Nerve Conduction Studies & Electromyography Flashcards
7 cards from real ABEM practice questions. Tap to flip, then mark Knew It or Still Learning — missed cards come back until you master them.
Read the first 7 Nerve Conduction Studies & Electromyography flashcards as text
In a patient with peroneal neuropathy at the fibular head, which NCS finding would confirm focal demyelination with conduction block?
Answer: CMAP amplitude drop of >50% stimulating above versus below the fibular head
Conduction block is defined by a >50% drop in CMAP amplitude or area comparing proximal to distal stimulation across the lesion site.
Which feature of motor unit action potentials (MUAPs) is most indicative of a chronic neurogenic process?
Answer: Increased duration, amplitude, and polyphasia
Chronic reinnervation leads to enlarged MUAPs with increased duration and amplitude due to collateral sprouting and fiber grouping.
The F-wave provides information primarily about which portion of the motor pathway?
Answer: Proximal nerve segments and nerve roots
F-waves reflect antidromic activation of anterior horn cells and return orthodromically, allowing assessment of proximal nerve segments.
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?
Answer: Myasthenia gravis
A decremental response of >10% at low-frequency (2–3 Hz) stimulation is the hallmark of myasthenia gravis due to postsynaptic acetylcholine receptor deficiency.
In radial neuropathy at the spiral groove, which muscle is typically SPARED on EMG?
Answer: Triceps
The branch to the triceps arises proximal to the spiral groove, so triceps is usually spared in radial neuropathy at that level.
Sensory nerve action potential (SNAP) amplitude reduction with normal conduction velocity is most consistent with:
Answer: Axonal sensory neuropathy
Axonal loss reduces the number of conducting fibers, lowering SNAP amplitude while velocity remains near normal until severe loss occurs.
Which EMG finding is most characteristic of amyotrophic lateral sclerosis (ALS)?
Answer: Widespread active denervation with neurogenic MUAPs in multiple regions
ALS shows widespread active denervation (fibrillations/PSWs) and large neurogenic MUAPs across multiple limb and bulbar regions due to upper and lower motor neuron loss.