EMG Flashcards
Electrophysiologic findings of compound muscle action potential conduction block and temporal dispersion, prolonged minimum F-wave latency, and reduced conduction velocity would most likely be seen in
a. Charcot-Marie-Tooth Disease.
b. myasthenic syndrome
c. Guillan-Barre syndrome
d. amyloidosis
c. All the findings mentioned are features associated with an acquired demyelinating condition such as Guillan-Barre syndrome or acute inflammatory demyelinating polyradiculoneuropathy (AIDP). Hereditary motor sensory neuropathies do not usually have temporal dispersion of compound muscle action potentials. Myasthenic syndrome is a neuromuscular junction disorder and amyloidosis is associated with a form of axonal peripheral neuropathy
You are treating a 48-year-old man who has two lumbar laminectomies for what you suspect is a recurrent right L5 radiculopathy. You perform an electromyogram to confirm the diagnosis, and it reveals 2+ positive waves and fibrillations with decreased recruitment in the right anterior tibialis. The patient informs you that he can only tolerate examination of one more muscle. Of the following you would choose:
A) extensor hallucis longus.
B) L5 paraspinals.
C) vastus medialis.
D) flexor digitorum longus
D) The history is suggestive of an L5 radiculopathy. Given the previous laminectomies, examining a single level of paraspinals would provide limited information. Although you cannot form any firm conclusions based on such a limited examination, study of the flexor digitorum longus will provide findings outside the peroneal, distribution and lend support to the clinical diagnosis.
Which technique may reduce stimulus artifact when performing sensory nerve conduction studies?
A) increasing the impedance of recording electrodes
b) increasing the stimulus duration
c) rotating the anode around the cathode
D) decreasing the low frequency filter
C) Rotating the anode around the cathode can decrease stimulus artifact. The other choices have no effect, or increase it
- Of the following, somatosensory studies would be the most useful in the diagnosis of
(a) tarsal tunnel syndrome.
(b) motor neuron disease.
(c) myasthenia gravis.
(d) multiple sclerosis.
- (d) Somatosensory studies can be helpful in the diagnosis of multiple sclerosis. Standard nerve
conduction studies and electromyography are far more useful in the diagnosis of the other
disorders.
- You are treating a 48-year-old man who has had two lumbar laminectomies for what you suspect is
a recurrent right L5 radiculopathy. You perform an electromyogram to confirm the diagnosis, and
it reveals 2+ positive waves and fibrillations with decreased recruitment in the right anterior
tibialis. The patient informs you that he can only tolerate the examination of one more muscle. Of
the following you would choose
(a) extensor hallucis longus.
(b) L5 paraspinals.
(c) vastus medialis.
(d) flexor digitorum longus.
- (d) The history is suggestive of an L5 radiculopathy. Given the previous laminectomies, examining a
single level of paraspinals would provide limited information. Although you cannot form any firm
conclusions based on such a limited examination, study of the flexor digitorum longus will provide
findings outside the peroneal distribution and could lend support to the clinical diagnosis.
- The number of phases of a motor unit is related to the
(a) conduction time through collateral nerve sprouts.
(b) sweep speed setting on the oscilloscope screen.
(c) sensitivity (gain) setting on the oscilloscope screen.
(d) central conduction time and the state of the upper motor neuron.
- (a) The number of phases of the motor unit potential represents the synchronization of firing of the
individual muscle fibers in a motor unit and is related to conduction time through collateral sprouts
of the nerve. The number of phases is increased under conditions in which some sprouts are poorly
myelinated and conduction is slow and less synchronous. The other factors do not affect the
number of phases of a motor unit potential.
- During the electromyographic evaluation of a patient, you note discharges consistent with
myotonia and small motor units in the distal muscles of the upper and lower extremities. The most
likely diagnosis is
(a) paramyotonia congenita.
(b) myotonic dystrophy.
(c) myotonia congenita.
(d) hyperkalemic periodic paralysis.
- (b) Although myotonic discharges may be seen in all of these disorders, myotonic dystrophy is the
only one with low-amplitude motor units in the distal muscles.
