EDT EDX Testing Techniques 3 — Questions and Answers
Question 1: Which finding on needle EMG is characteristic of acute denervation occurring within 1–3 weeks of injury?
- Fibrillation potentials and positive sharp waves (Correct answer)
- Large polyphasic motor unit potentials
- Reduced interference pattern only
- Fasciculation potentials with normal MUPs
Correct answer: Fibrillation potentials and positive sharp waves
Fibrillation potentials and positive sharp waves appear 1–3 weeks after axonal injury as denervated muscle fibers develop spontaneous instability of their resting membrane potential.
Question 2: The F-wave latency is used clinically to evaluate which segment of the peripheral nervous system?
- Proximal nerve and nerve root segments (Correct answer)
- Distal nerve terminals
- Neuromuscular junction
- Sensory dorsal root ganglion
Correct answer: Proximal nerve and nerve root segments
F-waves travel antidromically to the anterior horn cell and back, making their latency sensitive to conduction abnormalities in proximal nerve segments and nerve roots.
Question 3: What technical error is most likely if a CMAP has an initial positive deflection before the main negative peak?
- The G1 electrode is placed over the tendon rather than the muscle belly (Correct answer)
- The sweep speed is set too fast
- The stimulation intensity is supramaximal
- The high-frequency filter is set too low
Correct answer: The G1 electrode is placed over the tendon rather than the muscle belly
An initial positive deflection (volume-conducted artifact) occurs when G1 is not placed over the end-plate zone or is off the muscle belly, causing the wavefront to travel toward rather than away from the electrode.
Question 4: During ulnar motor nerve conduction, which muscle is used as the standard recording site?
- Abductor digiti minimi (Correct answer)
- First dorsal interosseous
- Flexor carpi ulnaris
- Hypothenar eminence broadly
Correct answer: Abductor digiti minimi
The abductor digiti minimi (ADM) is the standard recording muscle for ulnar motor nerve conduction studies due to its reliable anatomy and consistent innervation.
Question 5: A patient's median sensory SNAP is absent at the wrist but present with near-nerve recording. This most likely indicates:
- Low-amplitude signal below surface electrode threshold, not true absence (Correct answer)
- Complete axonal loss of sensory fibers
- Martin-Gruber anastomosis
- Submaximal stimulation artifact
Correct answer: Low-amplitude signal below surface electrode threshold, not true absence
Near-nerve needle recording is far more sensitive than surface electrodes; a signal present only with near-nerve recording indicates a very low-amplitude potential, not true absence of sensory axons.
Question 6: Which sweep speed is most appropriate for displaying motor unit potential (MUP) morphology during needle EMG?
- 10 ms/division (Correct answer)
- 100 ms/division
- 1 ms/division
- 500 ms/division
Correct answer: 10 ms/division
A sweep speed of 10 ms/division provides sufficient time resolution to assess MUP duration, amplitude, phases, and turns without compressing or stretching the waveform.
Question 7: Conduction block on motor nerve conduction study is defined as a CMAP amplitude reduction of greater than what percentage between proximal and distal stimulation sites?
- 50% (Correct answer)
- 20%
- 30%
- 10%
Correct answer: 50%
A proximal-to-distal CMAP amplitude drop exceeding 50% (with <15% area reduction from temporal dispersion) is the widely accepted criterion for conduction block indicating focal demyelination.
EDT EDX Testing Techniques 3
Which finding on needle EMG is characteristic of acute denervation occurring within 1–3 weeks of injury?