EMG Nerve Conduction Studies 2 — Questions and Answers
Question 1: Which pattern of NCS findings is most characteristic of a demyelinating polyneuropathy?
- Reduced amplitudes with normal conduction velocities
- Prolonged latencies, slowed conduction velocities, and possible conduction block (Correct answer)
- Absent F-waves with normal distal motor studies
- Incremental CMAP response to repetitive nerve stimulation
Correct answer: Prolonged latencies, slowed conduction velocities, and possible conduction block
Demyelinating neuropathies primarily affect the myelin sheath, causing markedly prolonged latencies, reduced conduction velocities (often below 70–75% of the lower limit of normal), and may show conduction block or temporal dispersion.
Question 2: At low-frequency repetitive nerve stimulation (2–3 Hz), a greater than 10% decremental response in CMAP amplitude is characteristic of which condition?
- Lambert-Eaton Myasthenic Syndrome
- Myasthenia Gravis (Correct answer)
- Botulism (early stage)
- Myotonic dystrophy
Correct answer: Myasthenia Gravis
Myasthenia Gravis shows a decremental CMAP response at low-frequency RNS due to progressive depletion of acetylcholine at the postsynaptic neuromuscular junction with repetitive stimulation.
Question 3: High-frequency repetitive nerve stimulation (20–50 Hz) produces a significant incremental (facilitated) CMAP response exceeding 100% in which condition?
- Myasthenia Gravis
- Lambert-Eaton Myasthenic Syndrome (Correct answer)
- Congenital myasthenic syndrome
- Botulism (chronic stage)
Correct answer: Lambert-Eaton Myasthenic Syndrome
In Lambert-Eaton Myasthenic Syndrome, high-frequency RNS causes presynaptic calcium accumulation that triggers release of additional acetylcholine, producing the characteristic incremental CMAP response.
Question 4: When performing a median-to-ulnar sensory comparison study across the palm, a latency difference greater than how many milliseconds between the two nerves suggests carpal tunnel syndrome?
- 0.2 ms
- 0.4 ms (Correct answer)
- 1.0 ms
- 2.0 ms
Correct answer: 0.4 ms
A median-to-ulnar sensory latency difference greater than 0.4 ms across the same palm distance is considered abnormal and significantly increases sensitivity for detecting mild carpal tunnel syndrome.
Question 5: In axonal polyneuropathy, what is the expected NCS pattern?
- Slowed NCV with preserved CMAP and SNAP amplitudes
- Normal or near-normal NCV with reduced CMAP and SNAP amplitudes (Correct answer)
- Conduction block at typical entrapment sites
- Prolonged distal latencies with normal conduction velocities
Correct answer: Normal or near-normal NCV with reduced CMAP and SNAP amplitudes
Axonal neuropathies primarily destroy nerve axons, reducing CMAP and SNAP amplitudes while conduction velocity in surviving fibers remains relatively preserved.
Question 6: The reference (inactive) recording electrode in motor nerve conduction studies is optimally placed:
- Proximal to the active electrode along the nerve trunk
- Over an electrically inactive area such as a tendon or bony prominence (Correct answer)
- At the stimulation site cathode
- At a point equidistant between the two stimulation sites
Correct answer: Over an electrically inactive area such as a tendon or bony prominence
Placing the reference electrode over an electrically inactive site (tendon or bone) minimizes spurious electrical signals and ensures the CMAP recording reflects true muscle activity at the active electrode.
Question 7: The medial antebrachial cutaneous nerve, commonly studied in brachial plexus evaluations, arises from which structure?
- Posterior cord of the brachial plexus
- Lateral cord of the brachial plexus
- Medial cord of the brachial plexus (Correct answer)
- C5-C6 nerve roots directly
Correct answer: Medial cord of the brachial plexus
The medial antebrachial cutaneous nerve originates from the medial cord of the brachial plexus (C8-T1) and its SNAP is evaluated to help localize lesions to the medial cord.
EMG Nerve Conduction Studies 2
Which pattern of NCS findings is most characteristic of a demyelinating polyneuropathy?