Prevalence of Alzheimer’s disease in very elderly people

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Abstract
Background: No previous autopsy-controlled, prospective, and population-based studies are available on the prevalence of AD in very elderly people. Objective: To study the point prevalence of neuropathologically defined AD in a population of people at least 85 years of age, stratified according to their APOE genotype. Methods: A population-based sample of 532 (of a total population of 601) elderly Finnish individuals, aged 85 years or more, were clinically tested for dementia in 1991 (with follow-up studies of the survivors in 1994, 1996, and 1999) and genotyped for APOE. An autopsy involving neuropathologic diagnosis of AD according to modified consensus criteria was 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 clinically diagnosed AD was 16%. There was a highly significant (p < 0.001) association between the APOE ε4 allele and AD: Sixty-three percent of APOE ε4 carriers and 20% of noncarriers had neuropathologic AD. The respective figures in subjects aged 90 years or more were 71 and 22%. Conclusions: The prevalence of neuropathologically defined AD is higher than that reported in most previous studies based on clinical diagnosis. The discrepancy between the neuropathologic and clinical diagnoses of AD in very elderly subjects may affect the results of population-based studies. The APOE genotype has a strong effect on the prevalence of neuropathologically defined AD, even after 90 years of age.