Nerve Conduction Studies & Electromyography Flashcards

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

Answer: SNAP amplitude

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

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

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

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.

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?

Answer: Sciatic neuropathy proximal to the short head branch

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.

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

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

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

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

Answer: Higher median CMAP amplitude with elbow stimulation than wrist stimulation

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.

Which electrodiagnostic finding is most characteristic of inclusion body myositis (IBM)?

Answer: Mixed myopathic and neurogenic MUAP features with fibrillations

IBM uniquely shows mixed myopathic and neurogenic EMG features, with both small short and large long-duration MUAPs alongside active denervation potentials.

When performing sural nerve conduction studies, which recording technique is preferred to maximize SNAP amplitude?

Answer: Near-nerve needle recording

Near-nerve needle recording places the electrode directly adjacent to the sural nerve, maximizing signal amplitude and improving detection of small or absent responses.