Shortening of distal motor latency in anode distal stimulation

Objective Distal motor latency (DML) is shortened when the anode is held distally instead of the cathode by increasing the stimulus intensity. The objective of this study was to clarify the mechanism responsible for this shortening. Methods In seven healthy subjects, compound muscle action potential (CMAP) was obtained from the thenar muscle by bipolar stimulation of the median nerve at the wrist, and the intensity at which the first motor units were stimulated was defined as the threshold. Bipolar stimulation with extended interpole distance was employed to identify the generating site of the CMAP and F-wave. Results The shortening of DML was dependent on the stimulus intensity and threshold. For the low threshold condition, the CMAP generating site was replaced from the proximal cathodal pole to the distal anodal pole by increasing the stimulus intensity. The generating site of the F-wave remained at the proximal cathodal pole irrespective of stimulus intensity. Conclusions Replacement of the generating site results in the shortening of DML. When the F-wave is recorded after being induced by anode distal stimulation, CMAP should not be simultaneously evaluated. Significance This study clarified the generation sites of CMAP and the F-wave when induced by anode distal stimulation.