Effect of carpal tunnel syndrome on median nerve proximal conduction estimated by F-waves.

Slowing of median nerve proximal motor conduction in patients with carpal tunnel syndrome (CTS) could be considered as an indicator of an additional proximal lesion (double crush syndrome). The effect of CTS on proximal conduction was assessed by comparing motor velocities calculated by F-waves obtained from muscles with the same root and nerve supply but different median branches, one emerging before the carpal tunnel (pronator quadratus muscle) and one passing through the tunnel (abductor pollicis brevis). Data were obtained from 26 patients with CTS and 21 age-matched healthy subjects. In the control group, the proximal (spinal cord and elbow) F-wave maximal velocity calculated when recording from abductor pollicis brevis (FCVmax-APB) was not different from the F-wave maximal velocity calculated when recording from pronator quadratus (FCVmax-PQ), while it was significantly different in the group of CTS patients, especially in patients with terminal motor latency greater than 4.5 ms (approximately 9% less, p = 0.001, Wilcoxon signed rank test). The study showed that median nerve proximal conduction velocity slowing in patients with CTS is restricted to the fibers that distally pass through the carpal tunnel and does not necessarily imply an additional proximal lesion. We suggest that comparison of FCVmax-APB and FCVmax-PQ could be useful when the question arises if a single (distal) or two (one distal, one proximal) lesions are responsible for a patient's symptoms.