The reason for forearm conduction slowing in carpal tunnel syndrome: an electrophysiological follow-up study after surgery

Background: The exact cause of decreased forearm median motor conduction velocity (FMMCV) in carpal tunnel syndrome (CTS) is still a subject of controversy. A conduction block or an axonal loss in the large myelinating fibers upon wrist compression, or retrograde axonal atrophy, is suspected. Methods: In order to attempt a determination of the cause, 10 patients with clinical symptoms and signs of CTS, confirmed using standard electrodiagnosis and with a slowed FMMCV <50 m/s, were included in this study. Serial standard median motor conduction studies were performed at baseline, 1 week, 2 weeks, 4 weeks, 8 weeks, and 12 weeks after endoscopic ligament release. Serial median motor distal latencies (MMDL), compound muscle action potential (CMAP) amplitudes, and FMMCV, were determined and compared. Results: Significant improvement in MMDL had occurred at the 1-week follow-up examination; however, no such improvement in FMMCV was observed. Furthermore, a significant increase in CMAP amplitude was evidenced beginning 4 weeks after surgery. The results revealed an improvement in median motor conduction, across the wrist segment, that did not parallel the increase in FMMCV, suggesting that a conduction block or axonal loss at wrist compression was not the likely cause of the decreased FMMCV. Conclusions: Retrograde axonal atrophy, not selective damage to the large myelinating fibers at the wrist, is the direct cause of decreased FMMCV in CTS.