- Dorsal ulnar cutaneous nerve conduction studies are most useful in differentiating
(a) ulnar neuropathy at the elbow from lower trunk plexopathy.
(b) ulnar neuropathy at the elbow from ulnar neuropathy at the wrist.
(c) lower trunk plexopathy from medial cord plexopathy.
(d) ulnar neuropathy at the cubital tunnel from ulnar neuropathy at the ulnar groove.
- (b) The fibers of the dorsal ulnar cutaneous nerve travel in the lower trunk and the medial cord of the
brachial plexus. This nerve travels with the ulnar nerve to the forearm, where it branches off
proximal to the wrist and supplies sensation to the ulnar aspect of the dorsum of the hand and
wrist. The dorsal ulnar cutaneous sensory nerve action potential amplitude could be decreased with
a lesion of the lower trunk, the medial cord, or the ulnar nerve at the elbow or proximal forearm,
and would not be useful in differentiating among them. It should be normal in ulnar neuropathy at
the wrist.
- The potentials above are
(a) complex repetitive discharges.
(b) myotonic discharges.
(c) neuromyotonia.
(d) myokymia.
- (b) The potentials noted are single-fiber discharges waxing and waning in frequency and amplitude.
This is characteristic of myotonic discharges.
- Comparing the results of electrodiagnostic studies on patients with clinical evidence of postpolio
syndrome and the results of those obtained in persons with a history of polio of similar severity and
duration since polio onset but no clinical evidence of postpolio syndrome, you find
(a) smaller motor units in the symptomatic group.
(b) more polyphasic motor units in the symptomatic group.
(c) more fasciculations in the symptomatic group.
(d) no significant differences between the groups.
- (d) There are no significant differences between the groups. Electrodiagnostic studies are not
performed to confirm the diagnosis of postpolio syndrome; this is a clinical diagnosis. They are
performed to rule out other disorders in the differential diagnosis.
- In testing a patient with suspected myasthenia gravis, needle electromyography (EMG) of the right
upper extremity and orbicularis oculi is normal. Repetitive stimulation of the right ulnar nerve at a
rate of 2/second shows no decrement before or immediately after 1 minute of exercise. A 4%
decrement is noted 2 minutes after exercise. Your next electrodiagnostic step should be
(a) EMG of the lower extremities.
(b) EMG of the frontalis before and after edrophonium (Tensilon).
(c) repetitive stimulation of recording from a proximal muscle.
(d) ulnar somatosensory evoked potentials.
- (c) In patients with myasthenia gravis, repetitive nerve studies recorded from proximal muscles are
more sensitive, though technically more difficult.
- In evaluating a hypotonic infant with electromyography you find low-amplitude, short-duration
motor units with early recruitment. Based on these findings, the LEAST likely diagnosis would be
(a) central core disease.
(b) nemaline myopathy.
(c) type II glycogenosis (acid maltase deficiency).
(d) infantile spinal muscular atrophy.
- (d) The motor unit changes noted are typically seen in myopathies. Spinal muscular atrophy is an
anterior horn cell disease.
- Which hypothesis does NOT explain a normal electromyograph (EMG) in a patient who has a
lumbar radiculopathy?
(a) Involvement of only the sensory root
(b) Limited sampling of muscles
(c) Oxycodone taken prior to the study
(d) Timing of the study
- (c) Pain medication has no effect on EMG findings. All the other choices can be an explanation for a
normal EMG in a patient who has a lumbar radiculopathy.
- A 27-year-old previously healthy woman awoke with severe right scapular and shoulder pain 4
weeks ago. There is no history of trauma. She has no constitutional symptoms. Three weeks ago,
her pain began resolving and scapular winging developed. Electromyography (EMG) reveals 3+
positive waves and fibrillations with markedly decreased recruitment in the right serratus anterior.
EMG of the right deltoid, biceps, pronator teres, abductor pollicis brevis, first dorsal interosseous,
and cervical paraspinals is normal, as is EMG of the left serratus anterior. The most likely
diagnosis is
(a) systemic lupus erythematosus.
