnar nerve innervates all of the hand intrinsic muscles, except for the LOAF muscles
—lumbricals 1/2, opponens pollicis, abductor pollicis brevis, and flexor pollicis brevis—which are
innervated by the median nerve. The median nerve sensory distribution is the palmar aspect of
the radial 3.5 digits and the corresponding palm. The palm is innervated by the palmar cutaneous
branch, which arises proximal to the carpal tunnel (approximately 4-8 cm proximal to the distal
wrist crease) and does not pass through the carpal tunnel. Carpal tunnel syndrome presents with
numbness in the radial 3.5 digits (sparing the palm), paresthesias, and/or weakness of the LOAF
muscles. Pain and paresthesias often have nocturnal worsening and patients frequently describe
shaking out their hand to relieve the symptoms. The median-innervated forearm muscles include
the pronator teres, flexor carpi radialis, flexor digitorum superficialis, and the anterior interosseous
innervated muscles, including the flexor digitorum profundus, flexor pollicis longus, and pronator
quadratus. The median-innervated forearm muscles are spared in carpal tunnel syndrome. On
examination, provocative maneuvers that may prove useful in diagnosing carpal tunnel syndrome
include the Tinel sign, Durkan compression test, Phalen maneuver, and reverse Phalen
maneuver. Ancillary studies including electrodiagnostics and ultrasound can aid in diagnosis.
Treatment options typically include bracing, carpal tunnel steroid injections, and surgical carpal
tunnel release.
The radial nerve innervates which of the following sets of muscles?
Answers:
A. Extensor Carpi Radialis Longus, Extensor Carpi Radialis Brevis, Brachioradialis
B. Pronator Teres, Flexor Carpi Radialis, Abductor Pollicis Brevis
C. Dorsal Interossei, Palmar Interossei, Abductor Digiti Minimi
D. Biceps Brachii, Coracobrachialis, Brachialis
E. Supraspinatus, Infraspinatus, Subscapularis
Extensor Carpi Radialis Longus, Extensor Carpi Radialis Brevis, Brachioradialis
The radial nerve arises from the posterior cord in the axilla. The nerve travels along the posterior
wall of the axilla supplying motor branches to the three heads of the triceps and sensation to the
posterior aspect of the arm. The nerve continues through the triangular interval between the long
head of the triceps and the humerus. The radial nerve then passes down the spiral groove and
wraps around the humerus to pass between the brachioradialis muscle and brachialis muscle to
enter the forearm anterior to the lateral epicondyle. In the proximal forearm, the radial nerve
terminates by dividing into the superficial radial (sensory) and the deep radial/posterior
interosseous nerve (PIN).
The radial nerve innervates the triceps brachii, brachioradialis, extensor carpi radialis longus, and
extensor carpi radialis brevis. The posterior cutaneous nerve of the arm and posterior cutaneous
nerve of the forearm are sensory branches of the radial nerve that supply sensation to the
posterior arm and forearm. The superficial radial nerve is a terminal branch of the radial nerve that
is a pure sensory nerve, supplying sensation to the dorsum of the radial 3.5 digits. The other
terminal branch of the radial nerve is the deep radial nerve/posterior interosseous nerve. This
nerve supplies the supinator, extensor carpi ulnaris, extensor digitorum communis, extensor digiti
minimi, abductor pollicis longus, extensor pollicis longus, extensor pollicis brevis, and extensor
indicis.
Potential compression points include the radial nerve at the spiral groove by the lateral
intermuscular septum, the posterior interosseous nerve at the supinator/Arcade of Frohse, and the
superficial radial nerve between the brachioradialis and extensor carpi radialis longus tendons.
Due to the course of the radial nerve in close association with the humerus, the radial nerve is
particularly prone to injury in association with mid-shaft humeral fractures.
Clinically, proximal radial nerve injuries are characterized by weakness or loss of elbow extension,
wrist extension, and finger extension. The triceps branches arise very proximally such that elbow
extension is preserved with injuries to the radial nerve in the mid-arm. A posterior interosseous
palsy is characterized by loss of finger extension, with preserved wrist extension but radial
deviation during wrist extension. This occurs due to the preservation of function in the extensor
carpi radialis longus and brevis (innervated by the radial nerve), with loss of function in the
extensor carpi ulnaris (innervated by the posterior interosseous nerve).
A winged scapula occurs after an injury to which of the following nerves?
Answers:
A. Suprascapular Nerve
B. Spinal Accessory Nerve
C. Lateral Pectoral Nerve
D. Upper Subscapular Nerve
E. Axillary Nerve
Spinal Accessory Nerve
Scapular winging can be categorized as lateral or medial winging. Medial winging is caused by
weakness of the serratus anterior muscle, innervated by the long thoracic nerve. Lateral winging
can be caused by weakness of the rhomboid muscles, innervated by the dorsal scapular nerve, or
weakness of the trapezius, innervated by the spinal accessory nerve. Winging related to a dorsal
scapular neuropathy is usually subtle. Winging is more than just cosmetic; patients with significant
winging often present with reduced active range of motion with shoulder abduction and/or forward
flexion.
The spinal accessory nerve is a pure motor nerve. It leaves the jugular foramen and innervates the
sternocleidomastoid and trapezius muscles. After innervating the sternocleidomastoid, it typically
emerges from under the sternocleidomastoid approximately 2/3 of the distance from the sternal
notch to the mastoid and runs through the posterior triangle of the neck where it is very superficial
and prone to injury during surgery in the area, including cervical lymph node dissection/biopsy.
The suprascapular nerve innervates the supraspinatus and infraspinatus muscles. The axillary
nerve innervates the teres minor and deltoid muscles. The upper subscapular nerve innervates
the subscapularis. The lateral pectoral nerve innervates the pectoralis major muscle.
A 52-year-old man is brought to the emergency department after sustaining a gunshot wound to
the region of the right clavicle. On examination, there is weakness in the right biceps, deltoid,
brachioradialis, supinator longus, supraspinatus, and infraspinatus muscles. Which of the following
is the most likely site of injury to the brachial plexus?
Answers:
A. Upper Trunk
B. Lower Trunk
C. Middle Trunk
D. Lateral Cord
E. Medial Cord
Upper Trunk
The brachial plexus can be divided into five segments: roots, trunks, divisions, cords, and
branches. The roots and trunks can be found in the supraclavicular space. The divisions typically
are retroclavicular. The cords and branches are infraclavicular. The branches typically arise at the
level of the axilla. The brachial plexus comprises the C5-T1 nerves. The C5 and C6 nerves
merge to form the upper trunk. The C7 nerve continues independently as the middle trunk. The
C8 and T1 nerves merge to form the lower trunk.
The biceps brachii is innervated by the musculocutaneous nerve (C5/C6, upper trunk, lateral
cord). The deltoid is innervated by the axillary nerve (C5/C6, upper trunk, posterior cord). The
brachioradialis and supinator are innervated by the radial nerve (C5/C6, upper trunk, posterior
cord). The supraspinatus and infraspinatus are innervated by the suprascapular nerve (C5/C6,
upper trunk). These muscles are all commonly innervated by the upper trunk. With an injury to
the upper trunk, the major motions that are lost include shoulder abduction, forward flexion, and
external rotation, elbow flexion, and supination.
The upper trunk ends in a trifurcation with the suprascapular nerve laterally, posterior division in
the middle, and anterior division medially. As an anatomic variant, the suprascapular nerve can
arise from the posterior division of the upper trunk. The anatomic arrangement of this trifurcation
is often misrepresented in many textbooks. The supraspinatus and infraspinatus are innervated by
the suprascapular nerve. The anterior division carries the motor innervation to the
coracobrachialis, biceps brachii, and brachialis through the musculocutaneous nerve. The
posterior division carries the motor innervation to the deltoid and teres minor through the axillary
nerve and to the brachioradialis and supinator through the radial nerve.
The rule of 3s can be used as a rough guide for the management of traumatic nerve injuries.
Open, sharp nerve injuries should be repaired within 3 days of the injury. Open, ragged nerve
injuries are typically repaired 3 weeks after the injury, allowing time for the zone of injury to fully
demarcate. In many instances, a more acute exploration is undertaken to tag the injured nerves to
be repaired later. Management of closed nerve injuries is typically delayed for 3 months, with
surgical management reserved for those patients not demonstrating clinical or electrodiagnostic
evidence of recovery. The management of closed injuries is the most nuanced. Gunshot wounds
rarely sever the nerve, and instead typically cause a concussive injury from the velocity of the
round or a heat injury related to the round traversing the tissue. Correspondingly, nerve injuries
related to gunshot wounds are typically managed similarly to closed injuries.
The dorsal cutaneous nerve of the forearm is a major sensory branch of which of the following
nerves?
Answers:
A. Median Nerve
B. Musculocutaneous Nerve
C. Ulnar Nerve
D. Radial Nerve
E. Axillary Nerve
Radial Nerve
The cutaneous sensation to the dorsal forearm is supplied by three major sensory nerves: medial
antebrachial cutaneous (branch of medial cord), lateral antebrachial cutaneous (continuation of
musculocutaneous nerve), and the dorsal (posterior) cutaneous nerve of the forearm (branch of
radial nerve, also called the posterior antebrachial cutaneous nerve). The dorsal (posterior)
cutaneous nerve of the forearm arises from the radial nerve in the proximal arm and courses with
the radial nerve through the mid and upper arm. The dorsal (posterior) cutaneous nerve of the
forearm diverges from the radial nerve as the radial nerve pierces the lateral intermuscular
septum, emerges superficially through a fascial hiatus proximal to the lateral epicondyle, gives off
a branch to the lateral epicondyle, and continues to its termination in the cutaneous tissue of the
dorsal forearm. The dorsal (posterior) cutaneous nerve of the forearm provides sensation to the
lateral epicondyle and the skin of the dorsal midline forearm between the elbow and wrist.
The sensory distribution of the radial nerve includes the posterior arm (posterior cutaneous nerve
of the arm), the posterior forearm (posterior cutaneous nerve of the forearm), and the dorsal-lateral
hand and dorsal radial 3.5 digits (superficial radial nerve). The sensory distribution of the ulnar
nerve includes the dorsal ulnar hand (dorsal cutaneous branch), the ulnar palm (palmar cutaneous
branch), and the dorsal and palmar ulnar 1.5 digits (superficial branch of the ulnar nerve). The
sensory distribution of the median nerve includes the radial palm (palmar cutaneous branch) and
the palmar aspect of the radial 3.5 digits. The sensory distribution of the musculocutaneous nerve
includes the lateral forearm via the lateral antebrachial cutaneous nerve, which is the continuation
of the musculocutaneous nerve distal to the motor branches to the biceps brachii and brachialis.
The sensory distribution of the axillary nerve is the area overlying the deltoid in the lateral upper
arm (superior lateral cutaneous nerve).
Injury to the upper (superior) trunk of the brachial plexus is suspected when a patient
demonstrates which of the following on physical examination?
Answers:
A. Loss of elbow flexion
B. Loss of finger flexion
C. Loss of shoulder shrug
D. Loss of wrist extension
E. Loss of elbow extension
Loss of elbow flexion
The most likely physical exam manifestation of an upper (superior) trunk injury would be loss of
elbow flexion.
The trunks of the brachial plexus are formed through the confluence or continuation of the C5
through T1 roots. The upper trunk is formed from the union of the C5 and C6 roots, the middle
trunk is the continuation of the C7 root, and the lower trunk is the union of the C8 and T1 roots.
Occasionally, the C4 root may contribute to the upper trunk or the T2 root may contribute to the
lower trunk, with these variations termed a pre-fixed (C4 contribution) or post-fixed (T2
contribution) brachial plexus. The upper trunk typically trifurcates into the anterior division,
posterior division, and the suprascapular nerve, though the suprascapular nerve may arise from
the posterior division of the upper trunk. The upper trunk’s contribution to the lateral cord provides
axons to the lateral pectoral nerve (pectoralis major), musculocutaneous nerve (coracobrachialis,
biceps brachii, brachialis), lateral antebrachial cutaneous nerve, and the median nerve (pronator
teres, flexor carpi radialis, sensory fibers in the median nerve). The upper trunk’s contribution to
the posterior cord provides axons to the upper and lower subscapular nerves (subscapularis and
teres major), thoracodorsal nerve (latissimus dorsi), axillary nerve (deltoid, teres minor) and radial
nerve (brachioradialis, supinator, extensor carpi radialis longus and brevis).
