EMG Qbank Flashcards
A patient’s right upper limb EMG/NCS results are as follows. Routine median, ulnar, and radial sensory nerve conduction studies are normal except for reduced amplitude of ulnar SNAP to digit 5. EMG reveals decreased recruitment and polyphasic potentials in triceps, extensor indicis proprius, flexor digitorum profundus to digits 2-5, flexor carpi ulnaris, abductor pollicis brevis, and abductor digiti minimi, but normal activity in supraspinatus, deltoid, biceps, brachioradialis, and flexor carpi radialis. Which of the following is the most likely diagnosis?
This is a lower trunk plexopathy. Radiculopathy would spare the SNAPs, which is affected here for the ulnar nerve. Middle trunk plexopathy would affect the flexor carpi radialis, which is spared here
During nerve conduction studies, why do we refer to the CMAP as the “compound” muscle action potential?
Because the CMAP records an electrical summation of all muscle fibers of a single muscle contraction
A previously healthy 53 year-old female presents with gradual onset weakness, worse with any type of exercise, climbing stairs, and worse as the day progresses. She does not smoke, drink alcohol, or use illicit drugs. She notices associated double vision at times, worse as the day progresses. She denies trauma or bowel/bladder abnormalities. Which of the following etiologies is most likely responsible for her symptoms?
Presence of antibodies against postsynaptic acetylcholine receptors
- myasthenia gravis (MG)
Which of the following is the main problem with performing the Phalen test with the elbows flexed instead of extended?
A positive test is less informative
-When performing the Phalen test (flexing the wrists, holding this posture, and eliciting hand numbness/tingling due to median nerve compression within the carpal tunnel), it is best performed with the patient’s elbows extended. Flexing the elbows stretches the ulnar nerve and may irritate an already irritated ulnar nerve, thus causing hand numbness and tingling NOT due to median nerve compression. Thus, a positive test in that case would not allow you to narrow it down between ulnar neuropathy and median neuropathy - it is less informative. The other answer choices are fictional.
Which of the following is the most likely cause of a patient’s lumbosacral plexopathy?
diabetes is the most likely to cause lumbosacral plexopathy.
During nerve conduction studies, sensory nerves are typically recorded in which way?
Antidromically
- SNAPs are typically recorded antidromically to obtain a clearer, “louder” signal from the nerve.
A 60 year-old female presents with proximal hip and shoulder weakness, periorbital erythematous rash, and erythematous papules over her metacarpophalangeal (MCP) and interphalangeal (IP) joints of the hands. Which of the following EMG findings is most likely present in this patient?
Early/increased recruitment
-patient presents with classic dermatomyositis (proximal muscle weakness with heliotrope rash and Gottron papules). This is a myopathic disease; thus, the EMG will show signs of myopathy (short-duration, small amplitude MUAPs with early/increased recruitment). Decreased recruitment is found in nerve disorders, not muscle disease. Giant MUAPs are found in post-polio syndrome. Decreased SNAP amplitudes can be found in any sensory nerve disease that affects axons.
Which of the following structures forms the lateral border of Guyon’s canal?
lateral border : hook of the Hamate
medial border : pisiform\
An EMG potential is described as polyphasic if it crosses the baseline a minimum of how many times?
Polyphasicity is defined as 5 or more phases. Phases are defined as the number of baseline crossings + 1. Thus, polyphasic potentials are defined as a baseline crossing of 4 or more times, which would equate to 5 or more phases.
Lowering the high frequency filter during NCS will cause which of the following changes?
Prolonged peak latency, prolonged onset latency
When a muscle recruits its first motor unit during initial contraction, it tends to fire at _____ Hz, and subsequent motor units are recruited for every ______ Hz increase in firing rate.
5, 5
-The initial motor unit is recruited at 5 Hz; every 5 Hz an additional motor unit is added to the recruitment pattern. Thus, your EMG screen should at some point show you 4 motor units firing at once with gradually increased muscle contraction by the patient: these units will be firing at 20, 15, 10, 5 Hz respectively.
Neurapraxia can be defined as which of the following?
Focal pressure on a nerve, leading to focal demyelination and conduction block
-This can become remyelinated over 2-3 weeks,
You are performing an EMG/NCS on a patient. The sensory nerve action potential (SNAP) to digit 2 and compound motor action potential (CMAP) to the abductor pollicis brevis are normal. The SNAP to digit 5 reveals prolonged latency. The CMAP to the abductor digiti minimi is normal. Needle EMG reveals no abnormalities. Which of the following is the most likely diagnosis?
