Neurophysiological evaluation of spinal excitability in patients affected by primitive restless legs syndrome

Abstract

Restless legs syndrome (RLS) is a frequent pathology, yet underrated and underestimated, affecting inasmuch as 5–10% of the population as a whole. However the pathophysiology has not been completely understood. The dopaminergic system has certainly a primary role, and some studies have highlighted a condition of spinal hyper-excitability. The aim of our study is to explore this hypothesis throughout the electrophysiological evaluation of the spinal and peripheral nervous system in primitives RLS patients. Among the patients affected by primitive RLS admitted to our Sleep Centerlab, we selected 15 women, and compared them with 17 control subjects of the same sex and age. All subjects had undergone ENG evaluation to exclude any secondary causes of lower limb paresthesia and to evaluate spinal excitability. According to a previous study, we considered two parameters which can be easily extracted from the routine tests normally carried out in the neurophysiopathology labs, the duration of F waves (FWD) of the Internal Popliteal (IPN) and ulnar nerves, and the relationship between FWD and the duration of the corresponding CMAP (CMAPD). None of the subjects (RLS and controls) included in our study presented alterations in the nerve conduction velocity. Compared to the control group, significantly higher values were found in the RLS patients for the FWD/CMAPD ratio average (p < 0.001 test Mann–Whitney) and for the FWD average for both nerves ulnar (p < 0.05 unpaired t-test) and IPN (p < 0.01 unpaired t-test). The results of our study confirm the absence of peripheral involvement in primitive RLS, while they indicate a spinal motorneuronal hyper-excitability, which seems widespread, as both IPN and ulnar nerve stimulation indicators are altered. Such condition could be due mainly to an alteration of the modulation in the interneuronal system. Presently, RLS diagnosis is based exclusively on clinical criteria. The FWD/CMAPD ratio can help to shed light on the pathogenesis of RLS, and can be used as an instrumental diagnostic indicator, easily obtainable and useful especially in cases of lower leg discomfort at night of unclear interpretation. The authors would like to thank Prof. F. Marrosu and Dr. M. Fraschini, University of Cagliari, Italy, and Prof. L. Parrino, University of Parma, Italy, for their important contribution.