Incidence of Clinically Diagnosed Subtypes of Dementia in an Elderly Population. Cambridge Project for Later Life

Background: In developed countries, most dementia appears to be due to Alzheimer's disease and vascular dementia. We report rates for incidence of subtypes of dementia based on clinical diagnosis.

Method: This study was a 2.4-year (s.d. 2.6 months) follow-up of a cohort aged 75 years and over, seen initially in a prevalence study of dementia. A screening interview in 1173 survivors was followed in a subsample of 461 respondents by a diagnostic interview 1.8 months after screening (s.d. 1.5 months). This comprised a standardised interview with respondent and informant, with venepuncture where possible. Clinical diagnoses of subtypes were made by specified criteria.

Results: The incidence of Alzheimer's disease of mild and greater severity was 2.7/1000 person-years at risk (1.6-4.4); in men 1.5 (0.8-2.7) and in women 3.3 (1.8-5.9). The incidence of vascular dementia was 1.2/100 person-years at risk (0.7-1.9); in men 1.1 (0.4-2.8) and in women 1.2 (0.7-2.0). Alzheimer's disease, but not vascular dementia, showed a marked increase with age, particularly in women. Rates for minimal dementia of different subtypes showed similar age and sex effects, but were much higher for Alzheimer's disease than vascular dementia.

Conclusions: The striking rise in incidence rates of dementia in the very old appear to be due to Alzheimer's disease, while rates for vascular dementia remain relatively constant. These trends are particularly marked for minimal dementia, but emphasise the importance of Alzheimer's disease in the community as a cause of cognitive decline of all degrees.