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 <50m/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.
Chang
Ming Hong
MH
Section of Neurology, Taichung Veterans General Hospital, No. 160, Chung Kang Road, Section 3, 40705, Taichung, Taiwan, ROC. cmh50@ms10.hinet.net
Wei
Shiew Jue
SJ
Chen
Lee Wei
LW
D002349:Carpal Tunnel Syndrome / Q000503:physiopathology* / Q000601:surgery; D004558:Electric Stimulation; D004576:Electromyography; D004594:Electrophysiology; D005071:Evoked Potentials; D005500:Follow-Up Studies; D005542:Forearm / Q000294:innervation / Q000503:physiopathology*; D006801:Humans; D008475:Median Nerve / Q000503:physiopathology; D018482:Muscle, Skeletal / Q000503:physiopathology; D009431:Neural Conduction / Q000502:physiology*; D011930:Reaction Time; D013997:Time Factors