Prevalence of dementia and dementing diseases in the old-old population in Japan: the Kurihara Project. Implications for Long-Term Care Insurance data

Background: There have been no reports on the prevalence of dementia among the old-old people in Japan. Methods: We studied the old-old population in Kurihara, northern Japan. Analysis 1 of Participants 1 (n = 590) was performed to evaluate the prevalence of dementia and dementing diseases by intensive evaluation including MRI. Analysis 2 aimed to determine a good indicator for detecting suspected dementia condition based on the Long-Term Care Insurance index. Analysis 3 of Participants 2 (n = 3915) aimed to estimate the prevalence of suspected dementia condition. Results: In Analysis 1, 73 people (12.4%) were diagnosed with dementia. The most common cause was Alzheimer's disease with cerebrovascular disease. In Analysis 2, level I of the Impairment Level of Dementia was found to be a good indicator of suspected dementia condition. In Analysis 3, the overall estimated prevalence of suspected dementia condition was 23.6%. In men, the ratio increased gradually from 75 to 87 years old to about 20%, increased to 40% at the age of 88 and became stable thereafter. In contrast, in women, the ratio increased from 75 to 95+ years old, reaching about 70%. Conclusions: The prevalence was higher than that reported previously. There was a difference between the sexes: an age-related increase occurred in men and an ageing-related increase in women. Alzheimer's disease with cerebrovascular disease was the most common cause, which coincided with the previous findings of individuals aged 65 years and older; however, the ratio of mixed dementia was greater.