Depressive symptoms and cognitive decline in late life: a prospective epidemiological study.

CONTEXT: Depression is associated with cognitive impairment and dementia. It is less clear whether depression contributes to further cognitive decline over time, independently of incipient dementia.
OBJECTIVE: To examine the relationship between depressive symptoms and subsequent cognitive decline in a cohort of nondemented older adults, some of whom remained dementia free during follow-up and others in whom incident dementia eventually developed.
DESIGN: Twelve-year prospective epidemiological study, including biennial measurement of cognition and depressive symptoms, biennial assessment of dementia, and comparison of cognitive function at baseline and over time in persons with and without baseline depressive symptoms in the dementia-free and eventual-dementia groups, using random-effects models.
SETTING: A largely blue-collar rural community. PARTICIPANTS: Population-based sample of 1265 adults 67 years and older without dementia at baseline. MAIN OUTCOME MEASURES: Scores over time on each of several cognitive test composites.
RESULTS: Among 1094 participants who remained dementia free, those with baseline depressive symptoms had significantly lower baseline scores on all cognitive composites than the nondepressed participants. Among the 171 individuals in whom dementia later developed, depression was associated with worse performance in some but not all baseline cognitive composites. Cognitive decline over time was minimal in the dementia-free group, whereas marked decline was seen in the eventual-dementia group. Depressive symptoms were not associated with rate of cognitive decline over time in either group.
CONCLUSIONS: Depressive symptoms are cross-sectionally associated with cognitive impairment but not subsequent cognitive decline. Substantial cognitive decline over time cannot be explained by depression and most likely reflects incipient dementia.