The prevalence of dementia in the elderly: a review.

We reviewed the findings of the Canadian Study of Health and Aging in the context of studies published between January 1986 and June 1993 that documented dementia and Alzheimer's disease prevalence. Studies were identified using a MEDLINE literature search. Additional references were selected from the bibliography of identified articles. Most reports of all types of dementia prevalence are within a narrow range for each of the age groups 65+, 75+ and 85+ years. By contrast, two recent reports on the prevalence of Alzheimer's disease have reported much higher estimates (10.3% and 15.3%) in the elderly (65+ years). A variety of threats to both validity and generalizability of the estimates are present in all studies. In community studies which employed clinical interviews most subjects were only mildly affected; the natural history of impairment of this group requires further study if the consequences of these findings are to be understood. There is important variability in the definition of the functional consequences of cognitive impairment in the elderly which affects both the diagnosis and staging of dementia.