ABSTRACT
Functional imaging can be used to investigate central dopamine and other receptor status in an effort to understand the link between RLS and PD, but reports on RLS have been conflicting and generally inconclusive. Eisenhsehr et al. (17) showed no differences in presynaptic (dopamine transporter) or postsynaptic (123I-IBZM) D2 receptor binding in patients with RLS. However, an intermittent decreased dopamine turnover with relatively normal intrasynaptic dopamine levels or dysfunction of alternative dopamine-dependent pathways such as the diencephalospinal pathways or orexin (hypocretin) neuropeptide-related neuronal dysfunction causing symptoms of RLS cannot be excluded (13). Orexin pathways project to the substantia nigra and also to the pedunculopontine nucleus (18). In another study, Linke et al. studied the striatal dopamine transporter with (123) I IPT (a tropane ligand) using single photon emission tomography (SPECT) in 28 RLS, 29 early PD patients, and 23 age-matched controls. They reported no difference in IPT binding between RLS and controls (19). This study, therefore, further disputes the link between RLS and PD based on nigrostriatal presynaptic dopaminergic dysfunction. A recent study by Mrowka et al. investigated RLS patients and controls using a three-dimensional ultrasound-based movement analysis before and after a levodopa test dose and [2 beta-carbomethoxy-3 beta-(4-iodophenyl) tropaine (betaCIT)]-SPECT scans (20). No significant change in movement analysis in response to levodopa or signals in caudate or putamenwere observed. However, a slight but significant change in the relative 123I-beta-CIT SPECT in putamen versus the caudate nucleus was observed with lower quotient of ratios in RLS. The clinical significance of this is unclear. Schmidaeur and colleagues showed reduced midbrain echogenecity suggesting reduced iron (20a) using transcranial Doppler ultrasound sonography in RLS patients compared to controls and PD. Therefore, overall there is no good evidence to suggest that PD and RLS have similar pathology.
