ABEM Nerve Conduction Studies & Electromyography 5

Question 1

Which nerve conduction study parameter is most useful for detecting early axonal polyneuropathy?

Accepted Answer

SNAP amplitude

Answer

SNAP amplitude is the earliest and most sensitive indicator of axonal polyneuropathy because sensory fibers are affected early and amplitude reflects fiber density.

Question 2

The needle EMG finding of reduced interference pattern at maximal effort is best interpreted as evidence of:

Accepted Answer

Poor patient effort or upper motor neuron disease or lower motor neuron loss

Answer

Reduced recruitment/interference pattern reflects fewer active motor units, seen in lower motor neuron disorders, poor effort, or upper motor neuron lesions limiting voluntary drive.

Question 3

In a patient with known diabetes presenting with foot drop, EMG shows denervation in the peroneal-innervated muscles and short head of the biceps femoris. What does biceps femoris short head involvement indicate?

Accepted Answer

Sciatic neuropathy proximal to the short head branch

Answer

The short head of biceps femoris is innervated by the peroneal division of the sciatic nerve proximal to the fibular head, so its involvement localizes the lesion above the fibular head.

Question 4

Which of the following correctly describes the difference between demyelination and axonal loss on NCS?

Accepted Answer

Demyelination slows conduction velocity and prolongs latency; axonal loss reduces amplitude

Answer

Demyelination impairs saltatory conduction, slowing velocity and prolonging latencies, while axonal loss reduces the number of conducting fibers, lowering CMAP and SNAP amplitudes.

Question 5

A Martin-Gruber anastomosis (MGA) is suspected on NCS. What is the characteristic electrodiagnostic finding?

Accepted Answer

Higher median CMAP amplitude with elbow stimulation than wrist stimulation

Answer

In MGA, median-innervated fibers cross to the ulnar nerve in the forearm; elbow stimulation of the median nerve activates more motor fibers (including those crossing), increasing CMAP amplitude compared to wrist stimulation.

(ABEM) American Board of Electrodiagnostic Medicine Practice Test

(ABEM) American Board of Electrodiagnostic Medicine Practice Test

ABEM Nerve Conduction Studies & Electromyography 5