Incidence studies have been relatively neglected in psychiatric epidemiology. They are particularly important for dementia, since prevalence rates are affected by length of survial, which itself falls with increasing age and presence of dementia.
Two-wave community study of 1195 elderly subjects aged older than 75 years, restudied 2.4 years after a community prevalence study. A two-stage method was used, comprising the Mini-Mental State Examination followed in a stratified sample by the Cambridge Examination for Mental Disorders of the Elderly (CAMDEX) interview. Incidence rates were based on personyears at risk.
Annual incidence rates for dementia were 2.3% for subjects initially aged 75 to 79 years, 4.6% for ages 80 to 84 years, and 8.5% for ages 85 to 89 years, approximately doubling every 5 years. Rates did not differ significantly by sex, educational level, or social class. Twice as many additional individuals received a diagnosis of minimal dementia not reaching case threshold.
The findings show high rates of new onset dementia, increasing markedly with age, and suggest rapid acceleration of one or more processes that is common in advanced age.