NCS Neuromuscular Disorders 2 β Questions and Answers
Question 1: In amyotrophic lateral sclerosis (ALS), EMG characteristically demonstrates which combination of findings?
- Fibrillations, fasciculations, and large polyphasic motor unit action potentials (Correct answer)
- Myopathic potentials with early recruitment and small MUAPs
- Absent sensory nerve action potentials with normal motor studies
- Conduction block at multiple sites with normal motor unit morphology
Correct answer: Fibrillations, fasciculations, and large polyphasic motor unit action potentials
ALS shows both acute denervation (fibrillations, positive sharp waves, fasciculations) and chronic reinnervation (large polyphasic MUAPs) due to ongoing motor neuron loss with surviving motor units compensating.
Question 2: Nerve conduction studies in acute inflammatory demyelinating polyneuropathy (AIDP/GBS) most characteristically show:
- Asymmetric axonal sensory loss with normal motor conduction velocities
- Normal NCS with abnormal EMG fibrillations only
- Prolonged distal latencies, slow conduction velocities, conduction block, and prolonged F-waves (Correct answer)
- Reduced CMAP amplitudes with normal distal latencies and conduction velocities
Correct answer: Prolonged distal latencies, slow conduction velocities, conduction block, and prolonged F-waves
AIDP is a demyelinating process; NCS shows hallmark demyelinating features including prolonged distal latencies, reduced conduction velocities, conduction block, temporal dispersion, and prolonged or absent F-waves.
Question 3: Low-frequency (3 Hz) repetitive nerve stimulation in myasthenia gravis typically shows:
- Decremental response exceeding 10% between the 1st and 4th/5th potentials (Correct answer)
- Incremental response exceeding 100% with successive stimuli
- No change in amplitude across the stimulus train
- Progressive facilitation followed by a late decrement after 50 Hz stimulation
Correct answer: Decremental response exceeding 10% between the 1st and 4th/5th potentials
MG produces a decremental response (>10% reduction by the 4thβ5th stimulus) at slow (2β5 Hz) repetitive stimulation due to progressive depletion of releasable acetylcholine quanta at the postsynaptic end-plate.
Question 4: CMT1A, the most common hereditary motor and sensory neuropathy, is caused by:
- Point mutation in the MPZ (myelin protein zero) gene on chromosome 1
- Mutation in GJB1 (connexin 32) gene on the X chromosome
- Deletion of SMN1 gene on chromosome 5q
- Duplication of the PMP22 gene on chromosome 17p11.2 (Correct answer)
Correct answer: Duplication of the PMP22 gene on chromosome 17p11.2
CMT1A results from a 1.5 Mb tandem duplication of chromosome 17p11.2 containing the PMP22 gene, causing overexpression of PMP22 and demyelinating neuropathy with markedly slow conduction velocities (<38 m/s).
Question 5: Lambert-Eaton Myasthenic Syndrome (LEMS) is electrophysiologically distinguished from myasthenia gravis by:
- Decremental response at low-frequency repetitive stimulation identical to MG
- Facilitation and >100% CMAP amplitude increment with high-frequency (50 Hz) stimulation or post-exercise (Correct answer)
- Absent F-waves on motor nerve conduction studies
- Normal repetitive stimulation with abnormal single-fiber EMG only
Correct answer: Facilitation and >100% CMAP amplitude increment with high-frequency (50 Hz) stimulation or post-exercise
LEMS involves P/Q-type voltage-gated calcium channel antibodies impairing presynaptic ACh release; high-frequency stimulation or brief exercise causes calcium accumulation and dramatic CMAP facilitation (>100% increment).
Question 6: In diabetic distal symmetric polyneuropathy, which fiber type is characteristically affected first?
- Large myelinated motor fibers causing early foot drop
- Large myelinated AΞ² sensory fibers causing early proprioception loss
- Small unmyelinated C fibers and small myelinated AΞ΄ fibers causing pain and temperature loss (Correct answer)
- Autonomic large myelinated fibers causing early orthostatic hypotension
Correct answer: Small unmyelinated C fibers and small myelinated AΞ΄ fibers causing pain and temperature loss
Diabetic polyneuropathy classically begins with small fiber involvement β burning pain, allodynia, and impaired temperature sensation β before large fiber deficits (proprioception, vibration, strength) develop.
Question 7: CIDP (Chronic Inflammatory Demyelinating Polyneuropathy) is distinguished from GBS most definitively by:
- Symptom progression beyond 8 weeks or a relapsing-remitting course (Correct answer)
- CSF albuminocytologic dissociation, which is absent in GBS
- Presence of multifocal conduction block on nerve conduction studies
- Positive anti-ganglioside antibodies such as anti-GQ1b
Correct answer: Symptom progression beyond 8 weeks or a relapsing-remitting course
By definition, CIDP progresses for >8 weeks (or relapses), whereas GBS peaks within 4 weeks then plateaus or improves; both can share CSF, NCS, and antibody findings.
NCS Neuromuscular Disorders 2
In amyotrophic lateral sclerosis (ALS), EMG characteristically demonstrates which combination of findings?