ABEM Nerve Conduction Studies & Electromyography 5 — Questions and Answers
Question 1: Which nerve conduction study parameter is most useful for detecting early axonal polyneuropathy?
- Distal motor latency
- Conduction velocity
- SNAP amplitude (Correct answer)
- F-wave latency
Correct 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.
Question 2: The needle EMG finding of reduced interference pattern at maximal effort is best interpreted as evidence of:
- Myopathy
- Poor patient effort or upper motor neuron disease or lower motor neuron loss (Correct answer)
- Normal variation in an elderly patient
- Neuromuscular junction disorder
Correct 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.
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?
- L4-L5 radiculopathy
- Sciatic neuropathy proximal to the short head branch (Correct answer)
- Peroneal neuropathy at the fibular head
- Lumbosacral plexopathy
Correct 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.
Question 4: Which of the following correctly describes the difference between demyelination and axonal loss on NCS?
- Demyelination reduces amplitude; axonal loss prolongs latency
- Demyelination slows conduction velocity and prolongs latency; axonal loss reduces amplitude (Correct answer)
- Both patterns produce identical NCS findings
- Axonal loss prolongs F-wave latency; demyelination does not affect F-waves
Correct 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.
Question 5: A Martin-Gruber anastomosis (MGA) is suspected on NCS. What is the characteristic electrodiagnostic finding?
- Higher median CMAP amplitude with wrist stimulation than elbow stimulation
- Higher ulnar CMAP amplitude with elbow stimulation than wrist stimulation
- Higher median CMAP amplitude with elbow stimulation than wrist stimulation (Correct answer)
- Absent median F-wave with preserved ulnar F-wave
Correct 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.
Question 6: Which electrodiagnostic finding is most characteristic of inclusion body myositis (IBM)?
- Pure myopathic picture with early recruitment and small MUAPs
- Mixed myopathic and neurogenic MUAP features with fibrillations (Correct answer)
- Decremental response on repetitive nerve stimulation
- Pure sensory neuropathy pattern
Correct 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.
Question 7: When performing sural nerve conduction studies, which recording technique is preferred to maximize SNAP amplitude?
- Near-nerve needle recording (Correct answer)
- Surface electrode recording with averaging
- Standard surface electrode recording without averaging
- Concentric needle electrode recording
Correct 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.
ABEM Nerve Conduction Studies & Electromyography 5
Which nerve conduction study parameter is most useful for detecting early axonal polyneuropathy?