Sleep disorders and the natural history of Parkinson's disease: The contribution of epidemiological studies

Background: Sleep disorders (SD) are one of the most frequent non-motor manifestations of Parkinson's disease (PD). Recent studies showed that SD may precede the onset of PD. Objectives: We reviewed current literature concerning 1) the incidence of PD among subjects with SD; and 2) the occurrence and possible clinical correlations of SD during the course of PD. Methods: A Medline search found 17 longitudinal studies. Results: The incidence of PD among patients with rapid eye-movement sleep behavioural disorders ranged from 20% to 65% of cases, within a wide interval of time (range: 2.2-13.3). The incidence of SD during PD progressively increased with disease duration in population-based studies but presented marked fluctuations in clinical based studies. Older age, male gender, dopaminergic treatment with higher dosage, cognitive impairment and hallucinations were associated with the onset of SD during PD. In the only population-based study among Japanese men excessive daytime sleepiness was associated with a threefold increased risk of developing PD. Conclusions: Available data suggest that SD could be the heralding clinical manifestation or a risk factor for PD onset. The prevalence of SD increases during the course of the PD and may be related to specific phenotype and rapid progression of PD. However, the current data are limited because of limited sample size and poor study design; prospective studies with larger sample size are warranted. (C) 2010 Elsevier Ltd. All rights reserved.