(b) compression neuropathy of the dorsal scapular nerve.
(c) idiopathic brachial neuropathy (neuralgic amyotrophy).
(d) C5 radiculopathy due to cervical disc herniation.
- (c) This is a classic history for neuralgic amyotrophy or idiopathic brachial plexopathy involving the
long thoracic nerve. In 30% of patients with neuralgic amyotrophy, EMG abnormalities can be
found in the asymptomatic upper extremity; however, the absence of such findings does not
obviously exclude the diagnosis. The findings are inconsistent with the other diagnoses.
- A 47-year-old soldier presents with left finger extensor weakness after repetitive wrist extension
exercises at the gym. Motor nerve conduction studies were as follows:
Extensor Indicis
Nerve Stimulation Site Amplitude(mV) Conduction Velocity (m/s)
L. Radial mid-forearm 6.0
L. Radial elbow 2.0 60
L. Radial spiral groove 2.0 65
R. Radial elbow 5.8
This patient has
(a) radial neuropathy just distal to the spiral groove with axonotmesis.
(b) radial neuropathy just distal to the spiral groove with neurapraxia.
(c) posterior interosseous neuropathy with axonotmesis.
(d) posterior interosseous neuropathy with neurapraxia.
- (d) There is conduction block across the mid-forearm consistent with a posterior interosseous
neuropathy with neurapraxia.
- A 50-year-old man complains of paresthesias of the right lateral 3 ½ digits and wrist pain. Nerve
conduction studies for the right arm (norm in parentheses) are as follows:
Motor
Nerve Distal Latency (ms) Amplitude (mV) Conduction Velocity (m/s)
R. Median 5.3 (5) 48 (>45)
R. Ulnar 3.7 (5) 52 (>45) forearm
8.5 50 across the elbow
Sensory
Nerve Stimulation Site Peak Latency(ms) Amplitude (
- (d) There is slowing of the median motor distal latency and the median sensory latency across the
wrist, findings consistent with carpal tunnel syndrome.
- Which technique may reduce stimulus artifact when performing sensory nerve conduction studies?
(a) Increasing the impedance of the recording electrodes
(b) Increasing the stimulus duration
(c) Rotating the anode around the cathode
(d) Decreasing the low frequency filter
- (c) Rotating the anode around the cathode can decrease stimulus artifact. The other choices have no
effect, or increase it.
- The type of recruitment in this graph (gain: 500 uV/div, Sweep: 10 ms/div ) is most often seen in
(a) normals.
(b) neuropathy.
(c) myopathy.
(d) poor patient effort.
- (b) There is a single motor unit firing at approximately 20Hz without another motor unit coming in.
This is an example of decreased recruitment, which may be seen in neuropathy. In myopathy one
may see early recruitment of motor units. In patients who give submaximal effort there may be only
1 motor unit seen on the screen, but the firing rate is less than 20Hz.
- Regarding the electrodiagnostic testing of a patient with definite myasthenia gravis, which statement
is TRUE?
(a) An increment on repetitive stimulation at 1Hz of up to 25% is expected.
(b) A stimulation rate of 2-3Hz is most useful in demonstrating a decrement.
(c) An initial low amplitude compound motor action potential after a supramaximal stimulus is
expected.
(d) Motor unit variability is reflected by decreased jitter during single fiber EMG.
- (b) A 2 to 3Hz stimulation is optimal for demonstrating a decremental response. At this rate there is no
build up of Ca++ concentration within the nerve terminal and the amount of acetylcholine in the
readily available stores diminishes, making failure of some of the neuromuscular junctions possible
in those patients with an already small safety factor. A decrement of up to 10% on 2 to 3Hz
repetitive stimulation is considered normal. Small CMAPs initially are more suggestive of
myasthenic (Lambert-Eaton) syndrome than of myasthenia gravis. Single fiber EMG reveals
increased jitter and may reveal blocking.
- The potentials shown in this graph are
(a) fibrillations.