Mild ulnar neuropathy
Which of the following muscles is most likely to develop weakness following a proximal humerus surgical neck fracture?
axillary nerve is at greatest risk of injury following a surgical neck fracture.
The only muscle listed that is innervated by the axillary nerve is teres minor. Deltoid (not listed) would have also been an acceptable answer.
Supraspinatus and infraspinatus - suprascapular nerve. ECRB - radial nerve.
You are reviewing EMG/NCS results of a patient. You notice prolonged latency, decreased conduction velocity, increased temporal dispersion, and normal amplitude on NCS. There is decreased recruitment on EMG. Which of the following conclusions is most likely correct?
A patient’s left upper limb EMG/NCS results are as follows. Sensory nerve action potential (SNAP) of the ulnar nerve to digit 5 exhibits decreased amplitude. EMG reveals decreased recruitment and 3+ fibrillations in pectoralis major, flexor digitorum superficialis, flexor carpi ulnaris, abductor pollicis brevis, and first dorsal interosseous, but normal activity in deltoid, biceps, brachioradialis, triceps, and extensor indicis proprius. Which of the following EMG/NCS findings would you also expect to find in this patient?
Decreased amplitude of medial antebrachial cutaneous nerve SNAP
You are reviewing EMG/NCS results of the right lower limb. The superficial fibular sensory nerve action potential (SNAP) is abnormal while the sural SNAP is normal. Needle EMG reveals decreased recruitment and +1 fibrillations in the fibularis longus and normal activity in tibialis anterior, extensor digitorum brevis, abductor hallucis, gastrocnemius, medial hamstrings, rectus femoris, tensor fascia lata, and lumbar paraspinals. Which of the following is the most likely diagnosis?
Superficial fibular neuropathy
- Sciatic neuropathy would involve the gastrocnemius and abductor hallucis, as well as tibialis anterior and extensor digitorum brevis theoretically, which are all spared here. Deep fibular neuropathy would spare the fibularis longus and affect the tibialis anterior and extensor digitorum brevis.
During a blink reflex study, when stimulating the left trigeminal nerve, you detect a single left eye blink response. This is known as which of the following?
The quick, ipsilateral blink response is called the R1 response, and we record it from the orbicularis oculi muscle ipsilaterally to the side of stimulation.
During a nerve conduction study you realize that the patient’s limb being studied is cold. Which of the following effects will this have on the waveform?
A cold limb will result in increased amplitude, prolonged latency (slow conduction velocity), and increased duration. When the limb is cold, channels stay open longer, causing a larger, heftier amplitude, and longer time of depolarizing.
During an exam, perhaps much like this one, when a question stem mentions EMG findings of painless myokymia in C5-C6/upper trunk muscles, you will astutely recall that the most likely etiology of these findings is which of the following?
Radiation plexopathy is classically the cause of myokymia found on EMG, which classically affects C5-C6/upper trunk muscles, and is usually a painless finding. Lower trunk plexopathy with pain is concerning for pancoast (lung) tumor compressing the lower trunk of the brachial plexus. Myotonia congenita will demonstrate myotonic discharges (divebomber sound).
Demyelination will most likely result in which of the following nerve conduction study (NCS) results?
Normal amplitude, prolonged latency, increased temporal dispersion
-Demyelination is the stripping of the myelin sheath around axons. As myelin increases conduction velocity of axons (decreasing their latency), removal of this myelin will cause slowed conduction velocity, prolonged latency (it takes a longer time for the action potential to reach the recording electrode), and increased temporal dispersion. Temporal dispersion is the widening of the SNAP or CMAP due to impulses reaching the recording electrode at widely varying times (due to demyelination decreasing the uniformity of signal transmission along a nerve’s axons). Amplitude decrease occurs with axonal loss. Amplitude increase occurs in cold limbs.
A stretch injury to a nerve that leads to axon loss with intact epineurium is also known as which of the following?
-This question describes axonotmesis. Even though the axons have died due to crush/stretch injury, the epineurium is still intact, which will serve as a guide path for the axons to regenerate along and ultimately find their target muscle fibers again.
Which of the following correctly states the innervation of the flexor digitorum superficialis of the forearm?
“C7, C8, middle and lower trunk, medial and lateral cord, median nerve” correctly states the FDS innervation.
A patient presents with left foot drop. On EMG/NCS, you note normal superficial fibular and sural nerve SNAPs (sensory nerve action potentials). EMG reveals 3+ positive sharp waves and decreased recruitment in the tibialis anterior (TA), extensor digitorum longus (EDL), extensor hallucis longus (EHL), and extensor digitorum brevis (EDB). EMG of the fibularis longus, abductor hallucis (AH), medial gastrocnemius, hamstrings, rectus femoris, thigh adductors, tensor fascia lata, and lumbar paraspinals is normal. Which of the following is the most likely diagnosis?
