ABEM Equipment Operation & Data Interpretation 4 — Questions and Answers
Question 1: A patient's median motor NCS shows normal distal latency and CMAP amplitude but the conduction velocity is 35 m/s (normal >49 m/s). This pattern most suggests:
- Focal carpal tunnel syndrome
- Diffuse demyelinating polyneuropathy (Correct answer)
- Axonal polyneuropathy
- Normal variant
Correct answer: Diffuse demyelinating polyneuropathy
Diffusely slowed conduction velocity below 75% of the lower limit of normal, with preserved amplitude, suggests a demyelinating polyneuropathy affecting the entire nerve.
Question 2: Complex repetitive discharges (CRDs) in needle EMG are characterized by:
- Regular, uniform discharges that start and stop abruptly with a 'machine-like' sound (Correct answer)
- Waxing and waning amplitude and frequency with a 'dive-bomber' sound
- Triphasic spikes firing at 1–2 Hz
- Grouped bursts of MUAPs during voluntary contraction
Correct answer: Regular, uniform discharges that start and stop abruptly with a 'machine-like' sound
CRDs are high-frequency (5–100 Hz), uniform, repetitive discharges of constant morphology that start and stop abruptly, producing a machine-like sound.
Question 3: During single-fiber EMG (SFEMG), 'jitter' is defined as:
- The amplitude variability of a single fiber action potential
- The variability in interpotential interval between two muscle fiber action potentials from the same motor unit (Correct answer)
- The number of fiber pairs recorded per needle position
- The mean firing rate of a single fiber
Correct answer: The variability in interpotential interval between two muscle fiber action potentials from the same motor unit
Jitter is the variability (MCD – mean consecutive difference) in the time interval between action potentials of two muscle fibers belonging to the same motor unit.
Question 4: In somatosensory evoked potentials (SSEPs), the N20 component recorded over the scalp following median nerve stimulation represents:
- Peripheral nerve conduction in the brachial plexus
- Thalamo-cortical radiation or primary somatosensory cortex activation (Correct answer)
- Spinal cord dorsal horn activity
- Brainstem nuclei activation
Correct answer: Thalamo-cortical radiation or primary somatosensory cortex activation
The N20 is the first cortical response generated in or near the primary somatosensory cortex (area 3b) following median nerve stimulation.
Question 5: When performing near-nerve sensory recording (intraneural technique) compared to surface electrode recording, the primary advantage is:
- Lower impedance and reduced setup time
- Higher amplitude SNAPs enabling detection of small or absent responses (Correct answer)
- Elimination of stimulus artifact
- Better patient comfort
Correct answer: Higher amplitude SNAPs enabling detection of small or absent responses
Near-nerve needle recording dramatically increases SNAP amplitude, allowing detection of responses too small to be recorded with surface electrodes.
Question 6: A myopathic motor unit action potential (MUAP) pattern is characterized by:
- Increased duration, increased amplitude, reduced recruitment
- Reduced duration, reduced amplitude, early recruitment with full interference pattern at low force (Correct answer)
- Normal duration but increased polyphasia only
- Large-amplitude, long-duration polyphasic MUAPs with reduced recruitment
Correct answer: Reduced duration, reduced amplitude, early recruitment with full interference pattern at low force
Myopathic MUAPs are small (low amplitude, short duration) because fewer muscle fibers per motor unit are functional, and early recruitment occurs as more units are needed to generate force.
Question 7: The sweep speed setting of 10 ms/division on the EMG display is most appropriate for recording:
- Individual MUAP morphology during needle EMG (Correct answer)
- Somatosensory evoked potentials with cortical components
- F-wave studies
- Blink reflex studies
Correct answer: Individual MUAP morphology during needle EMG
A sweep speed of 10 ms/division provides adequate time resolution to analyze individual MUAP duration and morphology during needle EMG.
ABEM Equipment Operation & Data Interpretation 4
A patient's median motor NCS shows normal distal latency and CMAP amplitude but the conduction velocity is 35 m/s (normal >49 m/s).
This pattern most suggests: