Static mechanical hyperalgesia without dynamic tactile allodynia in patients with restless legs syndrome

Abstract

Pain sensitivity was assessed in 11 patients (age 60 ± 10 years) with 'primary' restless leg syndrome (RLS) (disease duration 18 ± 15 years) and 11 age- and gender-matched healthy control subjects. Stimulus-response functions for pricking pain were obtained with seven calibrated punctate mechanical stimulators activating AS-high threshold mechano-nociceptors. Stimuli at the foot were significantly more painful than at the hand in both patients and healthy control subjects both in the morning and evening. Generally, pin-prick pain ratings in RLS patients were significantly elevated, by a factor of 5.3 in the upper limb and by a factor of 6.4 in the lower limb indicating a significant generalized static hyperalgesia more pronounced in the lower limb. In contrast, pain to light touch (allodynia = dynamic mechanical hyperalgesia) as tested by a battery of three gentle tactile stimuli was never reported. Acute single-dose dopaminergic treatment with 100 mg levodopa + 25 mg benserazide, 90 min prior to the evening measurements, largely resolved patients' RLS symptoms, but had no effect on pin-prick pain. Static hyperalgesia to pinprick, however, was significantly reversed (median reduction -74%) by long-term individually tailored dopaminergic treatment. Our study shows that patients with RLS exhibit a profound static mechanical hyperalgesia to pin-prick stimuli, but no dynamic mechanical hyperalgesia (allodynia). This type of hyperalgesia is probably mediated by central sensitization to Aδ-fibre high-threshold mechanoreceptor input, a hallmark sign of the hyperalgesia type of neuropathic pain. The reduction of hyperalgesia in RLS patients by long-term dopaminergic treatment suggests that the pathophysiology of RLS includes disturbed supraspinal pain modulation involving the basal ganglia and/or descending dopaminergic pathways.