Sensory and mixed nerve action potential temporal dispersion in median neuropathy at the wrist

This retrospective pilot study was undertaken to help determine the usefulness of measuring sensory nerve action potential and mixed nerve action potential temporal dispersion in median neuropathy at the wrist (MNW; i.e., carpal tunnel syndrome). The records were reviewed for 34 patients who were referred to an electrodiagnostic medicine laboratory with normal antidromic median sensory nerve action potential (recording from the index finger), median transcarpal mixed nerve action potential, and ulnar transcarpal mixed nerve action potential peak distal latencies (NO group) and 29 patients with prolongation (>2.2 ms) of the left median transcarpal mixed nerve action potential peak distal latency or relative prolongation of this response (>0.4 ms) compared with the ipsilateral normal ulnar transcarpal mixed nerve action potential peak distal latency (MNW group). By using the time difference between onset and negative peak as a measure of waveform temporal dispersion, mean +/- standard deviation of the median transcarpal mixed nerve action potential time difference for the MNW group (0.57 +/- 0.15 ms) was found to be greater than the NO group (0.44 +/- 0.09 ms; P < 0.01). No statistically significant differences were found for the median sensory nerve action potential time difference between the two groups or between the subgroup of MNW patients with concurrent prolongation of the median sensory nerve action potential peak distal latency and the NO group. These findings suggest that increased median transcarpal mixed nerve action potential temporal dispersion may occur in association with median transcarpal mixed nerve action potential peak distal latency prolongation in MNW. The small magnitude of this increase, however, makes the clinical usefulness of this observation unclear.