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 incide nce 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 y ears and over, seen initially in a prevalence study of dementia. A scr eening 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 A lzheimer's disease of mild and greater severity was 2.7/1000 person-ye ars 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 ag e, particularly in women. Rates for minimal dementia of different subt ypes showed similar age and sex effects, but were much higher for Alzh eimer's disease than vascular dementia. Conclusions. The striking rise in incidence rates of dementia in the very old appear to be due to Al zheimer'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 a s a cause of cognitive decline of all degrees.