A method for determining median nerve conduction velocity across the carpal tunnel.

Palmar stimulation was used to assess median nerve conduction across the carpal tunnel. In 50 hands from 25 control subjects, motor and sensory latencies in the wrist-to-palm segment (mean +/- SD: 1.15 +/- 0.21 msec and 1.12 +/- 0.21 msec respectively) were less than half the conventional terminal latencies in the wrist-to-muscle and wrist-to-digit segment (3.01 +/- 0.44 msec and 2.47 +/- 0.39 msec). Motor and sensory conduction velocities (MNCV and SNCV) in the wrist-to-palm segment (56.0 +/- 7.6 m/sec and 58.7 +/- 7.5 m/sec respectively) were comparable to those in the elbow-to-wrist segment (57.0 +/- 4.5 m/sec and 62.4 +/- 5.7 m/sec). In 20 symptomatic hands from 13 patients with mild carpal tunnel syndrome, delay in motor and sensory terminal latencies (3.91 +/- 0.67 msec and 2.90 +/- 0.57 msec) was primarily attributable to increased conduction time in the wrist-to-palm segment (1.96 +/- 0.59 msec and 1.58 +/- 0.49 msec) and not in the remaining more distal portions. Consequently, MNCV and SNCV were significantly (P less than 0.001) slowed when calculated in the segment across the carpal tunnel (36.6 +/- 11.2 m/sec and 44.9 +/- 11.8 m/sec), even though the conventional terminal latencies from the stimulus site at the wrist were often within normal limits.