Restless Legs Syndrome Introduction Restless Legs Syndrome (rls) Is a Frequent Neu- Rologic Disorder with a Prevalence of Approxi- Mately 10% in the General Caucasian

RESTLESS LEGS SYNDROME (RLS) IS A FREQUENT NEUROLOGIC DISORDER WITH A PREVALENCE OF APPROXIMATELY 10% IN THE GENERAL CAUCASIAN population.1,2 Essential features comprise the urge to move the legs, usually accompanied by unpleasant sensations; onset or worsening of symptoms during periods of rest or inactivity; partial or total relief by movement; and worsening of symptoms in the evening or at night.3 To date, few neurophysiologic studies have been published in patients with RLS. Bara-Jimenez et al reported increased spinal cord excitability based on disinhibited flexor reflex responses.4 Investigation of soleus H-reflex and H-reflex recovery curves yielded ambiguous results.5-7 Blink reflex and exteroceptive electromyographic suppression of temporalis and gastrocnemius muscles were found to be normal,5,7 as were somatosensory and auditory evoked potentials.8 Transcranial magnetic stimulation studies have shown normal motor excitability measures following single pulse stimulation but have revealed evidence of intracortical disinhibition in RLS.9-12 The auditory startle reaction, a brainstem reflex that occurs in response to an unexpected loud stimulus, is generated in structures of the caudal brainstem, from where the reflex responses propagate up the brainstem and down the spinal cord along slowly conducting reticulobulbar and reticulospinal pathways.13 The nucleus reticularis pontis caudalis is the central structure that integrates the pattern of auditory startle responses (ASRs) in various muscles. This nucleus, along with other structures, is under basal ganglia control.14 ASRs have been examined in various disorders with basal ganglia involvement15-20 and have been reported to be absent or reduced in progressive supranuclear palsy,15,17,18 accelerated with a tendency toward higher probability in Parkinson disease,15,18 and exaggerated in multiple system atrophy.18,19 Because nigral neuronal loss is common to these different disorders and because recent neuropathologic and immunohistochemical studies in RLS have suggested impaired iron acquisition in neuromelanin cells of the substantia nigra,21,22 we hypothesized that ASRs might also be abnormal in idiopathic RLS.