Prevalence of Alzheimer's disease in very elderly people - A prospective neuropathological study

Background: No previous autopsy-controlled, prospective, and population-bas ed studies are available on the prevalence of AD in very elderly people. Ob jective: To study the point prevalence of neuropathologically defined AD in a population of people at least 85 years of age, stratified according to t heir APOE genotype. Methods: A population-based sample of 532 (of a total p opulation of 601) elderly Finnish individuals, aged 85 years or more, were clinically tested for dementia in 1991 (with follow-up studies of the survi vors in 1994, 1996, and 1999) and genotyped for APOE. An autopsy involving neuropathologic diagnosis of AD according to modified consensus criteria wa s performed in 118 of 198 deceased subjects who had been demented on April 1, 1991, and in 62 of 201 nondemented individuals. Results: The prevalence of neuropathologically defined AD was 33%, whereas the prevalence of clinic ally diagnosed AD was 16%. There was a highly significant (p < 0.001) assoc iation between the APOE <epsilon>4 allele and AD: Sixty-three percent of AP OE epsilon4 carriers and 20% of noncarriers had neuropathologic AD. The res pective figures in subjects aged 90 years or more were 71 and 22%. Conclusi ons: The prevalence of neuropathologically defined AD is higher than that r eported in most previous studies based on clinical diagnosis. The discrepan cy between the neuropathologic and clinical diagnoses of AD in very elderly subjects may affect the results of population-based studies. The APOE geno type has a strong effect on the prevalence of neuropathologically defined A D, even after 90 years of age.