Background: The number of dementia patients is increasing worldwide, especially in Japan, which has the world's highest ageing population. The increase in the number of older people with dementia is a medical and socioeconomic problem that needs to be prevented, but the actual situation is still not fully understood. Methods: Four cross-sectional studies on dementia were conducted in 1997, 2004, 2012, and 2016 for complete enumeration of all residents aged 65 years and older. We examined the secular trends in the prevalence of all-cause dementia, Alzheimer's disease (AD), vascular dementia (VaD), and other/unclassified dementia. Results: The age-standardised prevalence of all-cause dementia significantly increased (4.5% in 1997, 5.7% in 2004, 5.3% in 2012, 9.5% in 2016; P for trend <0.05). Similar trends were observed for AD (1.7%, 3.0%, 2.5% and 4.9%, respectively; P for trend <0.05) and other/unclassified dementia (0.8%, 1.0%, 1.0% and 2.2%, respectively; P for trend <0.05), whereas no significant change in VaD was seen (2.1%, 1.8%, 1.8%, 2.4%, respectively; P for trend = 0.77). The crude prevalence of all-cause dementia and AD increased from 1997 to 2016 among participants aged 75–79 years and ≥85 years (all P for trend <0.05). Similar trends were observed for other/unclassified dementia among participants aged ≥80 years (all P for trend <0.05), but not in VaD. Conclusions: The prevalence of dementia has increased beyond the ageing of the population, suggesting that factors in addition to ageing are involved in the increase in the number of older people with dementia. To control the increase in the number of older people with dementia, elucidation of secular trends in the incidence, mortality, and prognosis of dementia as well as the factors that promote and protect against dementia, and development of preventive strategies are necessary.
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