Intertwined depressive and cognitive trajectories and the risk of dementia and death in older adults: a competing risk analysis.

Background: Depressive symptoms and cognitive impairment often interact, rendering their associations controversial. To date, their joint trajectories and associations with dementia and death remain underexplored. Aims: To explore the interactions between depressive symptoms and cognitive function, their developmental trajectories and the associations with all-cause dementia, Alzheimer's disease (AD) and all-cause death in older adults. Methods: Data were from the Health and Retirement Study. Depressive symptoms and cognitive function were measured using the 8-item Centre for Epidemiologic Studies Depression Scale and the Telephone Interview of Cognitive Status, respectively. All-cause dementia and AD were defined by self-reported or proxy-reported physician diagnoses. All-cause death was determined by interviews. The restricted cubic spline, group-based trajectory modelling and subdistribution hazard regression were used. Results: Significant interactions between depressive symptoms and cognitive function in 2010 in their association with new-onset all-cause dementia and AD from 2010 to 2020 were found, especially in women (p for interaction <0.05). Independent trajectory analysis showed that emerging or high (vs no) depressive trajectories and poor or rapidly decreased cognitive trajectories (vs very good) from 1996 to 2010 were at significantly higher risk of subsequent all-cause dementia, AD and all-cause death. 15 joint trajectories of depressive symptoms and cognitive function from 1996 to 2010 were determined, where rapidly decreased cognitive function was more common in those with no depressive symptoms. Compared with older adults with the trajectory of no depressive symptoms and very good cognitive function, those with the trajectory of no depressive symptoms but rapidly decreased cognitive function were much more likely to develop new-onset all-cause dementia and death, with subdistribution hazard ratios (95% confidence intervals) of 4.47 (2.99 to 6.67) and 1.84 (1.43 to 2.36), especially in women. Conclusions: To effectively mitigate the risk of dementia and death, it is crucial to acknowledge the importance of preventing cognitive decline in older adults without depressive symptoms, particularly in women.