ABEM Equipment Operation & Data Interpretation 5 — Questions and Answers
Question 1: Which of the following is the correct cathode placement rule for nerve conduction studies?
- Cathode proximal to the anode to avoid hyperpolarization under the anode blocking the impulse (Correct answer)
- Cathode distal to the anode so the stimulus travels orthodromically
- Cathode and anode positions do not affect the measured latency
- Cathode over the muscle belly and anode over the nerve trunk
Correct answer: Cathode proximal to the anode to avoid hyperpolarization under the anode blocking the impulse
The cathode (negative electrode) must be placed closer to the recording site so the action potential is initiated beneath the cathode and is not blocked by hyperpolarization under the proximal anode.
Question 2: In blink reflex testing, the R1 response is generated by which pathway?
- Trigeminal afferent → brainstem interneurons → bilateral facial nerve efferents
- Trigeminal afferent → oligosynaptic pontine circuit → ipsilateral facial nerve efferent (Correct answer)
- Corticospinal tract → facial nucleus → bilateral orbicularis oculi
- Ophthalmic afferent → spinal trigeminal nucleus → contralateral facial nerve
Correct answer: Trigeminal afferent → oligosynaptic pontine circuit → ipsilateral facial nerve efferent
R1 is an ipsilateral, early (9–13 ms) response mediated by a short oligosynaptic reflex arc through the pons between the trigeminal and facial nuclei.
Question 3: Prolonged insertional activity in needle EMG is most commonly associated with:
- Normal muscle in a cold limb
- Denervated or irritable muscle (Correct answer)
- Upper motor neuron lesion
- Neuromuscular junction blockade
Correct answer: Denervated or irritable muscle
Prolonged insertional activity reflects an irritable muscle membrane, most commonly seen in denervation, inflammatory myopathies, or ion channel disorders.
Question 4: Which nerve conduction parameter is MOST sensitive for detecting early axonal loss neuropathy?
- Conduction velocity
- Distal latency
- CMAP or SNAP amplitude (Correct answer)
- F-wave minimum latency
Correct answer: CMAP or SNAP amplitude
CMAP and SNAP amplitudes reflect the number of conducting axons and decline early in axonal loss, while conduction velocity and latency remain relatively normal until significant axon loss occurs.
Question 5: Myotonic discharges on needle EMG differ from complex repetitive discharges (CRDs) in that myotonic discharges:
- Fire at a constant frequency without variation
- Wax and wane in amplitude and frequency, producing a 'dive-bomber' sound (Correct answer)
- Are always polyphasic with more than five phases
- Occur only during voluntary contraction
Correct answer: Wax and wane in amplitude and frequency, producing a 'dive-bomber' sound
Myotonic discharges characteristically wax and wane in both amplitude and frequency, creating the classic 'dive-bomber' or revving motorcycle sound on the audio monitor.
Question 6: When the low-frequency (high-pass) filter is set too high (e.g., 500 Hz instead of 2 Hz) during sensory NCS, the SNAP will appear:
- Larger in amplitude with longer duration
- Smaller in amplitude and shorter in duration with possible loss of slow components (Correct answer)
- Identical because SNAPs are high-frequency signals
- Shifted to a longer latency
Correct answer: Smaller in amplitude and shorter in duration with possible loss of slow components
A high low-frequency filter removes slow, low-frequency components of the SNAP, reducing amplitude and duration and potentially making a borderline response undetectable.
Question 7: In late response studies, the A-wave (axon reflex) differs from the F-wave because the A-wave:
- Occurs only at supramaximal stimulation
- Has a constant latency and morphology with submaximal stimulation and represents ephaptic or branching axon activation (Correct answer)
- Is larger than the CMAP in amplitude
- Requires antidromic sensory conduction to generate
Correct answer: Has a constant latency and morphology with submaximal stimulation and represents ephaptic or branching axon activation
A-waves have a fixed latency and waveform that appear at submaximal stimulation, reflecting activation of a branch or ephaptic connection rather than the random retrograde motor neuron firing seen in F-waves.
ABEM Equipment Operation & Data Interpretation 5
Which of the following is the correct cathode placement rule for nerve conduction studies?