Electrodiagnosis Flashcards
Myotonic discharges produce a distinctive sound on electromyography (EMG). This sound results
from
(a) spontaneous firing of motor unit action potentials (MUAPs).
(b) intermittent blocking of muscle fibers during voluntary activity.
(c) firing of just a few muscle fibers per motor unit during voluntary activity.
(d) spontaneous firing of muscle fibers
Answer: (d)
Commentary: The classic myotonic discharge of waxing and waning in frequency and amplitude is often described as either a dive-bomber or revving engine. The source generator of a myotonic potential is muscle fiber, and thus may take the form of either positive wave or a brief spike potential. Neuromyotonia and myokymia result in spontaneous firing of motor unit action potentials as opposed to muscle fiber action potentials. Intermittent blocking of some muscle fibers within a motor unit result in unstable MUAPs, which are characterized by changes in either amplitude or number of phases from potential to potential. Unstable MUAPs are seen in primary neuromuscular junction (NMJ) disorders or with new and immature NMJs, as can be seen in early reinnervation. Mypopathic MUAPs result from a decreased number of muscle fibers firing within a motor unit. These signals are seen as short-duration, small amplitude, and polyphasic potentials.
2013
One advantage of a concentric needle compared to a monopolar needle is its
(a) higher amplitude of motor unit action potentials(MUAPS).
(b) decreased likelihood of electrical interference.
(c) ability to vary the recording surface size.
(d) longer duration of the MUAPs.
Answer: (b)
Commentary: The shaft of a concentric needle serves as the reference electrode, whereas an additional electrode (typically a surface electrode) is needed as a reference when using a
monopolar needle. MUAPs recorded from monopolar needles are slightly longer in duration and have higher amplitude, since they record from the entire area around the needle tip rather than only from the fibers facing the bevel. Because the concentric needle shaft serves as the reference electrode, the recording surface size is fixed and interference from surrounding muscles is minimized.
2012
Which type of study best differentiates a severe polyradiculopathy from amyotrophic lateral
sclerosis (ALS)?
(a) Motor nerve conduction studies of upper and lower extremities
(b) Needle electromyography of thoracic paraspinals or bulbar muscles
(c) Sensory nerve conduction studies of upper and lower extremities
(d) Needle electromyography of multiple extremities
Answer (b)
Commentary: Sensory nerve conduction studies are normal in both radiculopathy and motor
neuron disease. Motor nerve conduction studies are also often normal in both diseases. Both
diseases may demonstrate abnormal needle examination in multiple extremities. Thoracic
paraspinals and bulbar muscle examinations are most helpful in differentiating severe
polyradiculopathy from amyotrophic lateral sclerosis (ALS), since one would expect these studies
to be normal in radiculopathy but may be abnormal in ALS
2011
In a patient with a neuromuscular junction disorder, which electrodiagnostic results for compound
muscle action potential (CMAP), motor unit action potential (MUAP) or nerve action potential
(SNAP) may be misleading if the limb is cold?
(a) Diminished CMAP decrement on repetitive nerve stimulation
(b) Diminished polyphasia of the MUAP
(c) Shortened distal latency of the CMAP
(d) Decreased amplitude of the SNAP
Answer (a)
Commentary: In neuromuscular junction (NMJ) disorders, compound muscle action potential
(CMAP) decrement may be diminished if the limb is cold, likely due to decreased functioning of
acetylcholinesterase. Cool temperatures may alter results by slowing nerve conduction velocity,
prolonging distal latency, increasing amplitude and duration of sensory nerve action potential
(SNAP) and CMAP and motor unit action potential (MUAP), increasing phases of MUAP.
2011
A 55-year-old overweight man presents to clinic with complaints of numbness in his left leg. He
reports that he does not exercise and has an office job. He is diagnosed with meralgia
paresthetica. Which of the following is consistent with this diagnosis?
a) Peroneal motor F-wave study is abnormal.
b) Sensory nerve conduction studies reveal decreased amplitude on the affected side.
c) Electromyography findings show denervation in the vastus lateralis.
d) Exam reveals decreased sensation in the medial thigh.
Answer: (b)
Commentary:
Meralgia paresthetica presents with paresthesias in the lateral thigh. Sensory nerve conduction
studies of the lateral femoral cutaneous nerve may show a drop in the sensory nerve action
potential (SNAP) amplitude on the affected side compared to the asymptomatic, contralateral
study. Symptoms are confined to below the inguinal ligament and above the knee. Peroneal motor
nerve conduction studies and F waves should be normal and needle electromyography should not
show acute or chronic axonal motor loss, because the lateral femoral cutaneous nerve is purely
sensory
2011
Myotonic discharges produce a distinctive sound on electromyography (EMG). This sound results from
(a) spontaneous firing of motor unit action potentials (MUAPs).
(b) intermittent blocking of muscle fibers during voluntary activity.
(c) firing of just a few muscle fibers per motor unit during voluntary activity.
(d) spontaneous firing of muscle fibers.
Answer: (d)
Commentary: The classic myotonic discharge of waxing and waning in frequency and amplitude is often described as either a dive-bomber or revving engine. The source generator of a myotonic potential is muscle fiber, and thus may take the form of either positive wave or a brief spike potential. Neuromyotonia and myokymia result in spontaneous firing of motor unit action potentials as opposed to muscle fiber action potentials. Intermittent blocking of some muscle fibers within a motor unit result in unstable MUAPs, which are characterized by changes in either amplitude or number of phases from potential to potential. Unstable MUAPs are seen in primary neuromuscular junction (NMJ) disorders or with new and immature NMJs, as can be seen in early reinnervation. Mypopathic MUAPs result from a decreased number of muscle fibers firing within a motor unit. These signals are seen as short-duration, small amplitude, and polyphasic potentials
2013
Which safety practice is the most appropriate when performing an electrodiagnostic study on a
patient in a hospital bed?
(a) The device should be turned on after the placement of electrodes on the patient.
(b) An insulated extension cord should be used to connect the power line.
(c) More than 1 ground electrode should be attached to the patient.
(d) All electrical devices in contact with the patient should share a common ground
Answer: (d)
Commentary: It is important to have all the electrical devices that are in contact with the patient
plugged into the same outlet to share a common ground. Similarly, only 1 ground electrode
should be used on the patient. To avoid power surges, the device should be turned on prior to the
application of any electrodes to the patient and turned off after the removal of electrodes.
Extension cords can increase leakage currents and should be avoided.
2010
Which characteristic best describes fasciculation potentials?
(a) Semirhythmic in their firing pattern
(b) Morphologically the same as motor unit potentials
(c) Produced by ephaptic conduction between single muscle fibers
(d) Randomly firing single muscle fibers
Answer: (b)
Commentary: Fasciculation potentials are spontaneously firing motor unit potentials with the
same morphologic characteristics as that of a motor unit or polyphasic action potential. They
have an irregular firing pattern usually and the site of origin is unclear. Ephaptic conduction
between single muscle fibers is thought to be the mechanism for complex repetitive discharges.
Single muscle fiber potentials are much smaller and represent units such as a fibrillation potential.
2010
A 37-year-old man is sent to you for electrodiagnostic assessment for right lumbosacral
radiculopathy. Nerve conduction studies of the right leg are normal. Needle exam shows the
following:
NEEDLE ELECTROMYOGRAPHY
Muscle -- Abnormal Spontaneous Activity -- Recruitment Adductor longus 0 Normal Vastus medialis 0 Normal Tensor fascia lata 1+ Normal Semimembranosus 1+ Normal Biceps femoris (short head) 0 Normal Tibialis anterior 2+ Reduced Medial gastrocnemius 0 Normal Lumbar paraspinals 1+
Which root is most likely injured?
