A-delta and C-fibres function in primary restless legs syndrome

Abstract

The sensory symptoms that are reported in restless legs syndrome (RLS) suggest involvement of the peripheral nervous system (PNS) in general and of the small-fibre system in particular. We aimed to study the status of the small-fibre system in primary RLS. We investigated 10 patients with idiopathic RLS (mean time since disease onset: 11.4 +/- 12 years, mean International Restless Legs Syndrome Study Group [IRLSSG] score: 23.4 +/- 8). Five had a family history. All had normal results for laboratory tests, neurological examination, and a sural/deep-peroneal nerve conduction study. Lower-limb thulium YAG laser-evoked potentials (LEP) and skin sympathetic reflexes (SSR) were performed. The results were compared with data from 10 healthy subjects. The nociceptive thresholds were 293 +/- 62 mJ for patients and 333 +/- 77 mJ for controls. For patients, the vertex N2 and P2 latencies were 208 +/- 25 ms and 366 +/- 51 ms, respectively (controls: N2 = 235 +/- 41 ms; P2 = 373 +/- 44 ms). The N2-P2 amplitude was 19 +/- 6 microV for patients and 18 +/- 7 microV for controls. SSR were normal in all patients. No significant differences between patients and healthy subjects were observed. We failed to demonstrate any significant involvement of small fibres and spinothalamic tracts in idiopathic RLS. Even though sufferers of this specific form of RLS report sensory symptoms, pathogenesis appears to be dissociated from a PNS alteration.