Neuroanatomical localization of a brachial plexus injury requires careful consideration of muscles
innervated proximal and distal to the suspected level of injury. The upper trunk provides critical
distal innervation for muscles of elbow flexion (biceps brachii, brachialis, brachioradialis),
movement and stabilization of the shoulder joint (supraspinatus, infraspinatus, deltoid, teres major,
teres minor, subscapularis), and cutaneous sensation to the lateral arm, forearm, and hand.
However, nerves that branch off proximal to the formation of the upper trunk (root level), such as
the dorsal scapular nerve (rhomboid, levator scapulae), phrenic nerve (diaphragm), or long
thoracic nerve (serratus anterior), will not evidence injury on physical examination, imaging, or
electrodiagnostic testing in an injury at the level of the upper trunk. Involvement of these muscles
raises the concern for a more proximal pathology at the root level. Root level injuries may be preganglionic
(such as root avulsions) or post-ganglionic. Discerning pre-ganglionic versus postganglionic
injury is critical for consideration of nerve reconstruction. Nerve graft reconstruction
requires a viable and accessible proximal nerve stump that is unavailable in pre-ganglionic injuries.
Nerve transfers are the primary consideration for nerve reconstruction in patients with preganglionic
root level injuries. CT or MRI evidence of a pseudomeningocele should raise the
suspicion for pre-ganglionic injury, but presence or absence is not conclusive. Nerve conduction
studies demonstrating absent motor conduction and normal sensory conduction from an anesthetic
region suggests a pre-ganglionic nerve injury, as the afferent nerve fibers may be preserved
despite disconnection from the spinal cord given the location of their cell bodies in the dorsal root
ganglion rather than the spinal cord. Due to this, the sensory fibers do not undergo Wallerian
degeneration, allowing a preserved sensory nerve action potential, despite the disconnection from
the spinal cord resulting in anesthesia.
Major motions that are lost in a complete upper trunk injury include shoulder abduction, shoulder
external rotation, elbow flexion, and forearm supination. Elbow and wrist extension loss would be
unlikely despite the upper trunk contribution to the radial nerve as the triceps, extensor carpi
ulnaris, and extensor carpi radialis longus/brevis all receive significant contributions from
C7/middle trunk and/or C8/lower trunk. Finger flexion loss would be unlikely as the upper trunk
typically does not contribute innervation to the finger flexors (flexor digitorum superficialis, flexor
digitorum profundus, lumbricals). Shoulder shrug loss would be unlikely as this is primarily
mediated by the trapezius muscle, which is innervated by the spinal accessory nerve.
Traumatic posterior dislocation of the humeral head most frequently results in injury to which of the
following nerves?
Answers:
A. Suprascapular Nerve
B. Axillary Nerve
C. Musculocutaneous Nerve
D. Median Nerve
E. Ulnar Nerve
Axillary Nerve
The axillary nerve is the nerve most likely to be injured with a posterior dislocation of the humeral
head.
The axillary nerve arises as a terminal branch of the posterior cord of the brachial plexus. The
axillary nerve carries fibers originating from the C5 and C6 roots coursing through the posterior
division of the upper trunk. Originating from the posterior cord anterior to the scapula, the axillary
nerve travels posteriorly underneath the glenohumeral joint to enter the quadrangular space. The
quadrangular space (also known as the quadrilateral space) is bounded by the teres minor
(superior), teres major (inferior), long head of triceps (medial), and the humerus (lateral). The
axillary nerve traverses the quadrangular space with the posterior humeral circumflex vessels. The
axillary nerve divides within or as it exits the quadrangular space into an anterior and posterior
division. The anterior division innervates the anterior and middle deltoid, while the posterior
division innervates the teres minor and posterior deltoid and provides cutaneous sensation for the
lateral upper arm through the superior lateral cutaneous nerve. Given the close proximity to the
glenohumeral joint and its course from anterior to posterior through a constrained anatomical
corridor (quadrangular space), the axillary nerve is prone to stretch injury with glenohumeral
dislocations such as in this case.
The pronator teres muscle is innervated by which of the following nerves?
Answers:
A. Posterior Interosseous Nerve
B. Radial Nerve
C. Ulnar Nerve
D. Median Nerve
E. Anterior Interosseous Nerve
Median Nerve
The median nerve arises from contributions from the lateral and medial cords of the brachial
plexus, carrying fibers from C6-T1. The medial cord provides predominantly motor axons from C8
and T1, while most of the sensory contribution to the median nerve comes from the lateral cord
from C6 and C7. The median nerve does not innervate any muscles in the arm. The branch to the
pronator teres is the first branch off the median nerve just proximal to the elbow. The pronator
teres (C6, C7) is the main muscle responsible for forearm pronation.
Distal to the elbow, the median nerve gives off branches to innervate the flexor digitorum
superficialis and the flexor carpi radialis. The anterior interosseous nerve (AIN) branches from the
median nerve in the proximal forearm at the level of the pronator teres. The AIN is a motor nerve
that innervates three forearm muscles: flexor digitorum profundus (2nd and 3rd digits), flexor
pollicis longus, and pronator quadratus. An anterior interosseous nerve palsy is typified by the
inability to form an OK sign secondary to the inability to flex the interphalangeal joint of the thumb
(flexor pollicis longus) and the distal interphalangeal joint of the index finger (flexor digitorum
profundus). After giving off the anterior interosseous nerve, the median nerve continues down the
forearm eventually passing through the carpal tunnel to enter the hand. In the hand, the median
nerve innervates the LOAF muscles: lumbricals 1 and 2, opponens pollicis, abductor pollicis
brevis, and flexor pollicis brevis. The median nerve also provides sensory innervation to the radial
3.5 digits on the palmar surface, as well as the corresponding segment of the palm through the
palmar cutaneous branch, which arises proximal to the carpal tunnel and does not run through it.
Pronator syndrome results from compression of the median nerve between the two heads of the
pronator teres. Patients typically present with volar forearm pain and may have a positive Tinel
sign at the pronator teres. Atrophy of the muscles innervated by the median nerve is rare in this
syndrome. Those at risk for developing pronator syndrome include carpenters and tennis players
due to the repetitive forearm pronation. Trauma, peripheral nerve tumors, and secondary
compression may also be responsible for pronator syndrome. Compression of the median nerve
in the forearm can also occur related to the lacertus fibrosus and the sublimus arch of the flexor
digitorum superficialis.
The median nerve can also be compressed at the carpal tunnel. Carpal tunnel syndrome is the
most common entrapment neuropathy. In carpal tunnel syndrome, the LOAF muscles may be
weak and there may be numbness of the palmar aspect of the radial 3.5 digits. The median- and
anterior interosseous-innervated forearm muscles are spared in carpal tunnel syndrome.
Dysfunction of which of the following nerve roots is most likely to result in great toe extension
weakness?
Answers:
A. L5
B. L3
C. L2
D. S1
E. L4
L5
The common peroneal nerve is one of the two divisions of the sciatic nerve and is responsible for
toe extension, ankle dorsiflexion, and eversion. The tibial nerve is the other division of the sciatic
nerve and is responsible for toe flexion, ankle plantar flexion, and inversion.
At the nerve root level, dorsiflexion is primarily mediated by the L5 nerve, whereas plantar flexion
is primarily mediated by the S1 nerve. Inversion is also primarily mediated by the L5 nerve,
whereas eversion is primarily mediated by the L5 and S1 nerves.
When a patient presents with a foot drop (dorsiflexion weakness), the main differential diagnosis is
a common peroneal neuropathy versus an L5 radiculopathy. Dorsiflexion plus eversion weakness
would be typical of a common peroneal neuropathy. Inversion is spared in a common peroneal
neuropathy. Conversely, dorsiflexion weakness plus inversion weakness would be typical of an L5
radiculopathy. In this way, inversion and eversion can be utilized to help differentiate a common
peroneal neuropathy from an L5 radiculopathy. The extensor hallucis longus (great toe extension)
is innervated by the deep peroneal nerve, with L5 as the primary nerve root contributing to its
innervation. Thus, in both L5 radiculopathy and common peroneal neuropathy, toe extension
weakness can be a prominent finding.
Electrodiagnostics and imaging can then be used to support the suspected diagnosis. For a
common peroneal neuropathy, the typical compression point is at the fibular tunnel around the
fibular neck, where the nerve can be compressed by the deep fascia of the peroneus longus
muscle. L5 radiculopathy most commonly occurs related to L4-5 paracentral disc herniations, as
the disc compresses the traversing L5 nerve root. Less commonly, the L5 nerve root can be
compressed from an L5-S1 far lateral disc, as the disc compresses the exiting L5 nerve root.
Which of the following sets of muscles are innervated by the ulnar nerve?
Answers:
A. Lumbricals 1/2, Opponens Pollicis, Abductor Pollicis Brevis, Flexor Pollicis Brevis
B. Lumbricals 3/4, Palmar Interossei, Dorsal Interossei, Adductor Pollicis
C. Abductor Pollicis Longus, Extensor Digitorum Communis, Extensor Pollicis Longus,
Extensor Carpi Ulnaris
D. Flexor Pollicis Longus, Flexor Digitorum Profundus, Pronator Quadratus
E. Flexor Carpi Radialis, Flexor Digitorum Superficialis, Pronator Teres
Lumbricals 3/4, Palmar Interossei, Dorsal Interossei, Adductor Pollicis
The ulnar nerve arises from the medial cord of the brachial plexus with primary nerve fibers
coming from the C8 and T1 nerve roots. It then descends with the brachial artery up to the
insertion point of the coracobrachialis muscle. It subsequently pierces the medial intermuscular
septum and enters the posterior compartment of the arm where it then travels posteromedial to the
humerus and behind the medial epicondyle to enter the cubital tunnel. It then enters the anterior
compartment of the forearm between the two heads of the flexor carpi ulnaris. Once it reaches the
wrist, it enters the hand superficial to the flexor retinaculum and lateral to the pisiform bone
traversing Guyon’s canal. It then terminally divides into superficial and deep branches. Two major
sensory branches arise in the forearm, distal to the cubital tunnel but proximal to Guyon’s canal:
the palmar cutaneous branch and the dorsal cutaneous branch.
The two major zones of compression for the ulnar nerve are around the elbow and around Guyon’s
canal in the palm. Compression around the elbow, known as cubital tunnel syndrome, can occur
related to the medial intermuscular septum, the Arcade of Struthers, bony abnormalities of the
medial epicondyle, Osborne’s ligament, or Osborne’s fascia. The major sensory branches in the
forearm—the dorsal cutaneous and palmar cutaneous branches—can be used to help differentiate
cubital tunnel syndrome from Guyon’s canal syndrome. Sensory loss in the distribution of these
nerves suggests cubital tunnel syndrome, since these branches arise proximal to Guyon’s canal
and are spared in Guyon’s canal syndrome. In Guyon’s canal, compression can often occur
related to the presence of a ganglion cyst or related to external compression such as can occur
from a bicycle handlebar.
The ulnar nerve provides sensory function to the fifth digit and medial half of the fourth digit via the
terminal superficial branch of the ulnar nerve, to the corresponding area of the palm via the palmar
cutaneous branch, and to the dorsal medial portion of the hand via the dorsal cutaneous branch of
the ulnar nerve. The ulnar nerve supplies two muscles in the forearm: the flexor carpi ulnaris and
the medial half of the flexor digitorum profundus. In the hand, the deep branch of the ulnar nerve
supplies the hypothenar muscles, including the opponens digiti minimi, abductor digiti minimi,
flexor digiti minimi brevis, as well as lumbricals 3 and 4, dorsal interossei, palmar interossei,
adductor pollicis, and the deep head of the flexor pollicis brevis. The superficial branch provides
motor function to the palmaris brevis. All of the hand intrinsic muscles except for the LOAF
muscles—lumbricals 1 and 2, opponens policis, abductor pollicis brevis, and flexor pollicis brevis—
are supplied by the ulnar nerve. The LOAF muscles are innervated by the median nerve. In
carpal tunnel syndrome, one would expect to potentially find weakness of the LOAF muscles,
sparing the rest of the hand intrinsics. In ulnar neuropathy, one would expect to potentially find
weakness of the hand intrinsics sparing the LOAF muscles. In C8/T1 radiculopathy or lower trunk
plexopathy, one would expect to potentially find weakness of all of the hand intrinsic muscles.
Which of the following nerves is most likely to become entrapped within the quadrangular space?
Answers:
A. Suprascapular Nerve
B. Axillary Nerve
C. Radial Nerve
D. Musculocutaneous Nerve
E. Thoracodorsal Nerve
Axillary Nerve
The axillary nerve is the nerve most likely to be entrapped within the quadrangular space.