Deep fibular neuropathy is the best answer. Superficial fibular neuropathy would affect fibularis longus and superficial fibular SNAP, and spare all the affected muscles in this question. Distal sciatic neuropathy would affect gastrocnemius and AH, as well as fibularis longus, and potentially superficial fibular SNAP. L4 radiculopathy would indeed show normal SNAPs, but would also be expected to affect other muscles receiving L4 innervation, such as rectus femoris, hamstrings, and thigh adductors.
You are performing an EMG on a patient with hand numbness. You note prolonged latency of the sensory nerve action potential (SNAP) to digit 5. The compound motor action potential (CMAP) to the abductor digiti minimi (ADM) is normal when stimulating at the wrist, but reduced when stimulating just below the elbow. What is the next most appropriate step?
This question is a classic example of discovering a Martin-Gruber Anastomosis (MGA) in a patient. This is a normal anatomic variant by which median nerve fibers cross over into the ulnar nerve somewhere in the forearm to supply innervation to the ulnar nerve muscles of the hand. This patient seems like they have a conduction block of the ulnar nerve somewhere in the forearm - however, this is rare, and MGA is much more common and likely. In case of MGA, record over the abductor digiti minimi (as you are doing already in this study) and stimulate the median nerve at the elbow, and see if the amplitude is repaired in your “conduction block”. If all of the amplitude is repaired, then you have MGA, and not conduction block of the ulnar nerve. This is the most appropriate next step here, as conduction block of the ulnar nerve in the middle and distal forearm is rare.
While performing a nerve conduction study (NCS) you apply current to the nerve and detect a small amplitude. You then move distally on the limb and apply current again to the nerve. To your surprise, the amplitude is normal. What is the most likely explanation for this finding?
Partial conduction block
You are performing serial EMGs on a patient. Over time you notice that while he used to demonstrate polyphasic motor unit action potentials (MUAPs) in a particular muscle, on his most recent EMG these polyphasic potentials have been replaced by larger, non-polyphasic potentials. What is the most reasonable explanation for this finding?
Polyphasic potentials represent ongoing reinnervation due to collateral sprouting of existing axons. The polyphasic property arises due to non-uniform myelination of these collateral sprouts during this early reinnervation stage. Once myelination is complete, the motor unit’s branches all conduct the action potential at essentially the same speed, producing a nice, uniform “roller coaster” bump (uniform-appearing MUAP) rather than the polyphasic, serrated potential seen in unmyelinated collateral sprouts.
A patient’s right lower limb EMG/NCS results are as follows. Routine fibular and tibial motor/sensory nerve conduction studies are normal. EMG reveals decreased recruitment and polyphasic potentials in tibialis anterior, extensor hallucis longus, short head of biceps femoris, tensor fascia lata, fibularis longus, and tibialis posterior, but normal activity in vastus medialis, adductor longus, gastrocnemius, and abductor hallucis. Which of the following is the most likely diagnosis?
L5 radiculopathy with reinnervation is the best answer. All L5-innervated muscles are affected (with polyphasic potentials, indicating early reinnervation taking place), but all muscles that do not share any L5 innervation are spared.
Which of the following types of dermatomyositis is associated with cancer?
3
What does a polyphasic potential represent during an EMG?
Polyphasic potentials represent ongoing reinnervation due to collateral sprouting of existing axons. The polyphasic property arises due to non-uniform myelination of these collateral sprouts during this early reinnervation stage.
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During an EMG, with the muscle at rest, you decide to advance your needle until you hear a “seashell” sound. What does this sound represent?
Miniature endplate potentials
- The “seashell” sound is the sound of miniature endplate potentials (MEPPs), which are heard when the needle is very close to endplates, which is a painful needle location for the patient.
The sternocleidomastoid (SCM) shares its innervation with which of the following muscles?
The SCM and trapezius are innervated by C3, C4; spinal accessory nerve. The rhomboids are innervated by C4, C5; dorsal scapular nerve. It’s not important for you to know splenius capitis’ innervation, but it’s not the spinal accessory nerve.
While examining a patient you notice that their right scapula sits more medially than the left scapula. Needle EMG would most likely reveal abnormalities of musculature belonging to which nerve?
The two nerves associated with winged scapula (this patient has a medially winged scapula) are the long thoracic nerve, which innervates the serratus anterior and whose injury can cause a medially winged scapula, and the spinal accessory nerve, which innervates the trapezius and whose injury can cause a laterally winged scapula. The dorsal scapular nerve innervates the rhomboids, and the thoracodorsal nerve innervates the latissimus dorsi.
