ABEM Neuromuscular Disorders & Diagnostic Criteria 5

Question 1

A patient presents with pure sensory ataxia, absent deep tendon reflexes, and normal motor strength. NCS shows absent SNAPs with normal CMAPs. This pattern is most consistent with:

Accepted Answer

Sensory neuronopathy (dorsal root ganglionopathy)

Answer

Sensory neuronopathy affects the dorsal root ganglion, producing diffuse, non-length-dependent loss of all SNAPs with preserved CMAPs, causing sensory ataxia without motor involvement.

Question 2

Which of the following best characterizes the electrodiagnostic findings in botulism?

Accepted Answer

Low CMAP amplitudes with facilitation at high-frequency stimulation (50 Hz) similar to LEMS

Answer

Botulism, like LEMS, is a presynaptic NMJ disorder causing low baseline CMAPs with incremental response at high-frequency stimulation, though the facilitation is typically less dramatic than in LEMS.

Question 3

In a patient with suspected polymyositis, which finding on needle EMG would suggest an alternative diagnosis of IBM?

Accepted Answer

Mixed myopathic and neuropathic MUAP changes with finger flexor weakness

Answer

IBM classically involves early and severe finger flexor and quadriceps weakness with a mixed EMG pattern of myopathic and neuropathic features, distinguishing it from polymyositis which shows pure myopathic changes.

Question 4

On NCS, which finding most reliably indicates axonal degeneration rather than demyelination as the primary pathology?

Accepted Answer

Reduced CMAP and SNAP amplitudes with relatively preserved conduction velocity

Answer

Axonal loss reduces the number of conducting fibers, decreasing CMAP and SNAP amplitudes while conduction velocity (reflecting the fastest surviving fibers) is relatively preserved unless axonal loss is very severe.

Question 5

In thoracic outlet syndrome (TOS) with true neurogenic form, which electrodiagnostic pattern is most expected?

Accepted Answer

Low or absent medial antebrachial cutaneous SNAP and low ulnar CMAP amplitude with relatively preserved median motor studies

Answer

True neurogenic TOS predominantly affects the lower trunk (C8-T1), causing low medial antebrachial cutaneous SNAP amplitude and reduced ulnar CMAP, with the median motor studies relatively spared early.

(ABEM) American Board of Electrodiagnostic Medicine Practice Test

(ABEM) American Board of Electrodiagnostic Medicine Practice Test

ABEM Neuromuscular Disorders & Diagnostic Criteria 5