Thermal hypoesthesia differentiates secondary RLS due to small fibre neuropathy from primary RLS

Objective: The aim of our study was to assess thermal and mechanical perception and pain thresholds in primary idiopathic Restless Legs Syndrome (RLS) and secondary RLS due to small fiber neuropathy.

Methods: 21 patients (age 53.3±8.4, n=3 male) with primary RLS and 13 patients (age 62.4±7.6, n=1 male) with secondary RLS due to small fiber neuropathy were compared with 20 healthy subjects (58±7; n=2 male). Differential diagnosis of secondary RLS was made based on clinical symptoms, particularly burning feet. A comprehensive quantitative sensory testing (QST) protocol encompassing thermal and mechanical detection and pain thresholds as devised by the German Research Network on Neuropathic Pain (DFNS) was performed upon the dorsum of both feet between 2 pm and 1 am while RLS symptoms were present in all patients.

Results: Patients with idiopathic RLS showed hyperalgesias to heat (p<0.05), blunt pressure (PPT: p<0.001), pin-prick (p<0.001), and vibratory hyperesthesia (VDT: p<0.01). Patients with secondary RLS due to small fiber neuropathy showed thermal hypoesthesia to cold (A delta fiber mediated) and warm (C fiber mediated) (all p<0.001), and hyperalgesias to blunt pressure (p<0.05) and pin-prick (p<0.001).

Discussion: Patients with secondary RLS due small fiber neuropathy showed significantly increased thermal detection thresholds as compared with idiopathic RLS and healthy controls. Previously demonstrated static mechanical hyperalgesia was confirmed for primary and secondary RLS patients. Furthermore, our results suggest that heat pain hyperalgesia may differentiate primary RLS from secondary RLS due to small fiber neuropathy.