You are performing an EMG/NCS on a patient. NCS reveals abnormal sensory nerve action potential (SNAP) to digit 5. Dorsal ulnar cutaneous nerve (DUC) SNAP latency is prolonged. Which of the following is the most likely diagnosis?
Complex repetitive discharges occur via which of the following mechanisms?
A CRD is an involuntary discharge that is very wide and serrated (complex in appearance, and repetitive in firing) and occurs due to a motor unit being denervated and then reinnervated by another motor neuron, which itself then becomes denervated. Ephaptic transmission is the process by which these muscle fibers all fire regularly together. CRDs are seen in chronic radiculopathy, anterior horn cell disease, and some normal patients.
A 45 year-old female presents with 3 months of right foot drop and leg pain. On exam, tinel at the fibular head is positive. Nerve conduction studies of the right lower limb (NCS) reveal decreased amplitude of sural and superficial fibular SNAPs (sensory nerve action potential) and decreased amplitude of extensor digitorum brevis (EDB) CMAP (compound muscle action potential). EMG reveals decreased recruitment of tibialis anterior (TA), EDB, abductor hallucis (AH), fibularis longus, and semitendinosus. EMG of lumbar paraspinals, gluteus maximus, tensor fascia lata (TFL), rectus femoris, and thigh adductors is normal. Which of the following is the most likely diagnosis?
Proximal sciatic neuropathy is the best choice, as all sciatic nerve-innervated muscles (some hamstrings and the gastroc/soleus are untested) are affected on EMG, as well as both tibial and fibular SNAPs being affected as well, indicating that most parsimonious explanation is a proximal sciatic neuropathy. A distal sciatic neuropathy, such as at the knee, would spare the hamstrings, which are affected in this case. A radiculopathy would show normal SNAPs (abnormal in this case). Common fibular neuropathy would spare the hamstrings and the abductor hallucis, both of which are affected in this case.
A patient’s right lower limb EMG/NCS results are as follows. Routine fibular and tibial motor/sensory nerve conduction studies are normal. EMG reveals decreased recruitment and polyphasic potentials in tensor fascia lata, rectus femoris, adductor longus, semimembranosus, and tibialis anterior, but normal activity in short head of biceps femoris, fibularis longus, gastrocnemius, and abductor hallucis. Which of the following is the most likely diagnosis?
All muscles receiving some degree of L4 innervation are affected here, but all those without L4 innervation are spared. Note that the short head of biceps femoris is innervated by L5-S1, common fibular division of sciatic nerve, thus differentiating it from the medial hamstrings and long head of biceps femoris, which have L4-L5-S1, tibial division of sciatic nerve innervation. Note also that typically an S1 radiculopathy question will refer to an abnormal H-reflex as partial evidence of an S1 radiculopathic process.
You are performing an EMG on a patient with hand numbness. You note prolonged latency of the sensory nerve action potential (SNAP) to digit 5. The compound motor action potential (CMAP) to the abductor digiti minimi (ADM) is normal when stimulating at the wrist, but reduced when stimulating above the elbow and near the axilla. What is the next best step?
This question demonstrates low CMAP amplitude when stimulating proximally, but normal when stimulating distally, indicating that conduction block is taking place somewhere between the above-elbow stimulation site and the wrist. The next best step would be to stimulate below the elbow and see if the amplitude is normal again. If normal, then you have confirmed a conduction block somewhere at the elbow (between the above-elbow site and below-elbow site). If abnormal, then the conduction block is somewhere in the forearm between the below-elbow site and the wrist. If abnormal, again, this can also be a Martin-Gruber Anastomosis, the normal anatomic variant by which median nerve fibers cross over into the ulnar nerve somewhere in the forearm to supply innervation to the ulnar nerve muscles of the hand. In case of MGA, record over the abductor digiti minimi (as you are doing) and stimulate the median nerve at the elbow, and see if the amplitude is repaired in your “conduction block”. If all of the amplitude is repaired, then you have MGA, and not conduction block of the ulnar nerve
When needling a patient’s biceps brachii during an EMG study, you detect an involuntary, abrupt, regular signal that sounds like soldiers marching. What is the most likely diagnosis?
Radiation plexopathy
Which of the following muscles is NOT innervated by the posterior interosseous nerve?
The ECRL is innervated by the radial nerve, not the posterior interosseous nerve (which arises from the radial nerve).
During nerve conduction studies, the optimal lower limb temperature is which of the following?
