Additive effects of cognitive function and depressive symptoms on mortality in elderly community-living adults

Abstract

Background: Poor cognitive function and depressive symptoms are common in the elderly, frequently coexist, and are interrelated. Both risk factors are independently associated with mortality. Few studies have comprehensively described how the combination of poor cognitive function and depressive symptoms affect the risk for mortality. Our aim was to examine whether the combination of varying levels of cognitive function and depressive symptoms affect the risk of mortality in community-living elderly adults.

Methods: We studied 6301 elderly adults (mean age, 77 years; 62% women; 81% white) enrolled in the Asset and Health Dynamics Among the Oldest Old (AHEAD) study, a prospective study of community-living participants conducted from 1993 to 1995. Cognitive function and depressive symptoms were measured using two validated measures developed for the AHEAD study. On each measure, participants were divided into tertiles representing the best, middle, and worst scores, and then placed into one of nine mutually exclusive groups ranging from best functioning on both measures to worst functioning on both measures. Mortality rates were assessed in each of the nine groups. Cox proportional hazards models were used to control for potentially confounding characteristics such as demographics, education, income, smoking, alcohol consumption, comorbidity, and baseline functional impairment.

Results: During 2 years of follow-up, 9% (548) of the participants died. Together, cognitive function and depressive symptoms differentiated between elderly adults at markedly different risk for mortality, ranging from 3% in those with the best function on both measures to 16% in those with the worst function on both measures (p <.001). Furthermore, for each level of cognitive function, more depressive symptoms were associated with higher mortality rates, and for each level of depressive symptoms, worse cognitive function was associated with higher mortality rates. In participants with the best cognitive function, mortality rates were 3%, 5%, and 9% in participants with low, middle, and high depressive symptoms, respectively (p <.001 for trend). The corresponding rates were 6%, 7%, and 12% in participants with the middle level of cognitive function (p <.001 for trend), and 10%, 13%, and 16% in participants with the worst level of cognitive function (p <.001 for trend). After adjustment for confounders, participants with the worst function on both measures remained at considerably higher risk for death than participants with the best function on both measures (adjusted hazard ratio, 3.1; 95% confidence interval, 2.0-4.7).

Conclusions: Cognitive function and depressive symptoms can be used together to stratify elderly adults into groups that have significantly different rates of death. These two risk factors are associated with an increased risk in mortality in a progressive, additive manner.

Figure 1

Crude 2-year mortality rates by tertiles of cognitive function scores and depressive symptom scores in AHEAD participants (N = 6310). Symptom tertile is given on the x axis; percentage of participants is given on the y axis. AHEAD = Asset and Health Dynamics Among the Oldest Old.

Figure 2

Additive effects of poor cognitive function and depressive symptoms on 2-year mortality in AHEAD participants (N = 6310). Tertile of depressive symptoms is given on the x axis; tertile of cognitive function, on the y axis; and 2-year mortality rate, on the z axis. Sample sizes for each of the nine mutually exclusive groups are superimposed on each bar. In each depressive symptom tertile, worse scores for cognitive function were associated with higher mortality (p < .001); in each cognitive function tertile, worse scores for depression symptoms were associated with higher mortality (p < .001). AHEAD = Asset and Health Dynamics Among the Oldest Old.