Low thalamic monoamine transporter availability is related to excessive daytime sleepiness in early Parkinson's disease

Although excessive daytime sleepiness (EDS) is a frequent non-motor dysfunction in Parkinson's disease (PD), the exact pathophysiology remains elusive. This study investigates the relationship between daytime sleepiness and presynaptic monoamine transporter densities of the basal ganglia in patients with early PD. Sixty-four patients with early PD who were evaluated with positron emission tomography (PET) using F-18-N-(3-fluoropropyl)-2beta-carbon ethoxy-3beta-(4-iodophenyl) nortropane were enrolled. EDS was evaluated with the Epworth Sleepiness Scale (ESS); nocturnal disabilities and nighttime sleep problems were assessed with Parkinson's Disease Sleep Scale 2nd version. PET images were normalized, and the standardized uptake value ratios (SUVRs) for caudate, putamen, globus pallidus, thalamus, and ventral striatum were obtained. The associations between regional SUVRs and ESS scores were analyzed. Among the patients studied, 12 had EDS defined as ESS > 10. The SUVR of the thalamus demonstrated a significant inverse relationship with ESS score, and thalamic monoamine availability appeared to predict EDS when controlling for covariates. The findings suggest that disrupted dopaminergic and serotonergic modulation of the thalamus may be implicated in EDS in PD. This in vivo study might contribute to elucidation of the neurobiological mechanism of hypersomnolence in PD.