The optimal lower limb temperature during NCS is 30 degrees Celsius.
The nerve that provides cutaneous sensation to the medial leg below the knee arises via which of the following ways?
The saphenous nerve is described here, and is the continuation of the femoral nerve after it has innervated all its muscles.
During a patient examination for left shoulder pain you notice that the patient’s left scapula sits more medially than the right scapula. The nerve implicated in this patient’s condition receives its innervation from which trunk of the brachial plexus?
The long thoracic nerve innervates the serratus anterior. Injury to the long thoracic nerve can cause a medially winged scapula, as this patient demonstrates. The serratus anterior innervation is C5, C6, C7, long thoracic nerve. Thus, its innervation is pre-plexus, and does not have any trunks or cords that contribute to it.
A 62 year-old male with a past medical history of hypertension presents with progressive onset weakness and swallowing difficulties. He is a retired professional football player. He denies numbness/tingling, bowel/bladder dysfunction, or a family history of this problem. EMG/NCS reveals normal sensory nerve action potentials (SNAPs) and compound muscle action potentials (CMAPs) in 3 limbs. Needle exam reveals long-duration, large amplitude motor unit action potentials (MUAPs) with decreased recruitment and 2+ fibrillations in 3 limbs. Which of the following treatments is most likely appropriate?
This patient’s presentation and EMG findings are classic for ALS (amyotrophic lateral sclerosis). This is the death of anterior horn cells due to a not-yet clearly defined cause. It has been associated with a professional athletics history. Recall that anterior horn cells are entirely separate from sensory neurons, and, thus, the SNAPs of these patients will be normal. CMAPs can be normal vs. abnormal in these patients. EMG shows a “neuropathic” pattern of MUAPs as described. Riluzole has been shown to prolong survival in ALS patients. Rehabilitation with submaximal exercise is recommended. ALS carries a poor prognosis, and most patients experience progressive disability and death within a few years of onset. Nusinersen is indicated in patients with Spinal Muscular Atrophy.
The medial antebrachial cutaneous nerve (MAC) arises in which of the following ways?
The MAC arises as a branch of the medial cord of the brachial plexus.
In Erb palsy, which of the following EMG/nerve conduction study findings is most likely to be discovered? SNAP: sensory nerve action potential.
Abnormal lateral antebrachial cutaneous nerve SNAP
-Erb palsy is a C5, C6/upper trunk brachial plexopathy commonly caused by trauma as an adult or obstetrical trauma as an infant due to traction forces on these roots. Because the C5 and C6 roots are injured, changes can be expected to be found in any downstream SNAPs, CMAPs, or EMG needling of C5, C6 muscles. The Waiter’s Tip position is classic for this: arm is adducted, internally rotated, pronated, wrist flexed, all due to C5/C6 muscle weakness. There is sensory loss over the lateral arm and dorsolateral forearm. The musculocutaneous nerve (C5, C6, upper trunk) terminates as the lateral antebrachial cutaneous nerve (LAC); thus, if the C5,C6/upper trunk is injured, an abnormal LAC SNAP may be detected. The median sensory fibers to the thumb involve C5/C6 fibers; thus, this median SNAP would be abnormal on NCS. The extensor indicis proprius (EIP) does not contain C5, C6, or upper trunk innervation, and thus would show normal recruitment in this case. The medial antebrachial cutaneous (MAC) SNAP would be normal as well, as it arises from the lower trunk of the brachial plexus, which itself arises from C8 and T1 nerve roots.
Which of the following muscles does not receive its innervation from the lateral cord of the brachial plexus?
Flexor carpi ulnaris
-Biceps brachii, flexor carpi radialis, and pronator teres all receive lateral cord innervation. Flexor carpi ulnaris receives medial cord innervation.
A 47 year-old male presents to your musculoskeletal clinic with complaints of 3 months of left shoulder pain. He works in the post office handling heavy packages. On exam, you note no pain with empty can, and no pain with resisted external or internal rotation of the shoulder. When abducting the arm 90 degrees and flexing the elbow 90 degrees, and then performing resisted external rotation of the shoulder, this reproduces the patient’s pain. What is the innervation of the muscle being tested?
The physical exam maneuver described is how one isolates and tests the teres minor (one could also argue infraspinatus, which is difficult to isolate from teres minor on physical examination), which is innervated by C5,C6, upper trunk, posterior cord, axillary nerve. The dorsal scapular nerve innervates the levator scapula, rhomboid minor, and rhomboid major, whose physical examination tests would involve scapular elevation and/or medial translation. The upper and lower subscapular nerves innervate the subscapularis and teres major (lower subscapular nerve → teres major), tested differently from how this question describes.
