Median nerve residual latency: normal value and use in diagnosis of carpal tunnel syndrome.

If normal variability in nerve conduction velocity (NCV) were eliminated from distal latency measurements, standard deviations of distal latencies should be reduced and normal ranges narrowed. This should be especially useful in diagnosing carpal tunnel syndrome. In this study, the median motor distal latencies, the sensory distal peak latencies, motor NCVs and motor residual latencies were determined in 100 normal subjects. Residual latency was calculated by dividing the distance between the cathode and active electrode by the motor NCV and subtracting the quotient from the motor distal latency. Results showed that the standard deviation and normal range of the residual latency were smaller than those of the motor distal latency or sensory distal latency. Data were also compared by age decade. Residual latencies hold constant into the 9th decade, while motor distal and sensory distal latencies increase with age. Three patients with recent onset of carpal tunnel syndrome are described who had prolonged residual latencies as their only abnormalities. Residual latency determination is particularly attractive because it does not require additional stimulation of the median nerve and the additional discomfort associated with it.