(b) myopathic motor units.
(c) end plate spikes.
(d) complex repetitive discharges.
- (c) The duration of these potentials is approximately 5ms, too short for a motor unit. The initial
deflection is negative, distinguishing this potential as an end plate spike rather than a fibrillation.
- Which muscle is innervated by the peroneal division of the sciatic nerve?
(a) Adductor magnus–anterior part
(b) Piriformis
(c) Semimembranosus
(d) Biceps femoris- short head
- (d) The anterior part of adductor magnus is innervated by the obturator nerve. The piriformis receives
its own branch off the lumbosacral plexus. The semimembranosus is innervated by the tibial
division of the sciatic nerve. Only the short head of the biceps femoris is innervated by the peroneal
division of the sciatic nerve.
- Surface electrodes for recording antidromic sural nerve conduction studies are best placed
(a) posterior to the medial malleolus.
(b) posterior to the lateral malleolus.
(c) anterior to the medial malleolus.
(d) anterior to the lateral malleolus.
- (b) The sural nerve travels posterior to the lateral malleolus and is best recorded over this area.
- A 40-year-old patient presents with weakness and sensory loss in the left arm after a motor vehicle
accident. An EMG study 4 weeks after the injury shows the following results:
Muscle Positive Waves Fibrillations Fasciculations Recruitment
L. Deltoid 2+ 2+ 1+ mod decreased
L. Biceps 0 0 0 normal
L. Latissimus dorsi 2+ 1+ 1+ mild decreased
L. Triceps 2+ 2+ 1+ mod decreased
L. Pronator teres 0 0 0 normal
L. Abd pollicis brevis 0 0 0 normal
L. 1st dorsal interosseous 0 0 0 normal
L. Paraspinals 0 0 0
Based on these findings what is the cause of the patient’s weakness?
(a) C6 radiculopathy
(b) Upper trunk plexopathy
(c) Posterior cord plexopathy
(d) Lateral cord plexopathy
- (c) The abnormalities noted are in a posterior cord distribution.
- A 34-year-old pregnant woman with nocturnal paresthesias of the right lateral 3 digits and pain in
the wrist and forearm is seen for electrodiagnostic studies. The studies (norms in parentheses) reveal
the following results:
Nerve Latency (ms) Amplitude (mV) Conduction Velocity (m/s)
Motor studies
R. Median – Wrist 4.2 (5)
R. Median – Elbow 4 (>5) 45 (>45)
R. Ulnar – Wrist 3.4 (5)
R. Ulnar – Elbow 6 (>5) 55 (>45)
L. Median – Wrist 3.9 (5)
L. Median – Elbow 7 (>5) 52 (>45)
L. Ulnar – Wrist 3.3 (5)
L. Ulnar – Elbow 7 (>5) 56 (>45)
Muscle Positive Waves Fibrillations Recruitment
R. Deltoid 0 0 normal
R. Biceps 0 0 normal
R. Triceps 0 0 normal
R. Pronator teres 2+ 2+ decreased
R. Flex carpi radialis 2+ 2+ decreased
R. Flex carpi ulnaris 0 0 normal
R. Flex pollicis longus 2+ 2+ decreased
R. Abd pollicis brevis 2+ 2+ decreased
R. 1st Dorsal interosseous 0 0 normal
R. Cervical paraspinals 0 0
The patient’s symptoms are most likely due to an entrapment of the
(a) median nerve at the wrist (carpal tunnel syndrome).
(b) median nerve at the pronator teres muscle.
(c) median nerve at the ligament of Struthers.
(d) anterior interosseous nerve of the forearm.
- (c) An entrapment of the median nerve at the ligament of Struthers could involve all median innervated
muscles of the forearm, including the pronator teres. Pronator teres syndrome usually does not
involve the pronator teres since it is innervated from a branch of the median nerve that is more
proximal. The patient does not have slowing of the median distal latencies suggestive of carpal
tunnel syndrome, and the EMG abnormalities include abnormalities in more muscles than can be
explained by an anterior interosseous neuropathy.