When performing an ultrasound-guided corticosteroid injection into the 1st extensor compartment, which of the following is the most likely potential complication?
The superficial radial nerve runs right along the 1st extensor compartment, and is susceptible to stretching/compression/crush if not identified and accounted for prior to advancing the needle towards the 1st extensor compartment (which contains APL and EPB tendons). This procedure is commonly done for De Quervain Tenosynovitis.
A patient’s left upper limb EMG/NCS results are as follows. Routine median, ulnar, and radial sensory nerve conduction studies are normal. EMG reveals decreased recruitment and 4+ fibrillations in triceps, extensor indicis proprius, abductor pollicis brevis, and first dorsal interosseous, but normal resting activity and recruitment in deltoid, biceps, brachioradialis, and pronator teres. Which of the following is the most likely diagnosis?
C8 radiculopathy
A patient’s right upper limb EMG/NCS results are as follows. Routine median, ulnar, and radial sensory nerve conduction studies are normal. EMG reveals decreased recruitment and 4+ positive sharp waves in infraspinatus, brachialis, deltoid, brachioradialis, pronator teres, and triceps, but normal activity in extensor indicis proprius, flexor digitorum superficialis, abductor pollicis brevis, and first dorsal interosseous. Which of the following is the most likely diagnosis?
C6 radiculopathy
-C6 radiculopathy is the best answer. All muscles with some C6 innervation are affected here, showing active denervation (positive sharp waves). Indeed, many of these muscles have C5 innervation and are affected, but this continues in muscles without C5 innervation (pronator teres: C6-C7, triceps: C6-C7-C8); notably, muscles with C7 or C8 or T1 innervation (lacking any C6) are spared. Sensory nerve action potentials are also normal, indicating proximal (e.g. radicular) process rather than distal. Posterior cordopathy would spare pronator teres. Upper trunk plexopathy would show median sensory abnormalities on NCS, as well as lateral antebrachial cutaneous nerve SNAP abnormality (which was untested here). Please refer to the innervation chart at the beginning of the Upper and Lower Extremity Peripheral Nervous System Diseases chapters for a detailed innervation guide.
During an EMG, you ask the patient to very lightly begin contracting their extensor indicis proprius. Instantly you notice the EMG screen become flooded with many small, short-duration MUAPs. Which of the following is the most likely diagnosis?
Polymyositis
- This question describes a myopathic recruitment pattern, or increased recruitment pattern. This is typically seen in myopathies, not neuropathies; thus, polymyositis is correct.
You are reviewing left lower limb EMG/NCS findings and trying to propose a possible treatment plan to the patient. The patient is experiencing foot pain not relieved by NSAIDs or heat/ice. NCS reveals normal superficial fibular and sural SNAPs (sensory nerve action potentials), prolonged latency of medial and lateral plantar nerve SNAPs, and abnormal CMAP (compound muscle action potential) to abductor hallucis (AH). The CMAP to the extensor digitorum brevis (EDB) is normal. Needle EMG reveals decreased recruitment in AH, abductor digiti quinti pedis (ADQP), and lumbricals. EMG of tibialis anterior, EDB, gastrocnemius, fibularis longus, hamstrings, rectus femoris, tensor fascia lata, and lumbosacral paraspinals is normal. Which of the following is the most reasonable treatment strategy for this condition?
Bracing
-These findings suggest tarsal tunnel syndrome, a rare compression of the tibial nerve as it passes through the tarsal tunnel around the medial malleolus, underneath the flexor retinaculum. The flexor retinaculum is a common culprit that compresses the tibial nerve too tightly, causing the patient’s symptoms of plantar foot pain with numbness/tingling and possible foot muscle weakness. Surgical release of the flexor retinaculum (akin to carpal tunnel release) is the best option of these choices (a structural solution for a structural problem). US-guided tarsal tunnel injection could also be considered. Bracing would not do anything to decompress the tarsal tunnel. Gabapentin also would not decompress the tarsal tunnel, but mask the symptoms while the nerve compression continues, potentially destroying axons over time. Peripheral nerve stimulation is not appropriate for this reason also.
During EMG studies, decreased insertional activity can be seen in which of the following situations?
Muscle fibrosis can cause decreased insertional activity. Increased insertional activity can be seen in cases in which the muscle cell membrane is hyper-irritable, such as active denervation
A 58 year-old female complains of sudden-onset severe shoulder pain for 2 weeks, which then improves and is followed by progressive onset shoulder weakness. She denies trauma. Shoulder x-ray and MRI are negative. On exam she has weakness in shoulder abduction. EMG/NCS reveals decreased recruitment and +2 positive sharp waves in the supraspinatus and infraspinatus. Which of the following is the most reasonable advice you should give to this patient?