(a) L2
(b) L3
(c) L4
(d) L5
Answer: D
Commentary: The common root among the affected muscles is L5. It is often difficult to narrow
the involvement to a single root level.
2010
One advantage of a concentric needle compared to a monopolar needle is its
(a) higher amplitude of motor unit action potentials(MUAPS).
(b) decreased likelihood of electrical interference.
(c) ability to vary the recording surface size.
(d) longer duration of the MUAPs.
Answer: (b)
Commentary: The shaft of a concentric needle serves as the reference electrode, whereas an
additional electrode (typically a surface electrode) is needed as a reference when using a
monopolar needle. MUAPs recorded from monopolar needles are slightly longer in duration and
have higher amplitude, since they record from the entire area around the needle tip rather than
only from the fibers facing the bevel. Because the concentric needle shaft serves as the reference
electrode, the recording surface size is fixed and interference from surrounding muscles is
minimized.
2012
In patients with steroid myopathy, the needle electromyographic study usually reveals
(a) small motor unit action potentials (MUAPs) with early recruitment.
(b) small MUAPs with reduced recruitment.
(c) positive waves and fibrillation potentials in proximal muscles.
(d) normal MUAPs and normal recruitment.
Answer: (d)
Commentary: Needle examination in patients with steroid myopathy usually reveals normal
insertional activity and no abnormal spontaneous activity. Motor unit potential morphology and
recruitment do not reveal any abnormalities. This combination occurs because in steroid
myopathy the type 2 muscle fibers are preferentially affected, in contrast to the first-recruited
type 1 fibers.
2012
- Which finding would you expect in a 35-year-old man with type 1 hereditary motor sensory
neuropathy?
(a) Absence of ankle reflexes
(b) Pes planus
(c) Motor nerve conduction velocity slowing, with evidence of temporal dispersion
(d) Absence of electromyographic spontaneous activity
Answer: (a)
Commentary: Hereditary motor sensory polyneuropathy type 1 (HMSN-1, the demyelinating
form of Charcot-Marie-Tooth disease) presents in young adult life with insidious onset of distal
weakness and sensory loss. Clinically, it typically presents with pes cavus, hammertoes and foot
drop. Ankle reflexes are absent. Temporal dispersion would indicate an acquired, not hereditary,
process. Secondary axonal loss is expected, which would result in positive waves and
fibrillations.
2012
Cooling can produce physiological changes in the body. One of these changes is an increase in
(a) nerve conduction rate.
(b) stretch receptor sensitivity.
(c) elasticity of connective tissue.
(d) general sympathetic activity.
Answer: (d)
Commentary: Nerve conduction rate is slowed by cooling. The stretch receptor sensitivity in
muscles and tendons is reduced, and the elasticity of connective tissue diminishes with cooling.
The general sympathetic activity is increased with cooling of the body, and this may affect the
responses of the stretch receptors in a beneficial way.
2012
A 33-year-old woman who had a prolonged labor 6 weeks ago, reports pain in the groin radiating
along the medial aspect of the thigh. Needle electromyographic exam shows evidence of
denervation in the gracilis and adductor muscles. Most likely she has a lesion in the
(a) femoral nerve.
(b) obturator nerve.
(c) sciatic nerve.
(d) genitofemoral nerve.
Answer: (b)
Commentary: The nerve common to the affected muscles is the obturator nerve. There is some
innervation of the adductor magnus by a branch off the sciatic nerve, but the gracilis, adductor
brevis, and adductor longus are supplied by the obturator nerve. Injuries of the obturator nerve are
uncommon, but one cause in such cases is compression of the nerve between the fetal head and
the pelvic wall during prolonged labor.
2012
What do fibrillation potentials represent?
