EMG Clinical Applications & Pathology Flashcards

Which electrodiagnostic feature is most helpful in distinguishing axonal Charcot-Marie-Tooth (CMT2) from demyelinating CMT (CMT1)?

Answer: Markedly reduced CMAP amplitudes with relatively preserved conduction velocities in CMT2 versus uniformly slowed velocities in CMT1

CMT2 is axonal, producing reduced amplitudes with near-normal velocities, while CMT1 is demyelinating with uniformly slowed conduction velocities (often below 38 m/s in the upper limbs).

In myasthenia gravis, which electrodiagnostic test is most sensitive for confirming neuromuscular junction dysfunction?

Answer: Single-fiber EMG (SFEMG) measuring jitter

Single-fiber EMG measuring jitter is the most sensitive electrodiagnostic test for neuromuscular junction dysfunction, detecting abnormalities even when RNS is normal.

Which finding on needle EMG is most characteristic of Lambert-Eaton Myasthenic Syndrome (LEMS)?

Answer: Small baseline CMAP that markedly increases (>100%) after brief exercise or high-frequency stimulation

LEMS shows a small baseline CMAP with dramatic post-exercise or high-frequency facilitation (>100% increment) due to calcium-mediated acetylcholine vesicle release.

A patient with fatigable weakness, ptosis, and diplopia undergoes RNS testing showing a 12% CMAP decrement at 3 Hz. What is the most likely diagnosis?

Answer: Myasthenia gravis

A >10% CMAP decrement at low-frequency (3 Hz) repetitive nerve stimulation in the clinical context of fatigable ptosis and diplopia is consistent with myasthenia gravis.

Which EMG pattern is most consistent with inclusion body myositis (IBM)?

Answer: Mixed myopathic and neurogenic MUPs with fibrillations, affecting distal muscles (finger flexors, foot extensors) early

IBM characteristically shows a mixed myopathic-neurogenic pattern with both short and long MUPs, fibrillation potentials, and early distal involvement of finger flexors and foot extensors.

During EMG evaluation of suspected diabetic polyneuropathy, which nerve is typically studied first as a sensitive indicator of early disease?

Answer: Sural sensory nerve

The sural sensory nerve is among the longest and most vulnerable in length-dependent neuropathy; its reduced amplitude or absent response is often the earliest electrodiagnostic finding in diabetic polyneuropathy.