The Enigma of the Motor Nerve Conduction Study

In motor nerve conduction studies (MNCS), proximal stimulation should give a longer duration and lower amplitude compound muscle action potential (CMAP) due to higher temporal dispersion. Yet the CMAP waveforms at the distal and proximal stimulation sites appear remarkably similar. The objective of this study was to confirm this anomaly and investigate its possible cause by studying the median and ulnar nerves.

Recordings from 50 subjects with normal electrodiagnostic studies were reviewed. The conduction velocity (CV) was measured using different points on the negative phase of the CMAP including its peak and baseline crossing. Collision studies were performed in three healthy subjects to measure the dispersion when nerve action potentials (APs) propagated from elbow to wrist.

CV was relatively unaffected by the measurement point on the CMAP. The CMAP duration with elbow stimulation increased minimally compared to wrist stimulation. This was inconsistent with the dispersion of the AP from wrist to elbow measured in collision studies.

The insignificant change in the CMAP in spite of axon AP dispersion is an enigma. We hypothesize that the terminal conduction time (TCT) (i.e., conduction in terminal axon branches, neuromuscular transmission, etc.) is independent of axon CV, represents a significant portion of the latency, masks AP dispersion, and reduces CMAP dispersion. This yields similar CMAPs with distal and proximal stimulation. The onset latency at the distal stimulation site does not depend on CV. Thus, onset latency and CV may not reflect the conduction properties of the fastest conducting axons.