A. Spontaneous depolarization of the muscle fiber
B. Demyelination of peripheral nerves
C central nervous system pathology
d activation of the motor unit
A is correct
Fibrillation potentials are simply spontaneous depolarizations of a single muscle fiber seen in pathologic tissue and are often elicited with needle movement while performing electromyography (EMG). Fibrillation potentials are seen in neurogenic conditions with acute or chronic axonal loss, but also in myopathic disease. Fibrillations are not characteristic for central nervous system disease states or conduction block. Conduction block is evaluated on nerve conduction studies. However, fibrillations could be seen in cases of conduction block if axonal loss has occurred as well.
2014
A 35-year-old man presents with left foot drop due to a fracture of his left tibia in a motor vehicle crash. He is stable after reduction and plating. Since the surgery, he has weakness in his left leg and numbness along the dorsum of his foot. Electrodiagnostic testing yields the following results:
MOTOR NERVE CONDUCTION STUDIES (Normal values are in parentheses)
Nerve Stimulation Site Distal Latency (ms) Amplitude (mV) NCV (m/s)
L Peroneal (EDB) Ankle 4.4 (≤ 5.5) 5.1 (≥ 3.5) Below fibular head 4.8 (≥ 3.5) 46 (≥ 40) Knee 2.1 (≥ 3.5) 34 (≥ 40)
L Tibial (AH) Ankle 5.2 (≤ 6.0) 10.4 (≥ 3.5) Popliteal fossa 9.8 (≥ 3.5) 45 (≥ 40)
R Peroneal (EDB) Ankle 4.6 (≤5.5) 4.8 (≥ 3.5) Below fibular head 4.7 (≥ 3.5) 44 (≥ 40) Knee 4.7 distal latency 4.4 (≥ 3.5) 49 (≥ 40)
Abbreviations: EDB,extensor digitorum brevis; AH, abductor hallucis; NCV, nerve conduction velocity
SENSORY NERVE CONDUCTION STUDIES (Normal NCS values are in parentheses)
Nerve Stimulation site Recording Site Distal Latency (ms) Amplitude (µV)
L Superficial peroneal Leg - 12 cm Anterior ankle 3.3 (≤ 3.5) 4 (≥ 20)
R Superficial peroneal Leg - 12 cm Anterior ankle 3.2 (≤ 3.5) 21 (≥ 20)
L Sural sensory Calf - 14 cm Ankle 3.9 (≤ 4.4) 10 (≥ 8)
NEEDLE ELECTROMYOGRAPHY
Muscle
Abnormal Spontaneous Activity
Recruitment
L Tensor fascia lata
0
Normal
L Biceps femoris, long head
0
Normal
L Biceps femoris short head
0
Normal
L Anterior tibialis
2+
Reduced
L Peroneal longus
2+
Reduced
L Tibialis posterior
0
Normal
L Gastrocnemius
0
Normal
L Vastus medialis
0
Normal
L Lumbar paraspinals
0
Normal
R Anterior tibialis
0
Normal
R Gastrocnemius
0
Normal
R Vastus medialis
0
Normal
What injury do these test results reveal?
Option d is correct.
This patient presents with a left common peroneal mononeuropathy following a surgical procedure. There is evidence of focal slowing with partial conduction block at the fibular head and an abnormal superficial peroneal sensory NCS compared to the contralateral side. These findings would imply a compressive injury at the fibular head. Furthermore, needle EMG reveals abnormal spontaneous activity in muscles distal to the fibular head innervated by the branches of the left common peroneal nerve (superficial peroneal and deep peroneal nerves). The tibial nerve conduction studies are completely normal.
2014
Positive sharp waves and fibrillation potentials are most likely to be seen in which condition?
A Anterior horn cell disease
B Steroid myopathy
C Small fiber neuropathy
D Cerebellar ischemia
Option A is correct
Positive sharp waves (PSWs) and fibrillation potentials are seen in certain disorders of the muscles, neurogenic disease, and neuromuscular disorders. These may include but are not limited to inflammatory myositis, inclusion body myositis, rhabdomyolysis, anterior horn cell disorders, radiculopathies, plexopathies, peripheral neuropathies, myasthenia gravis, and botulism. Steroid myopathy does not result in muscle necrosis and thus abnormal spontaneous activity is not observed. Fibrillation potentials and PSWs are not characteristic for central nervous system pathology. Small fiber neuropathies will be undetected on needle EMG.