The axillary nerve arises as a terminal branch of the posterior cord of the brachial plexus. The
axillary nerve carries fibers originating from the C5 and C6 roots coursing through the posterior
division of the upper trunk. Originating from the posterior cord anterior to the scapula, the axillary
nerve travels posteriorly underneath the glenohumeral joint to enter the quadrangular space. The
quadrangular space (also known as the quadrilateral space) is bounded by the teres minor
(superior), teres major (inferior), long head of triceps (medial), and the humerus (lateral). The
axillary nerve traverses the quadrangular space with the posterior humeral circumflex vessels. The
axillary nerve divides within or as it exits the quadrangular space into an anterior and posterior
division. The anterior division innervates the anterior and middle deltoid, while the posterior
division innervates the teres minor and posterior deltoid and provides cutaneous sensation for the
lateral upper arm through the superior lateral cutaneous nerve.
As originally described, quadrilateral (or quadrangular) space syndrome presents with diffuse pain
around the shoulder, paresthesias in a non-dermatomal distribution, point tenderness above the
quadrangular space, and occlusion or narrowing of the posterior humeral circumflex artery in a
provocative position on angiogram. Fatty atrophy of the teres minor is common in this syndrome
and can be detected on MRI. Athletes that perform repeated overhead activities are most
susceptible to developing this syndrome, including baseball and volleyball players.
The quadrangular space is separated from the triangular interval by the teres major muscle. The
boundaries of the triangular interval include the teres major superiorly, long head of the triceps
medially, and the humerus laterally. The radial nerve passes through the triangular interval.
Forceful abduction of digits 2 through 5 of the hand results from action of the:
Answers:
A. Palmar Interossei—Radial Nerve
B. Palmar Interossei—Ulnar Nerve
C. Dorsal Interossei—Ulnar Nerve
D. Dorsal Interossei—Median Nerve
E. Palmar Interossei—Median Nerve
Dorsal Interossei—Ulnar Nerve
Abduction of digits 2-5 results from the action of the dorsal interossei, while adduction of digits 2-5
results from the action of the palmar interossei. Both dorsal and palmar interossei are innervated
by the ulnar nerve, specifically via the deep branch of the ulnar nerve.
The ulnar nerve arises from the medial cord, carrying fibers predominantly from the C8 and T1
roots, though there can be fibers from C7. The ulnar nerve provides motor innervation to the flexor
carpi ulnaris and the ulnar half of the flexor digitorum profundus in the forearm and then innervates
all of the hand intrinsic muscles except for the LOAF muscles—lumbricals 1 and 2, opponens
pollicis, abductor pollicis brevis, and flexor pollicis brevis—which are innervated by the median
nerve. The ulnar nerve provides sensation to the ulnar 1.5 digits through the terminal superficial
branch of the ulnar nerve and also provides sensation to the dorsal ulnar hand through the dorsal
cutaneous branch and to the palmar ulnar hand through the palmar cutaneous branch, both of
which arise in the forearm, proximal to Guyon’s canal.
Ulnar nerve entrapment occurs primarily in two places: 1) at or around the elbow, referred to as
cubital tunnel syndrome, and 2) in the palm, referred to as Guyon’s canal syndrome. Entrapment
of the ulnar nerve at the elbow is much more common than at the palm and represents the second
most common entrapment neuropathy in the upper extremity, following carpal tunnel syndrome.
Understanding the common points of compression for the ulnar nerve is important in surgical
treatment of ulnar entrapment neuropathy. Commonly described points of compression around the
elbow from proximal to distal include: 1. Arcade of Struthers – Musculoaponeurotic band in the upper arm extending from the medial
head of triceps to the medial intermuscular septum. This is not to be confused with the
Struthers ligament which is a fibrous band extending from a supracondylar process to the
medial epicondyle and associated with entrapment of the median nerve.
2. Medial intermuscular septum
3. Medial epicondyle
4. Osborne’s ligament – fibrous band extending from medial epicondyle to the olecranon
5. Osborne’s fascia – fascia bridging the two heads of the flexor carpi ulnaris muscle.
Entrapment at the elbow can be differentiated from entrapment at Guyon’s canal by examining the
dorsal and palmar hand. The branches that innervate the dorsal hand and the palm originate
distal to the cubital tunnel in the forearm and do not traverse Guyon’s canal. Thus, reduced
sensation on the dorsal hand and the palm are associated with cubital tunnel but not Guyon’s
canal syndrome, whereas both syndromes will have reduced sensation in the ulnar 1.5 digits.
Ulnar neuropathy should also be differentiated from more proximal etiologies, including thoracic
outlet syndrome and C8 radiculopathy. Ulnar neuropathy will present with sensory loss that does
not extend proximal to the wrist and that reliably splits the fourth digit, whereas C8 radiculopathy
will extend proximal to the wrist and will not split the fourth digit. With regard to motor function,
hand weakness related to an ulnar neuropathy will spare the LOAF muscles, whereas a C8
radiculopathy will involve all of the hand intrinsic muscles (both the ulnar and median innervated
muscles). Patients with severe ulnar neuropathy can present with characteristic exam findings. One such
finding is the Wartenberg sign. The Wartenberg sign is the inability to adduct digit 5. When the
palmar interossei are weak, the radial innervated finger extensors are unopposed, which have an
abducting effect on digit 5. Another such finding is the Froment sign. The patient is asked to pinch
a piece of paper between the thumb and index finger, with the interphalangeal joint of the thumb
extended. When the paper is pulled, the patient will flex the interphalangeal joint of the thumb to
try to maintain pinch on the paper. This occurs due to weakness of the adductor pollicis. The
patient attempts to compensate by using the flexor pollicis longus, which is innervated by the
anterior interosseous nerve. Finally, ulnar clawing can be observed. When asked to open the
hand, the proximal and distal interphalangeal joints of digits 4 and 5 remain flexed. There is also
typically hyperextension of the metacarpophalangeal joints of these digits. This occurs due to
weakness of lumbricals 3 and 4. The lumbricals action is complex. They flex the
metacarpophalangeal joint, while extending the proximal and distal interphalangeal joints.
The posterior interosseous nerve supplies motor function to which of the following muscles?
Answers:
A. Brachioradialis
B. Extensor Carpi Ulnaris
C. Extensor Carpi Radialis Longus
D. Triceps Brachii
E. Extensor Carpi Radialis Brevis
Extensor Carpi Ulnaris
The radial nerve arises from the posterior cord in the axilla. The nerve travels along the posterior
wall of the axilla supplying motor branches to the three heads of the triceps and sensation to the
posterior aspect of the arm. The nerve continues through the triangular interval between the long
head of the triceps and the humerus. The radial nerve then passes down the spiral groove and
wraps around the humerus to pass between the brachioradialis muscle and brachialis muscle to
enter the forearm anterior to the lateral epicondyle. In the proximal forearm, the radial nerve
terminates by dividing into the superficial radial (sensory) and the deep radial/posterior
interosseous nerve (PIN).
The radial nerve innervates the triceps brachii, brachioradialis, extensor carpi radialis longus, and
extensor carpi radialis brevis. The posterior cutaneous nerve of the arm and posterior cutaneous
nerve of the forearm are sensory branches of the radial nerve that supply sensation to the
posterior arm and forearm. The superficial radial nerve is a terminal branch of the radial nerve that
is a pure sensory nerve, supplying sensation to the dorsum of the radial 3.5 digits. The other
terminal branch of the radial nerve is the deep radial nerve/posterior interosseous nerve. This
nerve supplies the supinator, extensor carpi ulnaris, extensor digitorum communis, extensor digiti
minimi, abductor pollicis longus, extensor pollicis longus, extensor pollicis brevis, and extensor
indicis.
Potential compression points include the radial nerve at the spiral groove by the lateral
intermuscular septum, the posterior interosseous nerve at the supinator/Arcade of Frohse, and the
superficial radial nerve between the brachioradialis and extensor carpi radialis longus tendons.
Due to the course of the radial nerve in close association with the humerus, the radial nerve is
particularly prone to injury in association with mid-shaft humeral fractures.
Clinically, proximal radial nerve injuries are characterized by weakness or loss of elbow extension,
wrist extension, and finger extension. The triceps branches arise very proximally such that elbow
extension is preserved with injuries to the radial nerve in the mid-arm. A posterior interosseous
palsy is characterized by loss of finger extension, with preserved wrist extension but radial
deviation during wrist extension. This occurs due to the preservation of function in the extensor
carpi radialis longus and brevis (innervated by the radial nerve), with loss of function in the
extensor carpi ulnaris (innervated by the posterior interosseous nerve).
The posterior interosseus nerve supplies which of the following muscles?
Answers:
A. Extensor Carpi Radialis Brevis
B. Extensor Carpi Radialis Longus
C. Brachioradialis
D. Extensor Digitorum Communis
E. Triceps Brachii
Extensor Digitorum Communis
The radial nerve is formed from the posterior divisions of the brachial plexus (C5-T1) and is one of
two terminal branches of the posterior cord, the other being the axillary nerve. The radial nerve
runs behind the brachial artery and along the posterior border of the axilla. In the arm, it passes
between the long head of the triceps and the shaft of the humerus, beneath the teres major muscle
(triangular interval). This is not to be confused with the quadrangular space through which the
axillary nerve passes. At the mid-humeral level, the nerve crosses the posterior aspect of the
humerus moving from medial to lateral, lying in the spiral groove and traveling with the profunda
brachii artery. The radial nerve innervates the triceps brachii very proximally and then distal to the
spiral groove innervates the brachioradialis, extensor carpi radialis longus, and extensor carpi
radialis brevis. In the proximal forearm, the nerve bifurcates into the superficial radial nerve
(sensory) and the deep radial/posterior interosseous nerve (PIN).
The posterior interosseous nerve innervates the supinator and then passes through the arcade of
Frohse, a fibrous band between the two heads of the supinator. While entrapment of the PIN is
rare, this is one potential site of compression. The superficial muscle group—extensor carpi
ulnaris, extensor digitorum communis, and extensor digiti minimi—and deep muscle group—
abductor pollicis longus, extensor pollicis longus, extensor pollicis brevis, extensor indicis—are all
innervated by the PIN.
The pattern of motor and sensory loss can be particularly helpful for localizing radial nerve injuries
or lesions. A high radial nerve palsy occurring in the axilla is rare. These are distinguished from
more distal radial nerve injuries by the presence of triceps weakness (elbow extension) and loss of
sensation to the posterior arm. A radial nerve palsy at the spiral groove presents with wrist and
finger drop, weakness of brachioradialis, and loss of sensation in the posterior forearm and hand.
Injury to the radial nerve in this location can occur related to humerus fractures, external
compression (Saturday night palsy), or compression related to the lateral intermuscular septum.
Injuries to the PIN present with intact sensation and a finger drop, without a wrist drop. There is
radial deviation on wrist extension due to the preservation of function in the extensor carpi radialis
longus and brevis (innervated by the radial nerve), with loss of function in the extensor carpi
ulnaris (innervated by the posterior interosseous nerve). The posterior interosseous nerve can be
compressed at the supinator inlet (Arcade of Frohse), but also is commonly involved in Parsonage-
Turner syndrome. Entrapment of PIN is less common than Parsonage-Turner syndrome, so
patients presenting with a posterior interosseous neuropathy warrant additional evaluation for a
more widespread brachial plexopathy and for other etiologies, for example, tumors.
Radial nerve injuries in the spiral groove commonly spare which of the following nerves?
Answers:
A. Nerves to the Triceps
B. Nerve to the Extensor Carpi Radialis Longus
C. Nerve to the Brachioradialis
D. Superficial Radial Nerve
E. Posterior Interosseous Nerve
Nerves to the Triceps
The radial nerve is formed from the posterior divisions of the brachial plexus (C5-T1) and is one of
two terminal branches of the posterior cord, the other being the axillary nerve. The radial nerve
runs behind the brachial artery and along the posterior border of the axilla. In the arm, it passes
between the long head of the triceps and the shaft of the humerus, beneath the teres major muscle
(triangular interval). At the mid-humeral level, the nerve crosses the posterior aspect of the
humerus moving from medial to lateral, lying in the spiral groove and traveling with the profunda
brachii artery. The radial nerve innervates the triceps brachii very proximally and then distal to the
spiral groove innervates the brachioradialis, extensor carpi radialis longus, and extensor carpi
radialis brevis. In the proximal forearm, the nerve bifurcates into the superficial radial nerve
(sensory) and the deep radial/posterior interosseous nerve (PIN). The posterior interosseous nerve
innervates the supinator, extensor carpi ulnaris, extensor digitorum communis, extensor digiti
minimi, abductor pollicis longus, extensor pollicis longus, extensor pollicis brevis, and extensor
indicis.
