Epidemiology of dementia associated with Parkinson's disease
Dag Aarsland and Alexandra Bernadotte
Studies of the frequency of dementia in Parkinson’s disease (PD) have used a variety of methods and designs, and this may affect the outcome. Important methodological features include tests to assess cognition, definitions of dementia, and the criteria for selecting patients. For example, results vary according to whether attempts were made to identify all patients in a defined region or whether the study was based on convenience samples from hospital clinics. The optimal method for case identification is a door-to-door survey, but few studies have used this method to report the prevalence or incidence of PD with dementia (PD-D) in the general population. Most studies have been cross-sectional, providing an estimate of the proportion of PD patients who have dementia (point prevalence). For several reasons, including the higher mortality rate in people with PD-D versus PD patients without dementia , more accurate information regarding the true frequency of PD-D can be drawn from longitudinal studies. Such studies provide information on the incidence of PD-D. Furthermore, if a healthy control group is included, such studies can also deduce the relative increase in the risk of developing dementia related to PD pathology. In addition, by combining prevalence, incidence, and mortality rates, period prevalence, i.e. the proportion of people with dementia in a PD cohort during a specified time, provides important information concerning the total proportion of PD patients who will eventually develop dementia.
In a 1988 review of 27 studies representing 4336 patients with PD, Cummings  found a mean prevalence of dementia of 40%. Although the studies were critically considered, most were based on patients who had been referred to neurology clinics and might therefore not be representative of unselected PD populations. Also, at that time studies did not include the identification and exclusion of patients with dementia with Lewy bodies (DLB).
In 2005, a systematic review was conducted employing strict methodological inclusion and exclusion criteria; it included 13 studies with a total of 1767 patients . Of these, 554 were diagnosed with dementia, yielding a prevalence of 31.3% (95% confidence interval 29.2-33.6). This review also included 24 studies that explored the prevalence of dementia in the general population and included patients with PD. In this analysis 3-4% of patients with dementia in the general population had PD-D. The estimated prevalence of PD-D in the general population aged 65 years and over was found to be 0.3-0.5% . The results of studies published after this review are in line with these findings, reporting rates of dementia in PD of 48% , 35% , 23% , and 22% . In a recent German study of 886 people with PD, 28% were found to suffer from dementia, increasing from 13.8% in those under 65 years to 40.2% in those aged over 76 .