The cause of slowed forearm median conduction velocity in carpal tunnel syndrome: a Palmar stimulation study.

Objectives: To elucidate the etiopathogenesis of decreased forearm median motor conduction velocity (FMMCV) in carpal tunnel syndrome (CTS), we used segmental stimulation at the palm, wrist and antecubital fossa to determine conduction block at wrist and calculate and compare the segmental median motor conduction velocity (MMCV) to determine the pathogenesis.

Background: The cause of the decreased FMMCV in CTS remains unclear. Animal models have supported retrograde axonal atrophy as the cause. Some authors believe standard FMMCV, calculated by subtracting the distal latency, may not represent an exact assessment of FMMCV but rather the velocity of small fibers that persist throughout the carpal tunnel.

Subjects And Methods: Patients with clinical symptoms and signs of CTS which had been confirmed with standard electrodiagnosis, were included. The patients were divided into two groups: one with reduced FMMCV <50m/s (Group I, n=20) and the other with normal FMMCV>50m/s (Group II, n=40). Age-matched volunteers served as controls (n=60). We used palm, wrist and antecubital stimulation, and recorded compound muscle action potential (CMAP) amplitudes at the abductor pollicis brevis (APB) muscle. Based on a ratio of the CMAP amplitudes obtained from wrist and palm stimulation (W/P ratio) and the latency differences, we calculated the W/P ratio and the across wrist MMCV (AWMMCV) and FMMCV and compared and correlated them between two patient groups.

Results: There was no difference in median motor and sensory distal latency between Groups I and II. CMAP and sensory nerve action potential amplitudes were reduced in Group I compared with Group II, but the difference was only marginally significant. Four patients had a significant reduction of the W/P ratio in Group I, compared with 7 patients in Group II, which did not reach a significance. Sixteen patients (80%) in Group I demonstrated no conduction block. Furthermore, Group I showed significantly decreased FMMCV when compared with Group II; however, AWMMCV was not significantly reduced in Group I, suggesting that decreased FMMCV does not result from a decrease in AWMMCV.

Conclusions: There was no significant motor conduction block and no correlation of the FMMCV and AWMMCV in CTS patients with a decrease of FMMCV, suggesting retrograde axonal atrophy, and not selective conduction block of the large fibers at the wrist, is the direct cause of decreased FMMCV in CTS.