Clinically, proximal radial nerve injuries are characterized by weakness or loss of elbow extension,
wrist extension, and finger extension. The triceps branches arise very proximally such that elbow
extension is preserved with injuries to the radial nerve in the mid-arm. A posterior interosseous
palsy is characterized by loss of finger extension, with preserved wrist extension but radial
deviation during wrist extension. This occurs due to the preservation of function in the extensor
carpi radialis longus and brevis (innervated by the radial nerve), with loss of function in the
extensor carpi ulnaris (innervated by the posterior interosseous nerve).
While humeral shaft fractures account for only approximately 3% of all fractures, these have a high
association with radial nerve palsies. This incidence has been noted to be anywhere between 7
and 17% of all humeral shaft fractures, which makes this the most common nerve injury
associated with long bone fractures.
There is debate regarding timing of surgical exploration. Traditionally, radial nerve injuries
associated with closed humeral fractures have been treated expectantly, with ~70% spontaneous
recovery. When combined with those patients who underwent delayed surgical exploration due to
lack of recovery, the overall recovery rate rose to ~88%. In this series, the recovery rate when
early exploration was utilized was ~88%, suggesting that early exploration and delayed exploration
for those that do not spontaneously recover are equivalent. However, in some more updated
series, there has been some suggestion that early exploration may be better. In a larger, more
recent series, expectant management was noted to yield a 77% recovery rate, while early
exploration (<3 weeks from injury) yielded a 90% recovery rate. Thus, more recent data have
suggested that early exploration may provide more benefit than expectant management for radial
nerve injuries associated with humerus fractures. Regardless, a good outcome can be expected in
the majority of patients with radial nerve injuries associated with humeral fractures. When
examining for spontaneous recovery, the branch to the brachioradialis is the first branch distal to
the spiral groove. Thus, the brachioradialis is the muscle that is expected to show recovery first
and is critical to examine.
In patients who have radial nerve palsy following fracture of the midshaft of the humerus, the
muscle that usually regains function first is the:
Answers:
A. Extensor Carpi Radialis Longus
B. Brachioradialis
C. Extensor Digitorum Communis
D. Extensor Carpi Radialis Brevis
E. Abductor Pollicis Longus
Brachioradialis
The radial nerve originates from the posterior cord of the brachial plexus. It then traverses through
the triangular interval, where it ultimately reaches the radial sulcus on the posterior aspect of the
humerus. It subsequently travels down the arm between the lateral and medial heads of the triceps
until it reaches the lateral aspect of the arm at a point approximately 5 cm below the deltoid
tuberosity. At this point, the nerves pierces the lateral intermuscular septum to reach the anterior
compartment of the arm. It then crosses the elbow anterior to the lateral epicondyle and terminally
branches into the superficial radial nerve and the deep radial nerve/posterior interosseous nerve in
the proximal forearm.
The radial nerve innervates the triceps brachii, brachioradialis, extensor carpi radialis longus, and
extensor carpi radialis brevis. The posterior cutaneous nerve of the arm and posterior cutaneous
nerve of the forearm are sensory branches of the radial nerve that supply sensation to the
posterior arm and forearm. The superficial radial nerve is a terminal branch of the radial nerve that
is a pure sensory nerve, supplying sensation to the dorsum of the radial 3.5 digits and
corresponding dorsal hand. The other terminal branch of the radial nerve is the deep radial
nerve/posterior interosseous nerve. This nerve supplies the supinator, extensor carpi ulnaris,
extensor digitorum communis, extensor digiti minimi, abductor pollicis longus, extensor pollicis
longus, extensor pollicis brevis, and extensor indicis.
Potential compression points include the radial nerve at the spiral groove by the lateral
intermuscular septum, the posterior interosseous nerve at the supinator/Arcade of Frohse, and the
superficial radial nerve between the brachioradialis and extensor carpi radialis longus tendons.
Due to the course of the radial nerve in close association with the humerus, the radial nerve is
particularly prone to injury in association with mid-shaft humeral fractures.
Clinically, proximal radial nerve injuries are characterized by weakness or loss of elbow extension,
wrist extension, and finger extension. The triceps branches arise very proximally such that elbow
extension is preserved with injuries to the radial nerve in the mid-arm. A posterior interosseous
palsy is characterized by loss of finger extension, with preserved wrist extension but radial
deviation during wrist extension. This occurs due to the preservation of function in the extensor
carpi radialis longus and brevis (innervated by the radial nerve), with loss of function in the
extensor carpi ulnaris (innervated by the posterior interosseous nerve).
While humeral shaft fractures account for only approximately 3% of all fractures, these have a high
association with radial nerve palsies. This incidence has been noted to be anywhere between 7
and 17% of all humeral shaft fractures, which makes this the most common nerve injury
associated with long bone fractures.
There is debate regarding timing of surgical exploration. Traditionally, radial nerve injuries
associated with closed humeral fractures have been treated expectantly, with ~70% spontaneous
recovery. When combined with those patients who underwent delayed surgical exploration due to
lack of recovery, the overall recovery rate rose to ~88%. In this series, the recovery rate when
early exploration was utilized was ~88%, suggesting that early exploration and delayed exploration
for those that do not spontaneously recover are equivalent. However, in some more updated
series, there has been some suggestion that early exploration may be better. In a larger, more
recent series, expectant management was noted to yield a 77% recovery rate, while early
explorations (<3 weeks from injury) yielded a 90% recovery rate. Thus, more recent data have
suggested that early exploration may provide more benefit than expectant management in radial
nerve injury associated with humeral fractures. Regardless, a good outcome can be expected in
the majority of patients with radial nerve injuries associated with humeral fractures. When
examining for spontaneous recovery, the branch to the brachioradialis is the first branch distal to
the spiral groove. Thus, the brachioradialis is the muscle that is expected to show recovery first
and is critical to examine.
A 25-year-old woman is evaluated for progressive fatigue and discomfort in the upper back. The
photograph shown demonstrates the appearance of the patient’s back when she is asked to push
against resistance. Injury to which of the following nerves is most likely to be associated with this
patient’s condition
A. Suprascapular Nerve
B. Spinal Accessory Nerve
C. Axillary Nerve
D. Long Thoracic Nerve
E. Thoracodorsal Nerve
D. Long Thoracic Nerve
Scapular winging can be categorized as lateral or medial winging. Medial winging is caused by
weakness of the serratus anterior muscle, innervated by the long thoracic nerve. Lateral winging
can be caused by weakness of the rhomboid muscles, innervated by the dorsal scapular nerve, or
weakness of the trapezius, innervated by the spinal accessory nerve. Winging related to a dorsal
scapular neuropathy is usually subtle. Winging is more than just cosmetic, patients with significant
winging often present with reduced active range of motion with shoulder abduction and/or forward
flexion. The image in the question demonstrates medial winging consistent with a long thoracic
nerve palsy.
The spinal accessory nerve is a pure motor nerve. It leaves the jugular foramen and innervates the
sternocleidomastoid and trapezius muscles. After innervating the sternocleidomastoid, it typically
emerges from under the sternocleidomastoid approximately 2/3 of the distance from the sternal
notch to the mastoid and runs through the posterior triangle of the neck where it is very superficial
and prone to injury during surgery in the area, including cervical lymph node dissection/biopsy.
The dorsal scapular nerve arises directly from the C5 nerve. The dorsal scapular nerve pierces
the middle scalene muscle and then courses deep to the levator scapulae and rhomboid muscles.
The long thoracic nerve forms from the merger of branches from C5, C6, and C7. The long
thoracic nerve pierces the middle scalene muscle, runs over the first rib, and then descends along
the chest wall between the chest wall and the latissimus dorsi muscle. The long thoracic nerve is
prone to injury due to its long course along the chest wall. It is also susceptible to possible
compression in the middle scalene muscle.
Parsonage-Tuner syndrome, also referred to as idiopathic brachial plexopathy or neuralgic
amyotrophy, classically presents with sudden onset of severe shoulder or periscapular pain that
lasts from hours to a few weeks. Following resolution (or significant improvement) of the pain,
weakness then begins and progresses over a few days to weeks. While Parsonage-Turner
syndrome is poorly understood, it is thought to be an autoimmune inflammatory neuropathy. Often
a potential precipitator can be identified in the patient history, including surgery, trauma, viral
infection, vaccination, pregnancy, or significant stress.
For unclear reasons, Parsonage-Turner syndrome does have a predilection for certain nerves.
The most commonly involved nerves include the long thoracic nerve, suprascapular nerve, axillary
nerve, musculocutaneous nerve, posterior interosseous nerve, and anterior interosseous nerve.
The nerves are often affected in a patchy manner, displaying various degrees of involvement and
sparing of nerves from the same root or plexus level. Electromyography can help identify
muscles/nerves with subclinical involvement, which can help support the diagnosis.
After undergoing C3-4 anterior cervical discectomy and fusion, a patient wakes up with
ipsilateral slight upper eyelid droop and decreased facial sweating. These findings are most likely
caused by which of the following?
Answers:
A. Injury to the Hypoglossal Nerve
B. Injury to the C4 Nerve Root
C. Injury to the Recurrent Laryngeal Nerve
D. Injury to the C3 Nerve Root
E. Injury to the Sympathetic Chain
Injury to the Sympathetic Chain
The sympathetic chain courses over the anterior surface of the longus colli muscles in a loose
fascial plane. The sympathetic chain is located posteromedial from the carotid sheath. It courses
along the longus colli muscle lateral to the medial border of the musculature. During prolonged
retraction of the longus colli muscles, these fibers can be stretched leading to a transient or
potentially irreversible injury. Care should be taken to rest the lip of retractors under the edge of
the longus colli rather than on the surface of the muscle to help avoid injury to the sympathetic
chain. The findings classically associated with injury to the sympathetic chain are eponymously
known as Horner syndrome. These symptoms include ptosis, anhidrosis, and miosis. The
sympathetic chain is most at danger during exposure to the middle to lower aspect of the cervical
spine, as the fibers are positioned more medially at these levels.
Which of the following nerves passes through the quadrilateral space?
Answers:
A. Axillary Nerve
B. Dorsal Scapular Nerve
C. Radial Nerve
D. Ulnar Nerve
E. Suprascapular Nerve
Axillary Nerve
The axillary nerve arises as a terminal branch of the posterior cord of the brachial plexus. The
axillary nerve carries fibers originating from the C5 and C6 roots coursing through the posterior
division of the upper trunk. Originating from the posterior cord anterior to the scapula, the axillary
nerve travels posteriorly underneath the glenohumeral joint to enter the quadrangular space. The
quadrangular space (also known as the quadrilateral space) is bounded by the teres minor
(superior), teres major (inferior), long head of triceps (medial), and the humerus (lateral). The
axillary nerve traverses the quadrangular space with the posterior humeral circumflex vessels. The
axillary nerve divides within or as it exits the quadrangular space into an anterior and posterior
division. The anterior division innervates the anterior and middle deltoid, while the posterior
division innervates the teres minor and posterior deltoid and provides cutaneous sensation for the
lateral upper arm through the superior lateral cutaneous nerve.
As originally described, quadrilateral (or quadrangular) space syndrome presents with diffuse pain
around the shoulder, paresthesias in a non-dermatomal distribution, point tenderness above the
quadrangular space, and occlusion or narrowing of the posterior humeral circumflex artery in a
provocative position on angiogram. Fatty atrophy of the teres minor is common in this syndrome
and can be detected on MRI. Athletes that perform repeated overhead activities are most
susceptible to developing this syndrome, including baseball and volleyball players.
The quadrangular space is separated from the triangular interval by the teres major muscle. The
boundaries of the triangular interval include the teres major superiorly, long head of the triceps
medially, and the humerus laterally. The radial nerve passes through the triangular interval.
For each movement of the foot, select the nerve (A-E) most likely to be involved.
Dorsiflexion of the foot:
Answers:
A. Sural Nerve
B. Saphenous Nerve
C. Tibial Nerve
D. Superficial Peroneal Nerve
E. Deep Peroneal Nerve
Deep Peroneal Nerve
The deep peroneal nerve begins at the trifurcation of the common peroneal nerve into the deep
peroneal nerve, superficial peroneal nerve, and articular branch to the superior tibiofibular joint.
