Period (cumulative) prevalence
Since the mortality is higher among PD-D patients than PD patients without dementia , point prevalence is an underestimate of the true frequency of PD-D. Accordingly, reporting the cumulative proportion of PD patients who develop dementia with time provides a more accurate estimate of the frequency of PD-D. Some, but not all, longitudinal studies have controlled for the selected attrition due to death. Thus, merely adding up the number of patients who develop dementia before they die will underestimate the true proportion with dementia. Another potential bias is the interval between assessments, since attrition due to death increases with the duration of the interval.
The Sydney study  prospectively followed newly diagnosed PD patients to assess the frequency of dementia over more than 10 years. In that study, 149 patients with carefully diagnosed PD were recruited from neurologists for inclusion in a clinical trial. Patients were assessed at baseline with a comprehensive neuropsychological assessment, and 17% were classified as having dementia, defined as impairment of memory and two additional cognitive domains  (these patients would thus have been classified today as having DLB). After 3 and 5 years, 26 and 28%, respectively, had dementia . After 15 years, 48% of the evaluated patients had dementia and a further 36% had evidence of cognitive impairment; only 15% had no evidence of cognitive impairment . Recently, data from a 20-year follow-up were presented , reporting that 83% of the 30 survivors had dementia after 20 years, and altogether 75% had developed dementia prior to death. No attempt to control for selective attrition due to death was made.
The Stavanger Parkinson study  was based on a prevalence cohort of people with PD in south-western Norway, after a careful extensive search in the community. At baseline, the average duration of PD was 9 years, and 28% of the cohort had dementia. After 8 years, after adjustment for mortality, the cumulative prevalence of dementia was found to be 78% . Based on the 12-year follow-up period, Markov analysis was performed to enable a more precise estimate of the risk of developing dementia for an individual patient based on age, gender, and duration of PD . Without correcting for attrition due to death the proportion who developed dementia was stable at about 60%, but the cumulative prevalence steadily increased to 80-90% by the age of 90.
More specifically, at the age of 70 a man with PD but no dementia has a life expectancy of 8 years, of which 3 years would be expected to be with dementia. At any age, the life expectancy after onset of dementia was substantially reduced. At 12-year follow-up , only 10% of the population were alive and without dementia after having suffered from PD for an average of 19 years. In the CamPaIGN study, the cumulative probability of dementia after 10 years was 46% .