Nerve Conduction Studies: Technique, Interpretation, and Pitfalls
Get the basics right
Nerve conduction studies answer two questions: is the nerve normally myelinated, and is the axon intact? Motor studies stimulate supramaximally and record the compound muscle action potential (CMAP) with the active electrode over the motor point and the reference distally. Sensory studies record the sensory nerve action potential (SNAP) with the active electrode over the nerve and the reference 3-4 cm distally. Measure distance carefully, keep limb temperature above 32 C, and use supramaximal stimulation by increasing current until no further amplitude gain occurs. Set filters appropriately: motor 2 Hz-10 kHz; sensory 20 Hz-2 kHz or wider.
Key measures: distal latency (onset), amplitude (baseline-to-peak for CMAP; peak-to-peak for SNAP), conduction velocity between stimulation sites, F-wave (late response testing the proximal segment), H-reflex (afferent sensory, efferent motor; S1 root), and blink reflex. Demyelination produces slowed velocity, prolonged distal latency, prolonged F-wave, conduction block, and temporal dispersion. Axonal loss reduces amplitude with relatively preserved velocity and latency. Conduction block is greater than 50% CMAP amplitude drop with less than 30% duration increase; temporal dispersion is greater than 30% duration increase.
Traps: submaximal stimulation falsely reduces amplitude; co-stimulation of adjacent nerves; anode-break excitation; volume conduction; measuring latency to peak instead of onset; ignoring temperature, since cooling slows velocity about 2 m/s per degree C; failing to recognize Martin-Gruber anastomosis causing apparent conduction block; comparing amplitudes without checking baseline.
Worked example: median motor study shows distal latency 5.8 ms (ULN 4.2), CMAP 8 mV, and velocity 48 m/s (LLN 50). The markedly prolonged distal latency with near-normal velocity and amplitude suggests focal demyelination at the wrist, consistent with carpal tunnel syndrome, rather than axonal loss.
Rule of thumb: 'Amplitude reflects axons; latency and velocity reflect myelin.'