The trifurcation occurs at the inlet to the fibular tunnel around the junction of the fibular head and
neck. The deep peroneal nerve then courses deep to the extensor digitorum longus where it
ultimately resides on the anterior surface of the interosseous membrane. Once it reaches
approximately the midway point of the leg, it travels with the anterior tibial artery. It descends with
the artery anterior to the ankle joint and ultimately divides into lateral and medial terminal
branches. Potential compression points along the course of the nerve include at the fibular tunnel
inlet at the origin of the nerve, where compression can occur related to the deep fascia of the
peroneus longus muscle, at the anterior tarsal tunnel, where the compression is related to the
inferior extensor retinaculum, and at the midfoot, where compression can occur from the extensor
hallucis brevis tendon. Peroneal intraneural ganglion cysts also often preferentially impact the
deep peroneal nerve more than the superficial peroneal nerve due to the anatomic relationship
between the deep peroneal nerve and the articular branch to the superior tibiofibular joint, where
the deep peroneal nerve is closer anatomically than the superficial peroneal nerve.
The deep peroneal nerve provides motor fibers to the tibialis anterior, extensor hallucis longus,
extensor digitorum longus, peroneus tertius, extensor digitorum brevis, and extensor hallucis
brevis. The deep peroneal nerve is primarily responsible for innervating the muscles responsible
for dorsiflexion and toe extension. In addition, it provides sensation to the web space between the
first and second digits.
The saphenous nerve has no motor function. It is a pure sensory nerve that provides sensation to
the medial aspect of the leg, the infrapatellar region, and the medial ankle. The sural nerve is also
a pure sensory nerve which provides sensation to the lateral aspect of the calf and the lateral
aspect of the foot. The superficial and deep peroneal nerves arise from the common peroneal
nerve. The superficial peroneal nerve provides motor innervation to the peroneus longus and
brevis muscles. It also provides sensory fibers to the anterolateral aspect of the leg and most of
the dorsum of the foot, save the webspace between the first and second digits. The tibial nerve
has important motor and sensory innervations. The motor innervations include the gastrocnemius,
soleus, plantaris, tibialis posterior, flexor digitorum longus, flexor hallucis longus, abductor hallucis,
flexor digitorum brevis, flexor hallucis brevis, flexor digiti minimi, adductor hallucis, abductor digiti
minimi, interossei, and lumbricals. With regard to sensation, at or around the popliteal fossa, the
medial sural cutaneous nerve branches off of the tibial nerve to supply the lateral aspect of the
lower leg and the lateral aspect of the foot. The calcaneal nerve branch arises around the ankle
and supplies sensation to the posterior aspect of the heel. Finally, the medial and lateral plantar
branches supply the sole of the foot.
Inversion is mediated primarily by the tibialis posterior, which is innervated by the tibial nerve.
Eversion is mediated primarily by the peroneus longus and peroneus brevis, both of which are
supplied by the common peroneal nerve via the superficial peroneal nerve. Dorsiflexion is
mediated primarily by the tibialis anterior, which is supplied by the common peroneal nerve via the
deep peroneal nerve. Plantar flexion is mediated primarily by the gastrocnemius and soleus, both
of which are supplied by the tibial nerve.
Complete injury of the sciatic nerve at the level of the gluteal crease is most likely to result in:
Answers:
A. Loss of Plantar Flexion, Loss of Dorsiflexion, Preservation of Hamstring Function, Loss of
Knee Extension
B. Loss of Plantar Flexion, Preservation of Dorsiflexion, Preservation of Hamstring Function,
Preservation of Knee Extension
C. Loss of Plantar Flexion, Loss of Dorsiflexion, Loss of Hamstring Function, Loss of Knee
Extension
D. Loss of Plantar Flexion, Loss of Dorsiflexion, Loss of Hamstring Function, Preservation of
Knee Extension
E. Loss of Plantar Flexion, Loss of Dorsiflexion, Preservation of Hamstring Function,
Preservation of Knee Extension
Loss of Plantar Flexion, Loss of Dorsiflexion, Loss of Hamstring Function, Preservation of
Knee Extension
The sciatic nerve arises via the lumbosacral plexus from the L4, L5, S1, S2, and S3 spinal nerve
roots. The sciatic nerve is two nerves joined together that share the same epineurium – the
common peroneal (posterior divisions of L4-S2) and tibial nerve (anterior divisions L4-S3). It exits
the pelvis through the greater sciatic foramen, most commonly coursing under the piriformis
muscle, and runs deep to the gluteus maximus. The sciatic nerve then travels in the middle of the
posterior thigh between the hamstrings. Approximately two-thirds of the way down the thigh, the
sciatic nerve divides into the tibial and common peroneal nerves. The tibial nerve supplies muscles
that mediate plantar flexion, foot inversion, and toe flexion. Muscles responsible for dorsiflexion,
eversion, and toe extension are supplied by the common peroneal nerve.
The sciatic nerve innervates the musculature of the posterior compartment of the thigh, including
the hamstring muscles and a portion of the adductor magnus. The tibial division provides
innervation to the semitendinosus and semimembranosus medially, the long head of the biceps
femoris laterally, and the ischial half of the adductor magnus. The peroneal division supplies the
short head of the biceps femoris. The hamstrings function to flex the knee. The adductor magnus,
along with the obturator-innervated muscles, acts to adduct the hip.
The short head of the biceps femoris is the only peroneal-innervated muscle above the fibular
head. If a patient is suffering from a common peroneal neuropathy, the short head of the biceps
femoris will not be affected on electrodiagnostic testing, whereas injury to the peroneal division of
the sciatic nerve will involve not only the distal peroneal-innervated musculature but also the short
head of the biceps femoris. Therefore, this muscle is key to distinguishing a sciatic neuropathy
from a common peroneal neuropathy. Additionally, when the peroneal division of the sciatic nerve
is injured, the first muscle to potentially recover is the short head of the biceps femoris. Thus, this
muscle is also important in tracking recovery.
Hip abductors are innervated by the superior gluteal nerve (L4-S1), which is a branch of the
lumbosacral plexus. The obturator nerve (L2-L4) is primarily responsible for hip adduction. The
quadriceps are primarily responsible for knee extension and are supplied by the femoral nerve
(L2-4).
A 25-year-old man is brought to the emergency department after sustaining a gunshot wound to
the medial buttock. Operative exploration shows injury to the sciatic nerve. This patient is most
likely to exhibit difficulty with which of the following movements?
Answers:
A. Hip Adduction
B. Hip Abduction
C. Knee Flexion
D. Knee Extension
E. Hip Flexion
Knee Flexion
Knee flexion weakness would be the most likely finding in a patient with sciatic nerve injury at the
level of the buttock.
The sciatic nerve is formed via contributions from the L4-S3 roots and composed of distinct tibial
and peroneal components (divisions). The sciatic nerve descends through the posterior pelvis
toward the lower limb and exits the pelvis into the gluteal region via the greater sciatic foramen.
After exiting the sciatic foramen, the sciatic nerve typically courses underneath the piriformis
muscle before entering the posterior compartment of the thigh. However, several anatomical
variations have been described for the relationship of the sciatic nerve to the piriformis muscle. As
the nerve courses through the posterior thigh, it provides branches innervating the hamstring
muscles that are responsible for knee flexion (semitendinosus, biceps femoris, semimembranosus,
hamstring portion of adductor magnus). The tibial division of the sciatic nerve innervates the
semitendinosus, semimembranosus, hamstring portion of the adductor magnus, and long head of
the biceps femoris, while the peroneal division innervates the short head of the biceps femoris.
This is the only muscle innervated by the peroneal division/common peroneal nerve above the
knee. The sciatic nerve courses down the posterior thigh and divides into the common peroneal
nerve (lateral) and tibial nerve (medial) just proximal to the popliteal fossa. The common peroneal
nerve courses laterally through the popliteal fossa, around the fibular neck. At the inlet to the
fibular tunnel, the common peroneal nerve trifurcates into the superficial peroneal nerve, deep
peroneal nerve, and the articular branch to the superior tibiofibular joint. The superficial peroneal
nerve continues in the lateral compartment of the leg (most commonly) between the peroneus
longus and brevis, supplying sensory innervation to the anterolateral leg and dorsal aspect of the
foot (with the exception of a small area in the first web space between digits 1 and 2), as well as
motor innervation to the peroneus longus and brevis for foot eversion. The deep peroneal nerve
pierces the peroneus longus muscle and courses under the extensor digitorum longus to run
between the tibialis anterior and extensor digitorum longus in the superior portion of the leg and
between the tibialis anterior and extensor hallucis longus in the inferior leg. As it moves distally
through the anterior compartment of the leg on the anterior surface of the interosseous membrane,
it travels adjacent the anterior tibial artery to the dorsal foot. Along this route, it supplies motor
innervation to the tibialis anterior, extensor digitorum longus, extensor hallucis longus, extensor
digitorum brevis, extensor hallucis brevis, and peroneus tertius to enable ankle dorsiflexion and toe
extension. It also supplies cutaneous sensory innervation to a small area of skin in the first
webspace between digits 1 and 2. The tibial nerve after the sciatic bifurcation continues into the
deep posterior compartment of the leg. It innervates the gastrocnemius and soleus for plantar
flexion, the tibialis posterior for inversion, the flexor hallucis longus and flexor digitorum longus for toe flexion, as well as all of the foot intrinsic muscles except the extensor hallucis brevis and
extensor digitorum brevis. The tibial nerve also provides cutaneous sensation to the plantar foot
via the medial plantar nerve, lateral plantar nerve, and calcaneal nerve.
With proximal sciatic injuries, sensation is lost from the knee distal, with the exception of the
medial leg and the area around the instep of the foot, which are supplied by the saphenous nerve
(a branch of the femoral nerve). Proximal sciatic injuries are characterized by loss of knee flexion,
dorsiflexion, eversion, toe extension, plantar flexion, inversion, and toe flexion.
Hip flexion is primarily mediated by the iliopsoas, which comprises the psoas major and the iliacus
muscles. The psoas major is innervated by direct branches from the lumbosacral plexus, typically
L1-L3, while the iliacus is innervated by the femoral nerve. Hip adduction is primarily mediated by
the adductor longus and brevis, adductor portion of adductor magnus, and gracilis, which are
innervated by the obturator nerve, as well as the pectineus, which is innervated by the femoral
nerve. Hip abduction is primarily mediated by the gluteus medius, gluteus minimus, and tensor
fascia latae, innervated by the superior gluteal nerve. Knee extension is primarily mediated by the
quadriceps muscles (rectus femoris, vastus lateralis, vastus intermedius, vastus medialis),
innervated by the femoral nerve.
Which of the following is most characteristic of type 1 muscle fibers?
Answers:
A. Useful for powerful bursts
B. Rapid firing
C. High glycogen content
D. Primarily use aerobic respiration
E. Smooth muscle
Primarily use aerobic respiration
Human skeletal muscle can be subcategorized into three main types of muscle fibers based on
their type of metabolism and speed of contraction.
Type 1 fibers are designated slow oxidative muscle fibers that rely on aerobic oxidative
phosphorylation metabolism (and thus the use of oxygen and glucose for ATP energy production)
and contract relatively slowly with respect to actin-myosin cross-link cycling (dependent on how
quickly the muscle fibers hydrolyze ATP to break the actin-myosin cross-link).
Type 2 fibers can be stratified into type 2a and 2b. Type 2a fibers (or intermediary fibers) are fast
oxidative metabolizing fibers that also rely on aerobic metabolism. However, they are able to
contract faster and produce more power for contraction than type 1 fibers and, as a result, fatigue
more quickly as well. Type 2b muscle fibers utilize anaerobic glycolysis for ATP production and are
able to contract through actin-myosin cross-bridge formation and cycling the fastest of the three
fiber types. Accordingly, they also fatigue the quickest. Type 1 and 2a fibers are the most similar in
that they use oxidative aerobic metabolism. As a result, they will be most resistant to fatigue, have
a more robust capillary network for supplying glucose and oxygen, and have more mitochondria.
They are also typically smaller in diameter and have less glycogen than type 2b fibers. Moreover,
type 1 fibers will be used more for movement and functions that require less energy and occur
frequently, such as postural maintenance or fine delicate movements in the hands. Type 2b fibers
will be used for movement and functions that require a lot of energy and force over a short period
of time (powerful bursts) though are less precise, for example, quadriceps contraction used for
sprinting. As a result, these muscles produce and use energy quickly and will fatigue quickly as
well, with less mitochondria, a less rich capillary network, and less myoglobin.
A male infant born at 40 weeks gestation via vaginal delivery is noted to have an immobile right
arm at birth. Physical examination at the age of three months shows spontaneous flexion of the
wrist and fingers, but absent elbow flexion. Which of the following other motor functions is most
likely to be impaired?
