The Enigma of the Motor Nerve Conduction Study. Muscle Nerve 2025 Jan;71(1):87-95
Date
11/15/2024Pubmed ID
39545340DOI
10.1002/mus.28300Scopus ID
2-s2.0-85209195824 (requires institutional sign-in at Scopus site) 1 CitationAbstract
BACKGROUND: 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.
METHODS: 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.
RESULTS: 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.
DISCUSSION: 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.
Author List
Nandedkar SD, Stålberg EV, Barkhaus PEAuthor
Paul E. Barkhaus MD Adjunct Professor in the Neurology department at Medical College of WisconsinMESH terms used to index this publication - Major topics in bold
Action PotentialsAdult
Aged
Electric Stimulation
Electrodiagnosis
Electromyography
Female
Humans
Male
Median Nerve
Middle Aged
Motor Neurons
Muscle, Skeletal
Neural Conduction
Ulnar Nerve
Young Adult
