ABEM Neuromuscular Disorders & Diagnostic Criteria 2

Question 1

Which electrodiagnostic finding is most characteristic of Lambert-Eaton Myasthenic Syndrome (LEMS)?

Accepted Answer

Incremental response >100% at 50 Hz repetitive stimulation

Answer

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?

Accepted Answer

Fibrillation potentials, positive sharp waves, and large polyphasic MUAPs in multiple regions

Answer

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?

Accepted Answer

Absent or prolonged F-waves with normal distal motor latencies

Answer

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?

Accepted Answer

Coexistence of myotonic discharges with fibrillation potentials and small MUAPs

Answer

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:

Accepted Answer

Uniformly slowed below 38 m/s in upper extremity motor nerves

Answer

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.

(ABEM) American Board of Electrodiagnostic Medicine Practice Test

(ABEM) American Board of Electrodiagnostic Medicine Practice Test

ABEM Neuromuscular Disorders & Diagnostic Criteria 2