Answers:
A. Pronation
B. Finger Abduction
C. Elbow Extension
D. Supination
E. Finger Adduction
Supination
The upper trunk of the brachial plexus is formed by the merger of the ventral rami of the C5 and
C6 nerves. It most commonly terminally trifurcates into the anterior division of the upper trunk,
posterior division of the upper trunk, and the suprascapular nerve, though the suprascapular nerve
can arise from the posterior division. The upper trunk ultimately has axonal contributions to the
axillary, radial, musculocutaneous, median, and lateral pectoral nerves. In addition, the nerve to
the subclavius arises from the upper trunk.
Supination of the forearm is controlled by two muscles, the biceps brachii and the supinator. The
biceps brachii receives innervation via the musculocutaneous nerve, while the supinator receives
innervation from the radial nerve. The axons innervating these muscles originate in C5 and C6 and
thus are lost in an injury to the upper trunk.
The classic waiter’s tip posture is characteristic of an upper trunk injury. The shoulder is adducted
due to loss of the deltoid (axillary nerve) and supraspinatus (suprascapular nerve), internally
rotated due to loss of the infraspinatus (suprascapular nerve) and teres minor (axillary nerve), the
elbow is extended due to loss of the biceps brachii (musculocutaneous nerve), brachialis
(musculocutaneous nerve), and brachioradialis (radial nerve), and the forearm is pronated due
loss of the biceps brachii (musculocutaneous nerve) and supinator (radial nerve).
Pronation of the forearm is mediated by the pronator teres (median nerve) and pronator quadratus
(anterior interosseous nerve) muscles. Axons to the pronator teres originate predominantly in C6
and C7, whereas the axons to the pronator quadratus originate in C8 predominantly but also C7
and T1. Elbow extension is mediated by the triceps brachii, innervated by the radial nerve. Axons
to the triceps brachii originate in C7 and C8 predominantly, usually with some contribution from
C6. Finger abduction is mediated by the dorsal interossei, innervated by the ulnar nerve, while the
palmar interossei mediate finger adduction, also innervated by the ulnar nerve. Axons to the dorsal
and palmar interossei originate from C8 and T1.
Which of the following muscles is responsible for foot dorsiflexion?
Answers:
A. Tibialis Posterior
B. Gastrocnemius
C. Peroneus Longus
D. Tibialis Anterior
E. Soleus
Tibialis Anterior
Dorsiflexion is mediated primarily by the tibialis anterior muscle, which is innervated by the deep
peroneal nerve. The tibialis posterior muscle is primarily an invertor of the ankle and is innervated
by the tibial nerve. The peroneus longus is primarily an evertor of the ankle and is innervated by
the superficial peroneal nerve. Both the gastrocnemius and soleus mediate plantar flexion and are
innervated by the tibial nerve.
The differential diagnosis for a patient presenting with a foot drop would include a deep peroneal
neuropathy, common peroneal neuropathy, sciatic neuropathy, and L5 radiculopathy, with common
peroneal neuropathy and L5 radiculopathy most common. Deep peroneal neuropathy presents
with weakness of dorsiflexion and toe extension. Common peroneal neuropathy presents with
deep plus superficial peroneal innervated muscle weakness, so dorsiflexion, toe extension, and
eversion. Sciatic neuropathy presents with common peroneal plus tibial innervated muscle
weakness, so dorsiflexion, toe extension, eversion, plantar flexion, toe flexion, and inversion. L5
radiculopathy presents with weakness of dorsiflexion and toe extension as well as inversion.
Inversion and eversion can be used to help differentiate L5 radiculopathy from common peroneal
neuropathy. Dorsiflexion plus inversion weakness suggests an L5 radiculopathy, whereas
dorsiflexion plus eversion weakness, with sparing of inversion, suggests a common peroneal
neuropathy.
A 22-year-old man has weakness of elbow flexion and forearm pronation since being stabbed in
the right shoulder two weeks ago. Which of the following is the most likely site of injury?
Answers:
A. Medial Cord
B. Musculocutaneous Nerve
C. Median Nerve
D. Lateral Cord
E. Posterior Cord
Lateral Cord
Injury to the lateral cord is the most likely to present with weakness in elbow flexion and forearm
pronation after a penetrating, sharp injury at the shoulder.
The lateral cord of the brachial plexus arises from the anterior divisions of the upper and middle
trunks carrying fibers from the C5, C6, and C7 roots. The lateral cord gives off the lateral pectoral
nerve, musculocutaneous nerve, and the lateral cord contribution to the median nerve. The lateral
pectoral nerve arises from the proximal lateral cord and provides innervation to the pectoralis
major. The musculocutaneous nerve provides innervation to two of the three muscles responsible
for elbow flexion (brachialis, biceps brachii) and provides sensory innervation to the lateral forearm
through the lateral antebrachial cutaneous nerve. The lateral cord contribution of the median nerve
carries the bulk of the sensory nerve fibers for the median nerve, whereas the medial cord
contribution to the median nerve carries the bulk of its motor fibers. However, the lateral cord
contribution of the median nerve typically does provide the majority of the motor fibers that
innervate the pronator teres and flexor carpi radialis. A lateral cord injury would typically present
with weakness of elbow flexion and forearm pronation, as well as numbness in the lateral forearm
and lateral hand. An isolated lateral cord injury would not result in complete loss of elbow flexion,
as the third muscle of elbow flexion (brachioradialis) is innervated by the radial nerve, which arises
from the posterior cord.
In addition to the motor exam findings, the anatomic location of the stab injury helps guide
approximating the most likely component of the brachial plexus to be injured. Approximate external
landmarks for regions of the brachial plexus (roots, trunks, divisions, cords, branches) are the
following:
Roots – Above the clavicle at the level of the anterior scalene
Trunks – Above the clavicle, lateral to the anterior scalene
Divisions – At the level of the clavicle
Cords – Below the clavicle, medial to the coracoid process of the scapula (shoulder level)
Branches – Below the clavicle, within or distal to the axilla
The rule of 3s helps guide management of nerve injuries. Open, sharp injuries are typically
explored and repaired within 3 days of the injury. Open, ragged injuries are typically definitively
managed 3 weeks after the injury, though acutely the injury may be explored to tag the nerves.
Finally, closed injuries are typically managed non-surgically for 3 months and then evaluated for
surgical intervention. Though gunshot wounds are controversial, they are typically managed as
closed injuries, since gunshot injuries rarely transect the nerve but rather injure the nerves through
the concussive force or heat generated as the bullet traverses the tissue. Given the suspected
open, sharp nerve laceration, this patient should be managed with acute exploration within 3 days
of injury. If the nerve is found in discontinuity, nerve reconstruction with tension-free primary repair
is the preferred option. Primary repair is the preferred reconstruction option when healthy nerve
ends can be reapproximated in a tension-free manner. Inclusion of a nerve graft reduces the
likelihood of a good outcome, and the likelihood of a good outcome further declines as the graft
length increases. Increasingly, nerve transfers are also being utilized for nerve reconstruction,
particularly in the management of closed nerve injuries.
Compared with those in healthy persons, motor unit potentials in patients with myopathy are:
Answers:
A. Lower amplitude
B. Longer duration
C. Similar amplitude
D. Similar duration
E. Higher amplitude
Lower amplitude
Electromyography (EMG) is used to test the function of the peripheral nervous system and
consists of measuring and recording electrical properties of muscles at rest and with activation.
Testing is performed by inserting a recording needle electrode into a muscle to measure the action
potentials from the muscle fibers. Motor unit potentials (MUPs) are the sum of the action potentials
from all of the muscle fibers of a motor unit. Increased insertional activity, positive sharp waves,
and fibrillation potentials reflect muscle membrane instability/irritability. This can occur in the
setting of denervation or, alternatively, in the setting of inflammation or necrosis as can occur in
some myopathies. In myopathies, individual muscle fibers are dysfunctional. This leads to a
decrease in the size of the motor unit. The motor unit potentials become shorter duration, smaller
amplitude, and polyphasic. Because the motor units are smaller, more motor units are needed to
generate a similar amount of force, leading to an early recruitment pattern.
After fracturing his humerus and clavicle in a fall from a bicycle, a 14-year-old boy has wrist drop
but is able to extend his forearm at the elbow. Which of the following is the most likely location of
the injury in this patient?
Answers:
A. Posterior Cord around the Clavicle
B. Middle Trunk in the Supraclavicular Fossa
C. Posterior Interosseous Nerve in the Proximal Forearm
D. Radial Nerve around the Proximal Humerus
E. Radial Nerve around the Spiral Groove
Radial Nerve around the Spiral Groove
The brachial plexus is divided into five anatomical sections – roots, trunks, divisions, cords, and
branches. The roots and trunks are considered the supraclavicular brachial plexus and are above
the clavicle. The divisions lie directly posterior to the clavicle. The infraclavicular plexus is
composed of the cords and branches which lie distal to the clavicle, with the branches typically
arising around the axilla.
The radial nerve is formed from the posterior divisions of the brachial plexus (C5-T1) and is one of
two terminal branches of the posterior cord, the other being the axillary nerve. The radial nerve
runs behind the brachial artery and along the posterior border of the axilla. In the arm, it passes
between the long head of the triceps and the shaft of the humerus, beneath the teres major muscle
(triangular interval). At the mid-humeral level, the nerve crosses the posterior aspect of the
humerus moving from medial to lateral, lying in the spiral groove and traveling with the profunda
brachii artery. The radial nerve innervates the triceps brachii very proximal and then distal to the
spiral groove innervates the brachioradialis, extensor carpi radialis longus, and extensor carpi
radialis brevis. In the proximal forearm, the nerve bifurcates into the superficial radial nerve
(sensory) and the deep radial/posterior interosseous nerve (PIN). The posterior interosseous nerve
innervates the supinator, extensor carpi ulnaris, extensor digitorum communis, extensor digiti
minimi, abductor pollicis longus, extensor pollicis longus, extensor pollicis brevis, and extensor
indicis.
Potential compression points include the radial nerve at the spiral groove by the lateral
intermuscular septum, the posterior interosseous nerve at the supinator/Arcade of Frohse, and the
superficial radial nerve between the brachioradialis and extensor carpi radialis longus tendons.
Due to the course of the radial nerve in close association with the humerus, the radial nerve is
particularly prone to injury in association with mid-shaft humeral fractures.
Clinically, proximal radial nerve injuries are characterized by weakness or loss of elbow extension,
wrist extension, and finger extension. The triceps branches arise very proximally such that elbow
extension is preserved with injuries to the radial nerve in the mid-arm. A posterior interosseous
palsy is characterized by loss of finger extension, with preserved wrist extension but radial
deviation during wrist extension. This occurs due to the preservation of function in the extensor
carpi radialis longus and brevis (innervated by the radial nerve), with loss of function in the
extensor carpi ulnaris (innervated by the posterior interosseous nerve).
An injury to the middle trunk would typically not produce any complete loss of function due to
significant overlap with the upper and lower trunks, but may present with weakness of elbow
extension, wrist flexion, wrist extension, and/or pronation. Injury to the posterior cord would affect
both the radial nerve, with loss of elbow extension, wrist extension, and finger extension, and the
axillary nerve—the two terminal branches of the posterior cord—with weakness of shoulder
abduction and forward flexion. An injury to the radial nerve proximally around the proximal
humerus/axilla would affect the entire radial nerve and would present with weakness of elbow
extension, wrist extension, and finger extension. An injury to posterior interosseous nerve
preserves the radial nerve branches to the triceps, brachioradialis, extensor carpi radialis longus,
and extensor carpi radialis brevis and presents with a finger drop without a wrist drop. There is
radial deviation on wrist extension due to the loss of function in the extensor carpi ulnaris,
innervated by the posterior interosseous nerve.
An 18-year-old man is brought to the emergency department after falling off his bicycle and
fracturing his left humerus. Physical examination shows complete radial nerve palsy below the
middle of the left upper arm. He undergoes closed reduction and cast immobilization. Muscular
contraction and electromyographic evidence of reinnervation are most likely to occur first in which
of the following paralyzed muscles?
Answers:
A. Extensor Carpi Radialis Brevis
B. Brachioradialis
C. Triceps Brachii
D. Extensor Carpi Ulnaris
E. Extensor Carpi Radialis Longus
Brachioradialis
The posterior cord of the brachial plexus terminates as a bifurcation into the axillary nerve and the
radial nerve. Almost immediately after its origin, one or more branches supplying the triceps
brachii then emerge. The posterior cutaneous nerve of the arm also arises before the nerve exits
the axilla and supplies sensation to the posterior arm. The radial nerve leaves the axilla traversing
the triangular interval with the profunda brachii artery. This space is bounded by the humerus
laterally, the triceps (long head) medially, and the teres major superiorly. The radial nerve then
emerges posteriorly in the upper arm and winds around the humerus anteriorly in the spiral
groove, passing between the lateral and medial heads of the triceps where it is prone to injury.
