Neurology [2008] 70 (13) : 1017-1022 (Buter TC, van den Hout A, Matthews FE, Larsen JP, Brayne C, Aarsland D.) Complete abstract

The risk of dementia in Parkinson disease is high, with important clinical consequences for people with Parkinson's Disease. However, the absolute risk of dementia and how it affects survival in Parkinson's Disease were not known. Researchers assessed the prevalence of Parkinson's Disease with dementia as people age. Over 60% of people with Parkinson's Disease were eventually found to also have dementia. The cumulative incidence of dementia was found to steadily increase with age and duration of Parkinson's Disease. So for those that live until 90 years of age the likelihood of dementia increases to 80% to 90%. Women live with Parkinson's Disease longer than men and spend more years with dementia. At the age of 70, a man with Parkinson's Disease but no dementia has a life expectancy of 8 years, of which 5 years would be expected to be dementia free and 3 years would be expected to be with dementia. Although dementia was found to be common in Parkinson's Disease, and appear almost inevitable in the oldest of sufferers, no comparison was made with the general population to see if people without Parkinson's Disease were almost as likely to develop dementia. Parkinson's Disease is biochemically distinct from dementia. So there is no inevitable reason why they should coincide.