ABEM Nerve Conduction Studies & Electromyography 3 — Questions and Answers
Question 1: In a patient with peroneal neuropathy at the fibular head, which NCS finding would confirm focal demyelination with conduction block?
- Reduced CMAP amplitude distally with normal amplitude proximally
- CMAP amplitude drop of >50% stimulating above versus below the fibular head (Correct answer)
- Prolonged distal latency with normal proximal conduction velocity
- Absent sural sensory response
Correct 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.
Question 2: Which feature of motor unit action potentials (MUAPs) is most indicative of a chronic neurogenic process?
- Decreased duration and amplitude
- Increased duration, amplitude, and polyphasia (Correct answer)
- Fibrillation potentials and PSWs
- Reduced interference pattern
Correct answer: Increased duration, amplitude, and polyphasia
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?
- Distal terminal branches
- Neuromuscular junction
- Proximal nerve segments and nerve roots (Correct answer)
- Sensory afferents
Correct 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.
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?
- Lambert-Eaton myasthenic syndrome
- Myasthenia gravis (Correct answer)
- Botulism
- Congenital myasthenic syndrome
Correct 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.
Question 5: In radial neuropathy at the spiral groove, which muscle is typically SPARED on EMG?
- Extensor digitorum communis
- Extensor carpi radialis longus
- Brachioradialis
- Triceps (Correct answer)
Correct answer: Triceps
The branch to the triceps arises proximal to the spiral groove, so triceps is usually spared in radial neuropathy at that level.
Question 6: Sensory nerve action potential (SNAP) amplitude reduction with normal conduction velocity is most consistent with:
- Demyelinating sensory neuropathy
- Axonal sensory neuropathy (Correct answer)
- Focal conduction block in a sensory nerve
- Wallerian degeneration not yet completed
Correct answer: Axonal sensory neuropathy
Axonal loss reduces the number of conducting fibers, lowering SNAP amplitude while velocity remains near normal until severe loss occurs.
Question 7: Which EMG finding is most characteristic of amyotrophic lateral sclerosis (ALS)?
- Pure sensory NCS abnormalities
- Myopathic MUAPs with early recruitment
- Widespread active denervation with neurogenic MUAPs in multiple regions (Correct answer)
- Decremental response on repetitive stimulation
Correct 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.
ABEM Nerve Conduction Studies & Electromyography 3
In a patient with peroneal neuropathy at the fibular head, which NCS finding would confirm focal demyelination with conduction block?