This patient presents with classic Parsonage-Turner Syndrome (neuralgic amyotrophy, idiopathic brachial plexopathy). This typically presents as sudden shoulder pain for 2 weeks which gives way to weakness, classically in the suprascapular (this case), long thoracic, and/or anterior interosseous nerve territory. Serial EMGs are useful for prognosis, and most cases self-resolve within 1-2 years after onset. Early surgical options would not be indicated, as most cases self-resolve. PTS typically begins following a viral illness or surgery, and its etiology is not entirely defined at this time.
A patient presents with deep fibular neuropathy. Which of the following areas of the foot do you expect to demonstrate sensory abnormalities?
1st webspace, sparing the dorsum of the foot
-The deep fibular nerve innervates the skin between the 1st and 2nd toes (the 1st webspace). The superficial fibular nerve innervates the skin over the entire dorsum of the foot, except the 1st webspace (which is deep fibular nerve territory).
With LCL (lateral collateral ligament) injuries in the knee, it is important to rule out nerve injury most crucially in which of the following nerves?
The common fibular nerve wraps around the fibular head/neck near the LCL attachment, so with injuries to the LCL come injuries to other nearby structures, e.g. the common fibular nerve; thus, it is important to rule out injury to this nerve in a patient with LCL tear. High-yield functions to test would be foot dorsiflexion and eversion.
A 29 year-old female presents to your clinic with complaints of 2 months of bilateral hand numbness and tingling, worse at night. She is expecting her first child later this year. On exam, strength and sensation are intact. What is the next best step?
Bracing
-Pregnancy can cause carpal tunnel syndrome, simply due to fluid balance changes in the body. The first step is a carpal tunnel wrist brace that is to be worn during sleep, which prevents wrist flexion (wrist flexion causes increased pressure within the carpal tunnel, compressing and irritating the median nerve over time)
A 25 year-old male develops 2 weeks of progressive lower limb weakness. He has a history of recent gastroenteritis. Bilateral lower limb EMG/NCS reveals prolonged latency of bilateral superficial fibular nerve SNAPs (sensory nerve action potentials), but normal sural nerve SNAPs. The bilateral CMAPs to the EDB and AH (compound muscle action potentials to the extensor digitorum brevis and abductor hallucis) show prolonged latency but normal amplitude. EMG reveals decreased recruitment in the bilateral EDB, AH, tibialis anterior, gastrocnemius, and semimembranosus, but normal activity in rectus femoris and adductor longus. Which of the following additional EMG/NCS findings would most likely be discovered in this patient?
This patient presents with Guillain-Barre Syndrome (GBS), also referred to as AIDP (acute inflammatory demyelinating polyradiculopathy). GBS is caused by a recent infection (typically GI or URI) causing your immune system to confuse a foreign protein with a protein on your own myelin. Thus, your body destroys your own peripheral nerve myelin (demyelinating the nerves and prolonging the latency). The first EDX finding of GBS is delayed or absent F-waves. NCS reveals prolonged latency and sometimes reduced amplitude of SNAPs with sural nerve sparing, as the sural nerve is larger with more myelin than other peripheral sensory nerves, and thus is less affected than the other nerves. CMAPs show prolonged latency, decreased conduction velocity (CV), and typically normal amplitude (this is a demyelinating disease primarily, not axonal loss). EMG shows decreased recruitment but otherwise there should be no active denervation usually, but this can occur as a secondary feature of GBS weeks after onset. Treatment involves PT and early plasmapheresis/IVIG. Abnormal/increased temporal dispersion is also a feature of this disease on NCS, due to the widespread and more uniform demyelination of nerves taking place.
During a nerve conduction study, you decide to place the recording electrode over the patient’s soleus, and the reference electrode over their achilles tendon. You then stimulate in a proximal direction from the popliteal fossa. When you examine the waveform produced by this, you note prolonged latency. What is the most appropriate conclusion from these results?
There is damage somewhere along the pathway you stimulated
-The acquisition of the H reflex is described here. By performing this, the physician stimulates the Ia sensory afferent nerves and records over the muscle (soleus), sending the action potential proximally toward the spinal cord, triggering the spinal reflex arc, and sending the action potential back down distally along the motor nerve (sciatic → tibial) to make the muscle (soleus) contract. Prolonged latency of this clearly very long nerve pathway merely indicates that there is some kind of damage somewhere along the pathway that has caused the signal to take much longer to reach the recording electrode than is normal. The H reflex study is typically used to assess for S1 radiculopathy.
