ABEM Equipment Operation & Data Interpretation 3 — Questions and Answers
Question 1: A conduction block is defined in nerve conduction studies as a reduction in compound muscle action potential (CMAP) amplitude or area between proximal and distal stimulation sites of at least:
- 10%
- 20%
- 50% (Correct answer)
- 75%
Correct answer: 50%
A CMAP amplitude or area reduction of 50% or more between proximal and distal stimulation sites (with stable distal CMAP) is the accepted criterion for conduction block.
Question 2: In repetitive nerve stimulation (RNS) studies for myasthenia gravis, a decremental response is considered significant when the amplitude decrement exceeds:
- 5%
- 10% (Correct answer)
- 20%
- 30%
Correct answer: 10%
A decrement greater than 10% between the first and fourth or fifth response at 2–3 Hz stimulation is considered diagnostic of a postsynaptic NMJ disorder.
Question 3: Which filter setting change would INCREASE the duration of a recorded CMAP waveform?
- Raising the high-frequency filter from 10 kHz to 20 kHz
- Lowering the high-frequency filter from 10 kHz to 2 kHz (Correct answer)
- Raising the low-frequency filter from 2 Hz to 20 Hz
- Lowering the low-frequency filter from 2 Hz to 0.5 Hz
Correct answer: Lowering the high-frequency filter from 10 kHz to 2 kHz
Lowering the high-frequency filter removes fast-rise components and rounds the waveform, artificially prolonging its apparent duration.
Question 4: The H-reflex latency is used clinically to assess which nerve root level in the lower extremity?
- L3–L4
- L4–L5
- S1 (Correct answer)
- L2–L3
Correct answer: S1
The H-reflex recorded from the soleus via tibial nerve stimulation primarily evaluates the S1 afferent and efferent pathway.
Question 5: In needle EMG, fibrillation potentials are best characterized as:
- Polyphasic high-amplitude potentials seen during maximal contraction
- Spontaneous, regular, biphasic or triphasic potentials with amplitude 20–300 µV indicating denervated muscle fibers (Correct answer)
- Low-frequency repetitive discharges with a 'dive-bomber' sound
- Grouped discharges of normal MUAPs at rest
Correct answer: Spontaneous, regular, biphasic or triphasic potentials with amplitude 20–300 µV indicating denervated muscle fibers
Fibrillation potentials are spontaneous discharges of individual denervated muscle fibers, characteristically small (20–300 µV), regular, and biphasic or triphasic.
Question 6: What does 'temporal dispersion' of a CMAP refer to?
- Decreased amplitude due to axonal loss
- Broadening and phase cancellation of the CMAP with proximal stimulation due to differential slowing of axons (Correct answer)
- Absence of the CMAP at high stimulus intensities
- Increase in CMAP latency without change in morphology
Correct answer: Broadening and phase cancellation of the CMAP with proximal stimulation due to differential slowing of axons
Temporal dispersion occurs when axons within a nerve conduct at different velocities, causing arrival time spread and phase cancellation that broadens and reduces the proximal CMAP.
Question 7: When measuring sensory nerve action potential (SNAP) amplitude, the standard measurement is taken:
- From the peak of the positive deflection to baseline
- From baseline to the negative peak
- Peak-to-peak (positive to negative peak) (Correct answer)
- From the onset of the waveform to the end
Correct answer: Peak-to-peak (positive to negative peak)
SNAP amplitude is conventionally measured peak-to-peak (from the initial positive peak to the subsequent negative peak) to capture the full waveform height.
ABEM Equipment Operation & Data Interpretation 3
A conduction block is defined in nerve conduction studies as a reduction in compound muscle action potential (CMAP) amplitude or area between proximal and distal stimulation sites of at least: