Proximal slowing of nerve conduction velocity in carpal tunnel syndrome

Slowing of median nerve conduction velocity (NCV) over the forearm segment is often seen in patients with carpal tunnel syndrome (CTS). In order to evaluate the extent and significance of this retrograde slowing of NCV, we examined 132 CTS hands from 84 patients diagnosed by both clinical and electrophysiological findings and both hands of 50 normal subjects. The mean median motor NCV was relatively slow in the CTS group (55.8 +/- 4.4 m/s) as compared with normal subjects (60.0 +/- 3.4 m/s, p less than 0.001). Mean median motor NCV was slower in CTS with low median motor amplitudes (less than 6.0 mV), 50.9 +/- 5.7 vs 56.1 +/- 4.1 m/s, p less than 0.001; and absent median sensory responses, 51.8 +/- 5.3 vs 56.0 +/- 4.3 m/s, p less than 0.001. There were 14 CTS hands with abnormally slowed median motor NCV (less than 50 m/s). This group had significantly lower median sensory and motor amplitudes, and longer motor distal latencies and sensory wrist-palm conduction time as compared with CTS hands with normal median motor NCV (greater than 50 m/s), p less than 0.001 for each parameter. These findings indicate that the proximal median motor NCV is usually affected by the distal lesion of CTS. Furthermore, the motor nerve conduction velocity proximal to the wrist is reduced in proportion to the severity of the nerve lesion. Electromyographers should be aware of this phenomenon and avoid the error of lesion localization.