Median sensory nerve conduction block during wrist flexion in the carpal tunnel syndrome.

A new method for diagnosing the carpal tunnel syndrome (CTS) is introduced. We have determined the time (T50) necessary to reach a 50% reduction in the amplitude of the median antidromic sensory nerve action potential during wrist flexion. Ten patients with anamnestic and neurophysiological evidence of CTS (group CTS) and 20 patients with CTS suspected on clinical grounds (group CTS?) were compared with 10 normal subjects. During wrist flexion the sensory conduction in the median nerve became partially blocked in all patients, and also in 8 out of 10 controls. The T50 value in 8 out of 10 CTS patients and in 14 out of 20 CTS? patients was below the lowest recorded value in the control group. It was shown that ischemia caused the blocking of the median nerve. Determination of T50 of the median nerve SNAP during wrist flexion has the potential to add to the sensitivity and specificity of the electrophysiological diagnosis of CTS.