A 25 year-old female is involved in a motor vehicle accident. She suffers a left proximal femur fracture and requires orthopedic surgery to repair her left hip. She presents to your inpatient rehabilitation unit 2 weeks later for functional restoration. On exam, you note intact right lower limb strength, intact left hip flexion, knee extension, adduction, internal/external rotation, and abduction strength, but impaired knee flexion, dorsiflexion, and inversion. She has paresthesias in her posterior thigh and anterior shin. What is the most likely diagnosis?
This patient presents with findings concerning for sciatic neuropathy, either as a result of the trauma itself, or the surgical approach (posterior approach places sciatic nerve at risk). Proximal sciatic neuropathy will affect essentially all sciatic-innervated muscles, including hamstrings, and all muscles originating at or below the knee. Thus, knee flexion is impaired (hamstrings), dorsiflexion is impaired (common fibular nerve, which arises from sciatic nerve), and inversion is impaired (tibial nerve, which arises from sciatic nerve). Plantarflexion would also be impaired (gastroc/soleus, tibial nerve), but this was not mentioned in the exam. Femoral neuropathy would cause impaired knee extension (intact in this patient); Lumbosacral plexopathy would cause patchy abnormalities not likely cleanly narrowed down to a single nerve (as in this case); tibial and common fibular neuropathies would indeed cause the abnormalities discussed here, except the impaired knee flexion and paresthesias in the posterior thigh imply a proximal sciatic nerve pathology, leading to weak hamstrings.
During an upper limb EMG/NCS study you note normal sensory nerve action potentials (SNAPs) to digits 1, 2, and 5. Compound motor action potentials (CMAPs) to the abductor pollicis brevis (APB) and first dorsal interosseous (FDI) are normal. Needle EMG reveals decreased recruitment and abnormal spontaneous activity in the brachialis and biceps, with normal EMG of cervical paraspinals, triceps, brachioradialis, pronator teres, APB, and FDI. Which of the following would also most likely be found in this study?
This patient presents with findings consistent with musculocutaneous neuropathy. This nerve innervates the coracobrachialis (not discussed here), biceps brachii, and brachialis, hence the abnormal EMG to the biceps and brachialis. The musculocutaneous nerve terminates as the LAC; thus, LAC SNAP should be studied and would be expected to be abnormal compared to the contralateral side. Absent CMAP to EIP could be seen in posterior interosseous neuropathy or a proximal radial neuropathy. Radial neuropathy above the humerus midshaft could cause abnormal EMG to the anconeus. Abnormal CMAP to the FCR could be caused by median neuropathy, but not musculocutaneous neuropathy.
Which of the following is the key NCS/EMG finding, found in radiculopathies, that helps to differentiate a radiculopathy from more peripheral nerve lesions?
Normal SNAPs (sensory nerve action potentials) are the key NCS finding demonstrated in a radiculopathy. Because the dorsal root ganglion contains the cell bodies of the sensory neurons of a dermatome of a given limb, and these cells are bipolar neurons residing outside the spinal cord, any nerve injury proximal to these cell bodies (e.g. radiculopathy) will not affect the distally directed axons of these bipolar cells (i.e. the projections extending down the arm or leg from the dorsal root ganglion). Thus, these unaffected axons will conduct impulses quite normally, and SNAPs will be entirely normal in a pure radiculopathy. Paraspinal muscles corresponding to the affected nerve root level will theoretically show EMG changes (increased irritability, possible active denervation). CMAPs will be expected to potentially be abnormal, but this would not be the key differentiating feature of a radiculopathy.
In a nerve conduction study, the _______ is _______ charged, thus attracting _______ towards it.
On the stimulating electrode during a nerve conduction study, the cathode is negatively charged, which attracts positive sodium ions towards it; these sodium ions accumulate right outside the axon membrane where their concentration becomes so large that they trigger the voltage-gated sodium channels to open, thus initiating an action potential.
Which of the following muscles receives its innervation via the following neural pathway? C7, C8, T1; middle and lower trunk; medial and lateral cord; median nerve; anterior interosseous nerve.
PQ is innervated by the above pathway. FDS, FCR, and lumbricals 1 and 2 are all directly innervated by the median nerve, not the anterior interosseous nerve.
Normal motor recruitment follows which of the following principles?
Small motor units are recruited before large units. Type II motor units are fast twitch, powerful fibers that are recruited with maximal intensity, thus filling the EMG screen with motor units by the time Type II units are recruited.
