Neuromuscular Disorders & Diagnostic Criteria Flashcards

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

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.

In amyotrophic lateral sclerosis (ALS), which combination of EMG findings is most consistent with the diagnosis?

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.

A patient with Guillain-Barré syndrome (GBS) most commonly shows which early NCS pattern?

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.

Which feature best distinguishes myotonic dystrophy type 1 (DM1) from myotonia congenita on EMG?

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.

In Charcot-Marie-Tooth disease type 1A (CMT1A), motor nerve conduction velocities are typically:

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.

Which finding on repetitive nerve stimulation most reliably distinguishes myasthenia gravis (MG) from LEMS?

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%) at high-frequency stimulation.

Kennedy disease (spinal and bulbar muscular atrophy) is caused by a trinucleotide repeat expansion in which gene?

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.