Subtypes of carpal tunnel syndrome: median nerve F wave parameters

The carpal tunnel syndrome (CTS) provides a model for analyzing the effects of focal nerve injury on F waves. We studied 127 patients (164 CTS) with clinical and electrophysiological CTS and 35 healthy controls in order to determine the alteration of F wave parameters in different types of CTS and to evaluate the most predictive F wave abnormality for each type. Minimal, maximal and mean F wave latencies, F wave persistence and chronodispersion recorded from abductor pollicis brevis (APB) muscle with wrist stimulation were compared. Twenty-three patients (29 CTS) had prominent demyelinating type CTS, 37 patients (45 CTS) had prominent axonal type CTS and 60 patients (90 CTS) slight demyelinating CTS according to electrophysiological parameters. The amplitude of APB muscle and F wave persistence were correlated significantly (r: 0.36, P<0.001). Minimal F wave latency was more prolonged in demyelinating group than in the axonal and slight demyelinating groups (P=0.001). In conclusion, F wave determination, as a simple and valuable method, allows the discrimination between demyelinating injury and axonal degeneration and increases the diagnostic yield in CTS.