The nerve can be externally compressed in this location as can happen with Saturday night palsy,
can be compressed by the lateral intermuscular septum, and is prone to injury in this location,
typically in association with mid-shaft fractures of the humerus. Humeral fractures in this area are
typically treated with reduction (open or closed reduction) and fixed immobilization, either internal
via surgical plating and screws or external via casting or application of an external fixator.
Thorough neurological examination prior to treatment is essential to determine timing of radial
nerve injury. Weakness prior to fracture treatment is almost undoubtedly related to the inciting
trauma itself and can be managed with close clinical and electrodiagnostic follow-up. Radial nerve
related weakness whose onset is immediately post-surgical needs to be investigated to ensure
that no iatrogenic compression or laceration of the nerve has occurred from the placement of
instrumentation. Delayed radial nerve related weakness after cast application also needs to be
investigated to ensure no ongoing compression/entrapment is causing injury. The radial nerve
gives off a branch supplying the anconeus, as well as the posterior cutaneous nerve of the forearm
before piercing the lateral intermuscular septum beneath the lateral head of the triceps and
entering the lateral part of the anterior compartment of the arm. The posterior cutaneous nerve of
the forearm supplies sensation to the posterior aspect of the forearm. The radial nerve thencourses between the brachialis muscle and brachioradialis muscle and supplies motor innervation
to the brachioradialis. Thus, the motor branch to the brachioradialis is the first motor branch distal
to the spiral groove, making examination of the brachioradialis critical in detecting early recovery
following radial nerve injury at the spiral groove. The nerve branch to the extensor carpi radialis
longus then arises from the radial nerve several centimeters above the elbow. The radial nerve
then enters the lateral antecubital fossa beneath the brachioradialis and anterior to the lateral
epicondyle and gives off a motor branch to the extensor carpi radialis brevis. Immediately distal to
the antecubital fossa, the radial nerve begins to migrate posteriorly and bifurcates into its two
terminal branches—the deep radial nerve/posterior interosseus nerve (PIN) and the superficial
radial nerve. The superficial radial nerve continues distally in the forearm between the
brachioradialis above and the pronator teres below (lateral to the radial artery) to emerge between
the brachioradialis and extensor carpi radialis longus tendons and supply cutaneous sensation to
the dorsal aspect of the radial hand. The PIN, however, moves posteriorly immediately after its
takeoff diving between the two heads of the supinator (and supplying the muscle as it does so)
through the arcade of Frohse, the fibrous fascial arch of the superficial part of the supinator that
can be a site of entrapment for the PIN, to enter the posterior compartment of the arm. The
posterior interosseous nerve provides motor innervation to the supinator, extensor carpi ulnaris,
extensor digitorum communis, extensor digiti minimi, abductor pollicis longus, extensor pollicis
longus, extensor pollicis brevis, and extensor indicis.
Clinically, proximal radial nerve injuries are characterized by weakness or loss of elbow extension,
wrist extension, and finger extension. The triceps branches arise very proximally such that elbow
extension is preserved with injuries to the radial nerve in the mid-arm. A posterior interosseous
palsy is characterized by loss of finger extension, with preserved wrist extension but radial
deviation during wrist extension. This occurs due to the preservation of function in the extensor
carpi radialis longus and brevis (innervated by the radial nerve), with loss of function in the
extensor carpi ulnaris (innervated by the posterior interosseous nerve).
While humeral shaft fractures account for only approximately 3% of all fractures, these have a high
association with radial nerve palsies. This incidence has been noted to be anywhere between 7
and 17% of all humeral shaft fractures, which makes this the most common nerve injury
associated with long bone fractures.
There is debate regarding timing of surgical exploration. Traditionally, radial nerve injuries
associated with closed humeral fractures have been treated expectantly, with ~70% spontaneous
recovery. When combined with those patients who underwent delayed surgical exploration due to
lack of recovery, the overall recovery rate rose to ~88%. In this series, the recovery rate when
early exploration was utilized was ~88%, suggesting that early exploration and delayed exploration
for those that do not spontaneously recover are equivalent. However, in some more updated
series, there has been some suggestion that early exploration may be better. In a larger, more
recent series, expectant management was noted to yield a 77% recovery rate, while early
explorations (<3 weeks from injury) yielded a 90% recovery rate. Thus, more recent data have
suggested that early exploration may provide more benefit than expectant management in radial
nerve injury associated with humerus fracture. Regardless, a good outcome can be expected in
the majority of patients with radial nerve injuries associated with humeral fractures.
Which of the following nerves has the best prognosis for functional recovery following acute
complete laceration and primary surgical repair?
Answers:
A. Radial Nerve in the arm
B. Median Nerve in the arm
C. Tibial Nerve at the knee
D. Ulnar Nerve in the arm
E. Common peroneal Nerve at the knee
Tibial Nerve at the knee
The correct answer is the tibial nerve injury at the level of the knee. Primary re-approximation (8-0
or 10-0 Prolene) is recommended following acute nerve sharp transection. The primary factor
affecting functional recovery is the length of nerve distal to the injury (following Wallerian
degeneration, nerve fibers regenerate approximately 1 inch per month). Proximally innervated
muscles have a better chance for recovery than small distal muscles performing more delicate
movements. Motor-fiber predominant nerves tend to have better prognosis versus mixed motorsensory
nerves. Good tibial nerve recovery (100%) compared to common peroneal nerve (84%) is
also thought to be related to better blood supply and better nerve elasticity. Good recovery for
radial and median nerves (91%) was reportedly better than ulnar nerve recovery (73%). Of note,
secondary repair for median nerve outcomes (78%) were better than radial nerve (69%).
A 36-year-old man sustains a contusion to the left mid forearm. Examination shows weakness of
extension of the index finger and lesser weakness of the middle and ring fingers at the
metacarpophalangeal joints. Which of the following nerves has most likely been injured?
Answers:
A. Anterior Interosseous Nerve
B. Posterior Interosseous Nerve
C. Median Nerve
D. Radial Nerve
E. Ulnar Nerve
Posterior Interosseous Nerve
Injury to the posterior interosseous nerve is the most likely to present with metacarpophalangeal
joint extension weakness after a blunt injury at the mid-forearm.
In the proximal forearm, the radial nerve divides into the superficial radial nerve and the deep
radial nerve/posterior interosseous nerve. The nomenclature for the posterior interosseous nerve
can be confusing, with some articles using the terms deep radial nerve and posterior interosseous
nerve interchangeably and some using the term deep radial nerve for the portion of the nerve
proximal to the supinator and the term posterior interosseous nerve for the continuation of the
deep radial nerve distal to the supinator. The deep radial/posterior interosseous nerve is
susceptible to compression at the supinator inlet by the Arcade of Frohse and is also one of the
nerves that is commonly involved in Parsonage-Turner syndrome.
The deep radial/posterior interosseous nerve innervates the supinator, extensor carpi ulnaris,
extensor digitorum communis, and extensor digiti minimi, abductor pollicis longus, extensor pollicis
longus, extensor pollicis brevis, and extensor indicis. The nerve terminates as an articular branch
to the wrist joint and typically does not supply any cutaneous sensation. Deep radial/posterior
interosseous neuropathy presents with a finger drop, without a wrist drop. There is radial deviation
on wrist extension. This occurs because the extensor carpi radialis longus and brevis are
innervated more proximally by the radial nerve and are spared, whereas the extensor carpi ulnaris
is innervated by the posterior interosseous nerve and is lost. It is important to recognize that the
posterior interosseous-innervated finger extensors primarily act at the metacarpophalangeal
joints. Extension of the proximal and distal interphalangeal joints will be preserved in a posterior
interosseous neuropathy, since this movement is driven by the lumbricals, which are innervated by
the median and ulnar nerves.
The rule of 3s can be used as a rough guide for managing nerve injuries. An open, sharp
laceration should be explored and repaired within 3 days of the injury. An open, ragged injury
should be definitively repaired approximately 3 weeks after the injury. In some circumstances,
acute exploration is useful to tag the nerves to be repaired later. Finally, closed, blunt injuries are
considered for surgical intervention if there is no clinical or electrodiagnostic evidence of recovery
at 3 months following the injury. This decision is the most nuanced.
A 33-year-old woman is evaluated because of a progressive foot drop. On physical examination,
left ankle dorsiflexion and great toe extension are weak (antigravity, 3/5), but ankle inversion and
eversion are normal (5/5). Sensation is diminished in the web between the first and second toes,
but otherwise normal. Deep tendon reflexes are normal. Which of the following peripheral nerves is
most likely involved?
Answers:
A. Common Peroneal Nerve
B. Deep Peroneal Nerve
C. Femoral Nerve
D. Sciatic Nerve
E. Superficial Peroneal Nerve
Deep Peroneal Nerve
The deep peroneal nerve is a terminal branch of the common peroneal nerve. After the sciatic
bifurcation, the common peroneal nerve (CPN) continues deep and medial to the biceps femoris
muscle and tendon as it proceeds distally into the leg. It courses around the neck of the fibula
where it is usually quite superficial and, as a result, susceptible to injury. The common peroneal
nerve trifurcates into the superficial peroneal nerve, deep peroneal nerve, and the articular branch
to the superior tibiofibular joint around the fibular tunnel inlet at the fibular neck. At the fibular
tunnel inlet, the common peroneal nerve can be compressed by the deep fascia of the peroneus
longus. The superficial peroneal nerve continues in the lateral compartment of the leg (most
commonly) between the peroneus longus and brevis, supplying sensory innervation to the
anterolateral leg and dorsal aspect of the foot (with the exception of a small area in the first web
space between digits 1 and 2), as well as motor innervation to the peroneus longus and brevis for
foot eversion. The deep peroneal nerve pierces the peroneus longus muscle and courses under
the extensor digitorum longus to run between the tibialis anterior and extensor digitorum longus in
the superior portion of the leg and between the tibialis anterior and extensor hallucis longus in the
inferior leg. As it moves distally through the anterior compartment of the leg on the anterior surface
of the interosseous membrane, it travels adjacent the anterior tibial artery to the dorsal foot. Along
this route, it supplies motor innervation to the tibialis anterior, extensor digitorum longus, extensor
hallucis longus, extensor digitorum brevis, extensor hallucis brevis, and peroneus tertius to enable
ankle dorsiflexion and toe extension. It also supplies cutaneous sensory innervation to a small
area of skin in the first webspace between digits 1 and 2.
The common peroneal nerve is susceptible to compression at the fibular tunnel inlet by the deep
fascia of the peroneus longus. Risk factors for common peroneal neuropathy include habitual leg
crossing, diabetes mellitus, and rapid weight loss. The superficial peroneal nerve can be
compressed in the distal leg where it pierces the fascia to leave the lateral compartment (typically)
and enter the subcutaneous tissue. The deep peroneal nerve is susceptible to compression in the
anterior tarsal tunnel by the inferior extensor retinaculum or in the mid-foot by the tendon of the
extensor hallucis brevis. This should not be confused with what is commonly referred to as tarsal
tunnel syndrome. Tarsal tunnel syndrome should be more appropriately referred to as posterior
tarsal tunnel syndrome. Posterior tarsal tunnel syndrome involves compression of the tibial nerve
by the flexor retinaculum.
The common peroneal nerve is the nerve most susceptible to formation of an intraneural ganglion
cyst. Synovial fluid from a degenerative superior tibiofibular joint travels along the articular branch
into the common peroneal nerve to form the cyst. Due to the anatomic relationship at the
trifurcation of the common peroneal nerve, peroneal intraneural ganglion cysts often preferentially
affect the deep peroneal nerve over the superficial peroneal nerve. The deep peroneal nerve is
anatomically adjacent to the articular branch to the superior tibiofibular joint, which is the conduit
for formation of peroneal intraneural ganglion cysts.
A lesion of the left suprascapular nerve is most likely to result in which of the following deficits?
