Coexisting medical comorbidity and depression: Multiplicative effects on health outcomes in older adults

Background: Depression in the elderly is often associated with coexisting medical illnesses. We investigated the individual and combined impacts of depression and medical illnesses on disability and quality of life among community-living older persons. Methods: Cross-sectional and longitudinal analyses of data from 1,844 participants aged 55 and above of the Singapore Longitudinal Aging Study (SLAS-1). Baseline depressive symptoms (Geriatric Depressive Scale, GDS >= 5) and chronic medical comorbidity (>= 2) from self-reports were related to baseline and 2-year follow up instrumental and basic activities of daily living (IADL-BADL), and quality of life (Medical Outcomes Study 12-item Short Form (SF-12) physical component summary (PCS) and mental component summary (MCS) scores. Results: The prevalence of depressive symptoms was 11.4%. In main effect analyses of cross-sectional and longitudinal relationships, depression and medical comorbidity were individually associated with higher risk of IADL-BADL disability and lower PCS and MCS scores of quality of life, and only medical comorbidity was associated with increased risk of hospitalization. Significant interactive effects of depression and medical comorbidity were observed in longitudinal relationships with IADL-BADL disability (p = 0.03), PCS (p < 0.01), and MCS (p < 0.01) scores at follow up. The associations of medical comorbidity with increased odds of IADL-BADL disability and decreased SF-12 PCS and MCS scores were at least threefolds stronger among depressed than nondepressed individuals. Conclusion: Medical comorbidities and depression exert additive and multiplicative effects on functional disability and quality of life. The adverse impact and potential treatment benefits of coexisting mental and physical conditions should be seriously considered in clinical practice.