Reduced afferent-induced facilitation of primary motor cortex excitability in restless legs syndrome

Restless legs syndrome (RLS) is characterized by the association of an urge to move, and vesperal or nocturnal sensory symptoms; it is frequently associated with periodic limb movements. Evidence from imaging and electrophysiological studies suggests that RLS is linked to changes in sensorimotor integration. Nevertheless, the underlying mechanisms have not been characterized, and the cortical origin has yet to be confirmed. The objective of the present study was to establish whether or not sensorimotor integration in RLS patients is impaired in the evening. The time-dependent modulation of motor cortex excitability following peripheral electric nerve stimulation was studied in 14 idiopathic RLS patients, and 14 paired healthy controls. Different inter-stimulus intervals were used to measure short-latency and long-latency afferent inhibition (SAI and LAI) and afferent-induced facilitation (AIF). Motor evoked potentials were recorded from the first dorsal interosseous muscle in two experimental sessions (one in the morning and one in the evening). With the exception of LAI (which was present in the morning but absent in the evening in both healthy controls and RLS patients), no circadian variations were observed in sensorimotor integration. Although SAI was present in patients with RLS, AIF was disrupted (relative to controls) - suggesting the presence of an indirect sensorimotor integration disorder affecting the long corticocortical pathways in patients with RLS. The lack of circadian modulation in sensorimotor integration suggests that clinical circadian variations have other causes.