Answers:
A. Weakness of left shoulder internal rotation
B. Weakness of left elbow flexion
C. Weakness of left shoulder external rotation
D. Weakness of right shoulder internal rotation
E. Weakness of right shoulder external rotation
Weakness of left shoulder external rotation
The suprascapular nerve arises from the upper trunk carrying contributions from the C5 and C6
nerve roots. The suprascapular nerve arises either as a trifurcation of the upper trunk or can arise
directly from the posterior division of the upper trunk. Many anatomic diagrams misrepresent the
anatomic arrangement in this location. The suprascapular nerve is always the most posteriorlateral,
with the posterior division of the upper trunk next to it and the anterior division of the upper
trunk most anterior-medial. The suprascapular nerve courses dorsally, parallel to the inferior belly
of the omohyoid and deep to the trapezius toward the upper border of the scapula. At the upper
border of the scapula, the suprascapular nerve enters the supraspinous fossa of the scapula via
the suprascapular notch, traversing under the suprascapular ligament, and provides a branch
innervating the supraspinatus muscle. The supraspinatus functions to abduct the upper arm at the
shoulder and stabilize the shoulder joint as a component of the rotator cuff. The supraspinatus is
the main abductor of the shoulder over the first 15 degrees and then assists the deltoid beyond
that. The suprascapular nerve then courses laterally and caudally into the infraspinous fossa of
the scapula via the spinoglenoid notch and innervates the infraspinatus muscle. The infraspinatus
muscle functions to externally rotate the shoulder and stabilize the shoulder joint as a component
of the rotator cuff. Lesions of the suprascapular nerve at or proximal to the suprascapular notch,
which is a potential site of entrapment, present with weakness in both shoulder abduction
(supraspinatus) and external rotation (infraspinatus). However, lesions of the suprascapular nerve
can also present with isolated shoulder external rotation weakness due to injury distal to the
branch to supraspinatus, such as entrapment at the spinoglenoid notch.
Suprascapular neuropathy should be differentiated from a C5 radiculopathy or upper trunk
plexopathy. In a C5 radiculopathy or upper trunk plexopathy, the supraspinatus and infraspinatus
may be weak, but the deltoid and biceps will also be weak (typicall
In addition to entrapment neuropathies or suprascapular nerve lesions, the differential diagnosis
for patients presenting with weakness of muscles of the shoulder girdle, such as shoulder external
rotation, should include idiopathic brachial plexitis (Parsonage-Turner Syndrome or Neuralgic
Amyotrophy). Parsonage-Turner syndrome classically presents with acute onset, severe,
temporary upper extremity (usually shoulder or periscapular) pain that is followed by development
of weakness in shoulder girdle musculature. Though any nerve can be affected by Parsonage-
Turner syndrome, the long thoracic nerve, suprascapular nerve, and axillary nerve were
specifically noted to be commonly affected in the syndrome’s original description. The
pathophysiology of Parsonage-Turner Syndrome remains unclear but is thought to be an immunemediated
inflammatory condition. In many cases a potential precipitating event can be identified,
such as a viral illness, vaccine, pregnancy, trauma, or surgery.
Internal rotation of the shoulder is mediated by the pectoralis major (lateral pectoral nerve),
subscapularis (upper and lower subscapular nerves), teres major (lower subscapular nerve),
latissimus dorsi (thoracodorsal nerve) and the anterior deltoid (axillary nerve). The major internal
rotator is the subscapularis. Elbow flexion is mediated by the brachialis (musculocutaneous
nerve), biceps brachii (musculocutaneous nerve), and brachioradialis (radial nerve).y deltoid more than biceps if
secondary to a C5 radiculopathy). In an isolated suprascapular neuropathy, the weakness will be
limited to the supraspinatus and infraspinatus.
Electromyography of the deltoid muscle six weeks after an upper brachial plexus root avulsion will
show:
Answers:
A. Normal Motor Unit Action Potentials
B. Fibrillation Potentials
C. Decreased Amplitude of Motor Unit Action Potentials
D. Increased Amplitude of Motor Unit Action Potentials
E. Polyphasic Motor Unit Action Potentials
Fibrillation Potentials
When axons are disrupted, the distal axon degenerates through a process called Wallerian
degeneration. This degenerative process occurs in an anterograde fashion distal to the site of
injury. Within hours of the injury, axonal and myelin fragmentation occurs with breakdown of its
cytoskeletal components and subsequent loss of conductivity by 2-4 days postinjury, and the
terminal, injured, axonal end sealing itself off. Due to this process taking several days, immediately
after the injury action potentials can continue to be transmitted in the injured axon resulting in no
detectable electrophysiological abnormality, as long as the stimulating electrode and recording
electrode are either both proximal to the injury or both distal to the injury. Immediately after the
injury, conductivity across the injury will be lost. For mixed nerves, this means that a motor action
potential and sensory action potential can be detected and will be normal for several days after a
nerve transection, assuming the stimulation and recording electrodes are both placed distal to the
site of injury and not across the site of injury. Despite the motor action potential being normal, the
action potential cannot cross the site of transection, leading to loss of voluntary motor unit action
potentials, which occurs immediately at the time of the injury. Schwann cells respond to the injury
immediately by proliferating and upregulating gene expression essential to their function in
Wallerian degeneration. Macrophages, having migrated into the zone of injury from nearby blood
vessels, phagocytose the axonal and myelin debris preparing it for axonal regeneration. With the
macrophages, other inflammatory cells also gain entry into the site and begin phagocytosing the
debris; this can take several weeks to several months postinjury, depending on how severe the
injury is. Wallerian degeneration of the distal process continues over the subsequent weeks to
months. When injuries are more severe, the inflammatory response is typically more vigorous with
subsequent necrosis, cellular infiltration, and proliferation that correlates with the severity of the
initial injury. Endoneurial vascular disruption also occurs, resulting in hemorrhage, edema, and
influx of neutrophils and fibroblasts. The latter ultimately results in intraneural scarring with
collagen deposition. Positive sharp waves and fibrillation potentials typically take 10-14 days to
develop, as the muscle membrane becomes unstable due to denervation when Wallerian
degeneration reaches the neuromuscular junction.
As mentioned above, the process of Wallerian degeneration typically takes weeks to months.
Because of this, in the event of an intraoperative injury, stimulating the nerve distal to the site of
injury will still result in the detection and recording of action potentials due to the axons still being
in continuity and conducting the action potentials toward the recording electrode. In the same vein,
performing nerve conduction studies distal to the site of injury, including compound motor action
potentials (CMAPs) and sensory nerve action potentials (SNAPs), immediately after injury may not
detect any abnormality due to the fact that the axons conducting these action potentials have not
yet degenerated through Wallerian degeneration, assuming both the stimulating electrode and
recording electrode are distal to the site of injury. On the other hand, if the stimulating electrode
and the recording electrode are on opposite sides of the transected nerve, then no action
potentials will be recorded. In order for both SNAPs and CMAPs to be recorded, integrity of the
lower motor neuron, specifically the distal axonal segment, must be preserved. As such, both
SNAP and CMAP recordings will be affected in the event of a post-ganglionic injury. However,
SNAPs and CMAPs differ in the setting of a pre-ganglionic injury in that CMAPs will be affected,
but SNAPs will be preserved, due to the injury occurring proximal to the cell body and thus the
integrity of the distal axons being preserved.
At 6 weeks out from an upper trunk avulsion injury, we would expect to find no voluntary motor unit
action potentials and present fibrillation potentials and positive sharp waves in the deltoid on
electromyography. The fibrillation potentials and positive sharp waves typically take 2-3 weeks to
appear on EMG, whereas the lack of voluntary motor unit action potentials would be present
immediately after the injury.
If the deep peroneal nerve is transected, sensory loss is most likely to occur between which of the
following toes?
Answers:
A. Digits 3 and 5
B. Digits 1 and 2
C. Digits 4 and 5
D. Digits 3 and 4
E. Digits 2 and 3
Digits 1 and 2
The deep peroneal nerve is a terminal branch of the common peroneal nerve. After the sciatic
bifurcation, the common peroneal nerve (CPN) continues deep and medial to the biceps femoris
muscle and tendon as it proceeds distally into the leg. It courses around the neck of the fibula
where it is usually quite superficial and, as a result, susceptible to injury. The common peroneal
nerve trifurcates into the superficial peroneal nerve, deep peroneal nerve, and the articular branch
to the superior tibiofibular joint around the fibular tunnel inlet at the fibular neck. At the fibular
tunnel inlet, the common peroneal nerve can be compressed by the deep fascia of the peroneus
longus. The superficial peroneal nerve continues in the lateral compartment of the leg (most
commonly) between the peroneus longus and brevis, supplying sensory innervation to the
anterolateral leg and dorsal aspect of the foot (with the exception of a small area in the first web
space between digits 1 and 2), as well as motor innervation to the peroneus longus and brevis for
foot eversion. The deep peroneal nerve pierces the peroneus longus muscle and courses under
the extensor digitorum longus to run between the tibialis anterior and extensor digitorum longus in
the superior portion of the leg and between the tibialis anterior and extensor hallucis longus in the
inferior leg. As it moves distally through the anterior compartment of the leg on the anterior surface
of the interosseous membrane, it travels adjacent to the anterior tibial artery to the dorsal foot.
Along this route, it supplies motor innervation to the tibialis anterior, extensor digitorum longus,
extensor hallucis longus, extensor digitorum brevis, extensor hallucis brevis, and peroneus tertius
to enable ankle dorsiflexion and toe extension. It also supplies cutaneous sensory innervation to a
small area of skin in the first webspace, between digits 1 and 2.
The common peroneal nerve is susceptible to compression at the fibular tunnel inlet by the deep
fascia of the peroneus longus. The superficial peroneal nerve can be compressed in the distal leg
where it pierces the fascia to leave the lateral compartment (typically) and enter the subcutaneous
tissue. The deep peroneal nerve is susceptible to compression in the anterior tarsal tunnel by the
inferior extensor retinaculum or in the mid-foot by the tendon of the extensor hallucis brevis. This
should not be confused with what is commonly referred to as tarsal tunnel syndrome. Tarsal
tunnel syndrome should be more appropriately referred to as posterior tarsal tunnel syndrome.
Posterior tarsal tunnel syndrome involves compression of the tibial nerve by the flexor
retinaculum. Around the area of the tarsal tunnel, the tibial nerve branches into the calcaneal
branch, lateral plantar nerve, and medial plantar nerve, all of which individually enter fibrous
tunnels in which these branches can be compressed. The common peroneal nerve is the nerve
most susceptible to formation of an intraneural ganglion cyst. Due to the anatomic relationship at
the trifurcation of the common peroneal nerve, peroneal intraneural ganglion cysts often
preferentially affect the deep peroneal nerve over the superficial peroneal nerve. The deep
peroneal nerve is anatomically adjacent to the articular branch to the superior tibiofibular joint,
which is the conduit for formation of peroneal intraneural ganglion cysts.
Which of the following forearm muscles is innervated by the median nerve proximal to the
branching of the anterior interosseous nerve?
Answers:
A. Pronator Teres
B. Flexor Pollicis Brevis
C. Supinator
D. Pronator Quadratus
E. Flexor Pollicis Longus
Pronator Teres
The median nerve arises from contributions from the lateral and medial cords of the brachial
plexus, carrying fibers from C6-T1. The medial cord provides predominantly motor axons from C8
and T1, while most of the sensory contribution to the median nerve comes from the lateral cord
from C6 and C7. The median nerve does not innervate any muscles in the arm. The branch to the
pronator teres is the first branch off the median nerve just proximal to the elbow. The pronator
teres (C6,7) is the main muscle responsible for forearm pronation.
Distal to the elbow, the median nerve gives off branches to innervate the flexor digitorum
superficialis and the flexor carpi radialis. The anterior interosseous nerve (AIN) branches from the
median nerve in the proximal forearm at the level of the pronator teres. The AIN is a motor nerve
that innervates three forearm muscles: flexor digitorum profundus (2nd and 3rd digits), flexor
pollicis longus, and pronator quadratus. An anterior interosseous nerve palsy is typified by the
inability to form an ok sign secondary to the inability to flex the interphalangeal joint of the thumb
(flexor pollicis longus) and the distal interphalangeal joint of the index finger (flexor digitorum
profundus). The anterior interosseous nerve arises from the radial side of the median nerve,
whereas the branches to the flexor carpi radialis and flexor digitorum superficialis arise from the
ulnar side of the median nerve. After giving off the anterior interosseous nerve, the median nerve
continues down the forearm, eventually passing through the carpal tunnel to enter the hand. In the
hand, the median nerve innervates the LOAF muscles: lumbricals 1 and 2, opponens pollicis,
abductor pollicis brevis, and flexor pollicis brevis. The median nerve also provides sensory
innervation to the radial 3.5 digits on the palmar surface, as well as the corresponding segment of
the palm through the palmar cutaneous branch, which arises proximal to the carpal tunnel and
does not run through it.
The pronator quadratus and flexor pollicis longus are innervated by the anterior interosseous
nerve. The flexor pollicis brevis is innervated by the median nerve distal to the carpal tunnel. The
supinator is innervated by the deep radial/posterior interosseous nerve.
