Preserved sympathetic skin response at the distal phalanx in patients with carpal tunnel syndrome

Objective: To assess the viability of sympathetic sudomotor fibers in carpal tunnel syndrome (CTS). Methods: We recorded sympathetic skin response (SSR) with a multichannel recording system. Forty-four patients with CTS (51 hands), 7 patients (7 hands) with asymptomatic median mononeuropathy at the wrist (MMW), and 20 normal subjects (20 hands) were studied. We classified the patients into 4 grades of increasing severity. Results: In the hands of normal subjects, SSR was evoked easily at all recorded sites. SSR at the wrist in patients with asymptomatic MMW decreased in amplitude. SSR was markedly distorted at the wrist in severe grades of CTS. The SSR amplitude ratio (wrist/distal phalanx) decreased significantly with more severe grade (rh=−0.4; P<0.05), but the sensitivity was lower than that of other electrodiagnostic criteria. A patient with persistent allodynia at the wrist after surgery showed the slight recovery of SSR amplitude ratio; the other two patients without allodynia showed substantial recovery of SSR amplitude ratio within 24 weeks after surgery. Conclusion: SSR amplitude ratio is a poor indicator of CTS diagnosis, but may be useful in assessing the viability of sympathetic sudomotor fibers and may assist in evaluating the response to surgery.