ABEM Nerve Conduction Studies & Electromyography 3

Question 1

In a patient with peroneal neuropathy at the fibular head, which NCS finding would confirm focal demyelination with conduction block?

Accepted Answer

CMAP amplitude drop of >50% stimulating above versus below the fibular head

Answer

Conduction block is defined by a >50% drop in CMAP amplitude or area comparing proximal to distal stimulation across the lesion site.

Question 2

Which feature of motor unit action potentials (MUAPs) is most indicative of a chronic neurogenic process?

Accepted Answer

Increased duration, amplitude, and polyphasia

Answer

Chronic reinnervation leads to enlarged MUAPs with increased duration and amplitude due to collateral sprouting and fiber grouping.

Question 3

The F-wave provides information primarily about which portion of the motor pathway?

Accepted Answer

Proximal nerve segments and nerve roots

Answer

F-waves reflect antidromic activation of anterior horn cells and return orthodromically, allowing assessment of proximal nerve segments.

Question 4

Which condition is most likely if repetitive nerve stimulation at 3 Hz shows a >10% decrement in CMAP amplitude at the 4th or 5th response?

Accepted Answer

Myasthenia gravis

Answer

A decremental response of >10% at low-frequency (2–3 Hz) stimulation is the hallmark of myasthenia gravis due to postsynaptic acetylcholine receptor deficiency.

Question 5

In radial neuropathy at the spiral groove, which muscle is typically SPARED on EMG?

Accepted Answer

Triceps

Answer

The branch to the triceps arises proximal to the spiral groove, so triceps is usually spared in radial neuropathy at that level.

(ABEM) American Board of Electrodiagnostic Medicine Practice Test

(ABEM) American Board of Electrodiagnostic Medicine Practice Test

ABEM Nerve Conduction Studies & Electromyography 3