Substantial variation in the prevalence of dementia in Parkinson's disease (PDD) has been reported. The aim of this study was to review systematically and critically previous studies of the prevalence of PDD using PubMed to search the literature. Studies focusing on PD and PDD, as well as those examining on the epidemiology of dementia subtypes, were included. Predefined inclusion and exclusion criteria were used and the quality of the studies included was rated. Articles were included if: (1) the proportion of PDD among patients with either PD or dementia was reported in an original study; (2) patients had been subjected to prospective clinical examination; and (3) strategies to include all subjects with either PD or dementia within the community or hospital clinics within a geographical area were employed. Twelve studies of the prevalence of PD or PDD (1,767 patients included) and 24 prevalence studies of dementia subtypes (4,711 patients included) met the inclusion criteria. In the PD/PDD studies, the proportion (mean and 95% confidence interval) with PDD in PD was 24.5% (17.4-31.5). There were significant methodological variations between studies and in the four studies that matched the quality criteria most closely, the rate of PDD was 31.1% (20.1-42.1). The prevalence of PDD was estimated as 0.5% in subjects 65 years or older. The percentage of PDD among those with dementia was 3.6% (3.1-4.1), with an estimated prevalence of PDD of 0.2% in subjects aged 65 years or older. Despite methodological variation, this systematic review suggests that 24 to 31% of PD patients have dementia, and that 3 to 4% of the dementia in the population would be due to PDD. The estimated prevalence of PDD in the general population aged 65 years and over is 0.2 to 0.5%.
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