The relationship of median nerve F-wave parameters with severity and subtypes of carpal tunnel syndrome.

Purpose: F-wave analysis may help affirm or disprove a compression neuropathy. In this study we have analysed the effects of focal median nerve injury on F wave in Carpal Tunnel Syndrome (CTS).

Method: We studied 57 patients (100 hands) with clinical and electrophysiological CTS and 31 (62 hands) healthy subjects. Median nerve F-waves were evaluated following 10 supramaximal stimuli and recording from abductor pollicis brevis muscles. Minimum, maximum, and mean F-wave latencies, frequency of the F wave (Fp), chronodispersion (F_{CD}), mean F/M amplitude ratios (mF/M-amp) and F-wave conduction velocity (FwCV) were evaluated.

Results: The CTS patients showed prolonged F-wave latencies, decreased Fp, and an increase of the F_{CD} as compared with normative values. Absent F wave was presented in 8 of 34 hands (23.5%) with severe CTS patients. In the severe group, the FwCV was significantly slower (p< 0.001) and the mF/M-amp was significantly higher (p< 0.001) than that of mild and moderate groups, respectively. Also, the mF/M-amp was significantly greater and Fp was lower in the axonal type than in the demyelinating type. The F_{CD} was not different among groups. A strong positive correlations between mMDL with Fmin (r=0.81, p< 0.000), Fmean (r=0.80, p< 0.000) and Fmax (r=0.71, p< 0.000) were revealed.

Conclusion: Results support the differing effects of demyelinating and axonal injury on F-waves and suggest that the mF/M-amp ratio and FwCV, which is influenced by neuronal damages in the distal segment of the median nerve, is useful in the discrimination of CTS severity.