F-Wave duration as a specific and sensitive tool for the diagnosis of restless legs syndrome/Willis-Ekbom disease

Study Objectives: Restless legs syndrome, also known as Willis-Ekbom disease (RLS/WED), is a frequent condition, though its pathophysiology is not completely understood. The diagnosis of RLS/WED relies on clinical criteria, and the only instrumental tool, the suggested immobilization test, may lead to equivocal results. Recently, neurophysiological parameters related to F-wave duration have been proposed as a diagnostic aid. The aim of this study is to assess and compare the diagnostic values of these parameters in diagnosis of RLS/WED. Methods: Fifteen women affected by primary RLS/WED and 17 age-and sex-matched healthy subjects. A complete electroneurographic evaluation, including nerve conduction studies (NCS), cutaneous silent period (CSP), and F-wave parameters, namely amplitude, F-wave duration (FWD), and the ratio between FWD and duration of the corresponding compound muscle action potential (FWD/CMAPD). Results: No subject showed alterations of the NCS. However, FWD and FWD/CMAPD of both upper and lower limbs were significantly longer in patients than controls. Tibial FWD/CMAPD best discriminated RLS/WED patients from controls. A cutoff of 2.06 yielded a sensitivity of 69.2%, a specificity of 94.1%, a positive predictive power of 90%, and a negative predictive power of 80% (area under the curve = 0.817; 95% confidence interval = 0.674-0.959). The combination of ulnar or tibial FWD/CMAPD increases the sensitivity (85.7%) while slightly decreasing the specificity (87.5%, positive predictive value: 85.7%, negative predictive value: 87.5%). Conclusions: Lower limb FWD/CMAPD ratio may represent a supportive diagnostic tool, especially in cases of evening lower leg discomfort of unclear interpretation.