2014
Which statement is most correct regarding instrumentation for electrodiagnostic studies?
a. Signals pass from the filters to the amplifiers.
b.Cathode-ray tube subtracts common electrical activity.
c.Electrical signals are transmitted from 2 surface electrodes.
d Filters are used to increase the electrical noise.
Option c is correct.
Electrical signals are transmitted from skin surface electrodes or through a needle. These electrodes are referred to as E1 and E2 or the active and the reference. Differential amplifiers eliminate signal common to the 2 electrodes. Filters are used to remove excessive electrical noise after signal amplification. Cathode-ray tube permits the visual display of the electrical signal as a waveform.
2014
A 25 year old patient presents for electrodiagnostic testing after sustaining a right shoulder injury following a motorcycle crash 4 weeks ago. He has no active elbow, wrist or finger extension. The needle examination of the radial innervated muscles shows normal insertional activity, no abnormal spontaneous activity and absent motor recruitment. In the Sunderland classification system, this nerve injury is best described as?
a. myelin injury with no Wallerian degeneration
b. Wallerian degeneration with preservation of endoneurium
c. disruption of axon with intact perineurium
d complete discontinuity of the nerve
Option a is correct.
The injury described is limited to the myelin with preservation of the axon. This type of injury is classified as neurapraxia by the Seddon classification and as type 1 by the Sunderland classification. In axonal injury, one would typically observe positive sharp waves and fibrillation potential within 3 weeks after the injury. Sunderland type 2 and 3 are both forms of axonal injury and differ in their extent of injury to the supporting neural structures. Sunderland type 2 and 3 are classified as axonotmesis by the Seddon classification. Complete discontinuity of the nerve is classified as type 5 Sunderland and as neurotmesis by Seddon.
2014
Which electrodiagnostic finding is most consistent with neurogenic thoracic outlet syndrome?
(a) Small median motor response from the thenar muscles
(b) Abnormal response of the lateral antebrachial cutaneous nerve
(c) Abnormal median sensory responses
(d) Abnormal spontaneous activity in the pronator teres muscle
Answer: B
Commentary:Neurogenic thoracic outlet syndrome involves the lower trunk of the brachial
plexus; hence, sensory and motor loss develops in the C8–T1 distribution. Thumb abduction is
often affected. Sensory changes are usually in the distribution of the ulnar and medial
antebrachial cutaneous nerves
2009
Which electrodiagnostic criterion is included in the diagnosis of peripheral nerve demyelination?
(a) Conduction velocity reduced in at least 4 nerves
(b) Compound muscle action potential conduction block in at least 3 nerves
(c) Prolonged distal motor latencies in at least 4 nerves
(d) Prolonged F-wave latency or absent F wave
Answer: D
Commentary:The criteria require conduction velocity to be reduced in 2 or more nerves,
compound muscle action potential conduction block or abnormal temporal dispersion in 1 or more nerves, prolonged distal motor latencies in 2 or more nerves, and prolonged F wave or absent F wave. Three of these four criteria must be present.
2009
A 22-year-old woman with complaints of fluctuating weakness and abnormal fatigability that
improves with rest is sent for electrophysiologic testing. Which electrophysiologic finding on
routine testing would be most consistent with this clinical presentation?
(a) Small sensory nerve action potentials
(b) Slow motor nerve conduction velocities
(c) Variability in motor unit action potential amplitude
(d) Small compound muscle action potentials
Answer: C
Commentary:This patient’s presentation is most consistent with myasthenia gravis. The
incidence of this condition is bimodal and affects women more than men in the younger age
group. When one suspects myasthenia gravis the test of choice is repetitive stimulation.
