M-response quantification: a technique.

Median nerve responses in 13 patients and 9 normal subjects were evaluated using surface bipolar stimulation. The M response from surface electrodes placed upon the thenar eminence was recorded on a fiberoptic chart recorder and simultaneously subjected to true electronic integration. Comparison between the integrated M response and the M index (the product of the M response peak-to-peak amplitude and total duration) revealed a highly correlated linear regression. This linear relationship was present when stimulating both at the wrist (r2 = 0.96) and the antecubital fossa (r2 = 0.95). Statistically significant differences were observed between the integrated M response of 5 normal adult men and men with carpal tunnel syndrome (n 3, p less than 0.01) or anterior horn cell or neuropathic processes (n 4, p less than 0.001). Differences of borderline significance were noted between normal women (n = 4) and women with carpal tunnel syndrome (n 4, p less than 0.05) or anterior horn cell or neuropathic processes (n 2, p less than 0.01). There was no correlation between the integrated M response and the distal latency in patients with carpal tunnel syndrome. It is concluded that the M index is a highly reliable method of quantifying the M response as an approximation of integration in median nerve studies, but that this measurement holds little relationship with the presence of the distal neuropathic processes studied.