ABEM Neuromuscular Disorders & Diagnostic Criteria 2 — Questions and Answers
Question 1: Which electrodiagnostic finding is most characteristic of Lambert-Eaton Myasthenic Syndrome (LEMS)?
- Decremental response at 3 Hz repetitive stimulation
- Incremental response >100% at 50 Hz repetitive stimulation (Correct answer)
- Prolonged distal motor latency with normal CMAP amplitude
- Fibrillation potentials with absent SNAPs
Correct answer: Incremental response >100% at 50 Hz repetitive stimulation
LEMS shows a >100% incremental response (facilitation) at high-frequency (30–50 Hz) repetitive nerve stimulation due to presynaptic calcium channel antibodies increasing ACh release.
Question 2: In amyotrophic lateral sclerosis (ALS), which combination of EMG findings is most consistent with the diagnosis?
- Fibrillation potentials in one limb with normal motor unit morphology
- Fibrillation potentials, positive sharp waves, and large polyphasic MUAPs in multiple regions (Correct answer)
- Myotonic discharges with short-duration, low-amplitude MUAPs
- Reduced recruitment with normal MUAP morphology only
Correct answer: Fibrillation potentials, positive sharp waves, and large polyphasic MUAPs in multiple regions
ALS shows active denervation (fibrillations, PSWs) plus chronic reinnervation (large, polyphasic MUAPs with reduced recruitment) in multiple body regions reflecting upper and lower motor neuron involvement.
Question 3: A patient with Guillain-Barré syndrome (GBS) most commonly shows which early NCS pattern?
- Absent or prolonged F-waves with normal distal motor latencies (Correct answer)
- Uniform slowing across all nerve segments
- Reduced SNAP amplitudes with normal motor studies
- Conduction block at common entrapment sites only
Correct answer: Absent or prolonged F-waves with normal distal motor latencies
Early GBS preferentially affects nerve roots and proximal segments, causing absent or prolonged F-waves before distal latencies or conduction velocities become abnormal.
Question 4: Which feature best distinguishes myotonic dystrophy type 1 (DM1) from myotonia congenita on EMG?
- Presence of myotonic discharges
- Waxing and waning sound on loudspeaker
- Coexistence of myotonic discharges with fibrillation potentials and small MUAPs (Correct answer)
- Myotonic discharges only in distal muscles
Correct answer: Coexistence of myotonic discharges with fibrillation potentials and small MUAPs
DM1 shows myotonic discharges combined with myopathic changes (short-duration, low-amplitude MUAPs) and sometimes denervation features, reflecting its multisystem dystrophic nature rather than pure channelopathy.
Question 5: In Charcot-Marie-Tooth disease type 1A (CMT1A), motor nerve conduction velocities are typically:
- Normal or near-normal with low CMAP amplitudes
- Uniformly slowed below 38 m/s in upper extremity motor nerves (Correct answer)
- Slowed only in proximal nerve segments
- Variable with multifocal conduction blocks
Correct answer: Uniformly slowed below 38 m/s in upper extremity motor nerves
CMT1A (PMP22 duplication) causes uniform demyelination with MNCVs consistently below 38 m/s throughout all segments, distinguishing it from acquired demyelinating neuropathies with multifocal slowing.
Question 6: Which finding on repetitive nerve stimulation most reliably distinguishes myasthenia gravis (MG) from LEMS?
- A decremental response at 3 Hz distinguishes MG, while no response change occurs in LEMS
- MG shows >10% decrement at 3 Hz without significant post-exercise facilitation exceeding 100% (Correct answer)
- LEMS shows decrement at 3 Hz; MG shows increment at 50 Hz
- Both disorders show identical patterns on repetitive stimulation
Correct answer: MG shows >10% decrement at 3 Hz without significant post-exercise facilitation exceeding 100%
MG typically shows >10% decrement at low-frequency (2–3 Hz) stimulation with post-exercise facilitation of <100%, whereas LEMS shows dramatic facilitation (>100%) at high-frequency stimulation.
Question 7: Kennedy disease (spinal and bulbar muscular atrophy) is caused by a trinucleotide repeat expansion in which gene?
- SMN1
- Androgen receptor (AR) (Correct answer)
- DMPK
- FMR1
Correct answer: Androgen receptor (AR)
Kennedy disease results from CAG repeat expansion in the androgen receptor gene on the X chromosome, causing slowly progressive lower motor neuron degeneration with gynecomastia and sensory neuropathy.
ABEM Neuromuscular Disorders & Diagnostic Criteria 2
Which electrodiagnostic finding is most characteristic of Lambert-Eaton Myasthenic Syndrome (LEMS)?