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.