However, it is still important to assess for other possible problems and routine nerve conduction
and needle electromyographic examination should be performed. The sensory component of the peripheral nervous system lacks a neuromuscular junction and hence the sensory responses
should be normal. Motor amplitudes can be small, but this is usually only in severe cases. Motor
conduction velocities are normal, since this study assesses the conduction along the motor fibers.
Motor unit action potential amplitude variability is a characteristic abnormality observed during
routine electromyography in patients with neuromuscular junction disorders. This finding is due
to the variability in the total number of single muscle fibers being activated at any single time.
2009
Following repair of a right distal biceps tendon rupture, a 31-year-old construction worker
presents with problems extending his fingers. He had noticed swelling in the arm and forearm
before his cast was removed about 4 weeks ago. He does not have any sensory complaints and the
right superficial radial sensory nerve action potential is normal. Needle exam shows these data:
Muscle
Abnormal
Spontaneous
Activity
Recruitment
R Triceps 0 Normal
R Biceps 0 Normal
R Brachioradialis 0 Normal
R Extensor digitorum
communis
2+ Reduced
R Extensor indicis
proprius
2+ Reduced
R Flexor carpi ulnaris 0 Normal
R 1st dorsal
interosseous
0 Normal
R Extensor carpi
radialis
0 Normal
R Supraspinatus 0 Normal
R Cervical paraspinals 0 –
This patient most likely has a right
(a) radial mononeuropathy at the elbow.
(b) posterior interosseous neuropathy.
(c) posterior cord plexopathy.
(d) C7 and/or C8 radiculopathy
Answer: B
Commentary:The electrophysiologic findings are consistent with involvement of the right
posterior interosseous nerve (PIN). Typically in PIN injuries the triceps, brachioradialis, and
extensor carpi radialis longus/brevis muscles are spared.
2009
A 14-year-old girl is sent to you for electrodiagnostic evaluation of a left foot drop. The mother reports that the left foot drop has been present for about 2 months and that her daughter has lost about 40 pounds in the last 6 months. Nerve conduction studies and needle electromyography show the following data:
MOTOR NERVE CONDUCTION STUDIES Nerve Stimulation Site Distal Latency (ms) Amplitude (mV) NCV (m/s)
L Peroneal Ankle 4.2 1 Below knee 0.5 35 Above knee NR
R Peroneal Ankle 4.5 5.4 Below knee 5.0 47 Above knee 4.8 52
L Tibial Ankle 4.4 9 Knee 8.5 53
SENSORY NERVE CONDUCTION STUDIES Nerve Stimulation Site Distal Latency (ms) Amplitude (μV)
L Sural
Ankle - 14 cm
3.6
17
L Superficial peroneal
Ankle - 12 cm
NR
R Superficial peroneal
Ankle - 12 cm
3.1
15
Needle Electromyography
Muscle
Abnormal Spontaneous Activity
Recruitment
L Vastus medialis
0
Normal
L Semimembranosus
0
Normal
L Short head of biceps femoris
0
Normal
L Tibial anterior
3+
Reduced
L Medial gastrocnemius
0
None
L Peroneus longus
2+
Reduced
L Lumbar paraspinals
0
–
NCV = Nerve Conduction Velocity NR = No response
The patient’s symptoms are most likely due to left
(a) peroneal nerve injury at the fibular head.
(b) deep peroneal nerve injury.
(c) sciatic nerve injury.
(d) lumbosacral plexopathy.
(a)
The electrophysiologic findings are consistent with common peroneal nerve compression at the fibular head. The normal study of the short head of the biceps femoris points to a lesion distal to the innervation of this muscle, and hence a lesion at or below the level of the knee. Lack of involvement of muscles innervated by other nerves points away from a plexopathy or sciatic nerve injury. Excessive weight loss can often be a factor in patients with peroneal nerve compression lesions.
Ref: Dumitru D, Zwarts MJ. Focal peripheral neuropathies. In: Dumitru D, Amato AA, Zwarts
2008
