Our results from a population-based longitudinal study of 5,697 older people followed for 2 years provide data that clarify the relationship between cognitive impairment, depressive symptoms, and functional decline. First, cognitive impairment and depressive symptoms are independent predictors of ADL dependence in participants who are independent in all ADLs at baseline. In addition, in these participants, cognitive impairment and depressive symptoms were robust risk factors; those with both risk factors had a slightly greater risk of decline than those with either risk factor alone. Second, in participants who already had ADL dependence at baseline, cognitive impairment was associated with further decline, whereas depressive symptoms were not.
In participants who are independent in ADLs at baseline, cognitive impairment and depressive symptoms are independent predictors of functional decline, even after adjusting for each other. This suggests that both risk factors contribute to incident ADL dependence and do not simply act via each other. This is consistent with prior work by Gill et al. that demonstrates that the risk of functional dependence steadily increases as the degree of baseline vulnerability increases.11
Cognitive impairment and depressive symptoms may lead to an erosion of the physical skills needed to maintain functional independence and may leave a person less resistant to acute stressors, such as hospitalization, that often accelerate functional dependence in older people.21–23
In participants with baseline ADL dependence, cognitive impairment predicted further loss of ADL function, but depressive symptoms did not predict further functional decline, and, in fact, subjects with depressive symptoms, on average, demonstrated a slight improvement in ADL function, similar to subjects with neither condition. This may reflect fundamental differences in the natural history of cognitive impairment and depressive symptoms. In general, cognitively impaired participants deteriorate progressively over time. As a result, participants who have experienced functional decline secondary to cognitive impairment are likely to experience further functional decline as the cognitive impairment worsens. In contrast, depressive symptoms often improve, especially if effective treatment is administered. As a result, in participants with functional decline, the presence of depressive symptoms may be a marker for potential reversibility.
Prior studies that have evaluated depressive symptoms and cognitive function have suggested that depressive symptoms or cognitive impairment may predict functional decline, with varied effect sizes.4–13
The variance in prior reports may be due to lack of stratification by baseline ADL function, incomplete adjustment of confounders, or use of nonvalidated assessment of cognitive function or depressive symptoms. Our results describing incident ADL dependence are in accord with the few studies that have discussed the two risk factors together and have found cognitive impairment and depressive symptoms to be significant predictors in multivariate models.12,13
Because the current study undertook to stratify by ADL at baseline, adjusted for a large number of covariates, and used validated instruments for evaluation of cognitive impairment and depression, it was able to better clarify the independent and combined effects of cognitive impairment and depressive symptoms on functional decline.
Several limitations deserve comment. First, there is a potential attrition bias because of differential mortality, a factor common to studies of older people. For this cohort, participants who died had greater ADL dependence and more cognitive impairment and depressive symptoms at baseline. Second, the cognitive scale developed for AHEAD is unfamiliar, and thus the results may not be widely generalizable, but the AHEAD cognitive scale contains similar questions to the Mini-Mental State Examination, (MMSE)24
and has some evidence of construct validity.16
Alternatively, the MMSE itself has limitations, and the AHEAD cognitive scale may share these limitations. For example, the MMSE is insensitive to the cognitive effects of frontal system lesions. In particular, frontal system lesions may affect executive control functions, and executive control is not well measured by the MMSE.25
Because frontal system lesions are associated with disability26
there is a danger that cognitive deficits not detected by the AHEAD instrument may be mediating the apparent association between ADL function and depressive symptoms. Finally, there are potential limitations in the measurement of our primary outcome, change in ADL function, based on self-reports, but we do not believe that these limitations lessen the generalizability of these findings, because there is some evidence that the assessment of ADL is reliable in older populations, even those with cognitive problems.28,29
We also did not have information on the severity of ADL dependence, because dependence was assessed as a dichotomous yes/no response. Future studies could develop a gradient of dependence including questions about the nature of the help received, how often it was received, and how much activity could be performed if alone.
By describing the relationships between depressive symptoms, cognitive impairment, and functional decline, this study adds further insight into the factors responsible for functional decline. In addition, cognition and depressive symptoms may serve as targets for intervention, but this requires further study. Studies that address whether treatment of early depression by antidepressant medication, improving social networks, and addressing the potential effects of alcohol intake are warranted to determine whether these can prevent functional decline. Similarly, using current therapies and the development of new therapies for treating cognitive impairment may reduce concurrent ADL dependence and prevent increased decline.
This study suggests that depressive symptoms and cognitive impairment are independent predictors of incident ADL dependence. In addition, cognitive impairment is a more robust predictor of incident functional decline than are depressive symptoms because cognitive impairment predicts functional decline in subjects with and without baseline ADL dependence. Following from these results, early and adequate evaluation and treatment of cognitive problems and depressive symptoms are suggested. Future studies on the effect of treatment for depression and cognitive problems on functional decline outcome are warranted.