Cognitive Decline among a Dementia-Free Japanese Elderly Population: Radiation Effects Research Foundation Adult Health Study

Objective: Although it is known that accelerated decline in cognitive function precedes dementia onset by several years among demented subjects, enough knowledge has not been obtained about cognitive decline among subjects without dementia. We investigated age, gender, education, radiation and birth cohort effects in global cognition decline among dementia-free subjects. Method: This study examined cognitive decline among about 1700 dementia-free subjects between 1992 and 2011 through biennial medical check-up. Participants were from the Adult Health Study (AHS), consisting of atomic bomb survivors and their controls, aged 60 years or older at study-start (1992). We evaluated cognitive performance with the Cognitive Abilities Screening Instrument (CASI). Results: Averaging 8.6 years of follow-up, many subjects experienced cognitive decline. Older ages were related to both decreased initial levels and faster rates of decline in cognitive function. More years of formal education associated with higher initial levels, but was not found to affect cognitive decline over time. Additionally, education had a stronger positive effect on initial levels of women than that of men. Finally, radiation was not found to affect cognitive function at baseline or over time. Conclusions: The longitudinal study of the AHS provides a better understanding of the decline of cognition among dementia-free subjects. The results are also useful as reference at dementia screening.