Practically every medical illness or acute medical event is followed by an increased risk of depression in both young adults and elderly people. Ischemic heart disease, stroke, cancer, endocrine diseases, chronic lung disease, kidney disease, arthritis, hip fracture, and neurodegenerative disease have all been shown to increase ones risk for depression. In a meta-analysis across specialty medical settings, Dew observed an increased prevalence of major depressive disorder (MDD) in practically every illness compared to the community rate in the Epidemiologic Catchment Area study. Conversely, elderly people with MDD have a high rate of comorbid medical illness. Descriptive data from a sample of 449 elderly patients at the Western Psychiatric Institute and Clinic in Pittsburgh, showed that most elderly people with depression had 2–4 comorbid medical conditions; >50% had hypertension, and almost 50% had osteoarthritis (R Mantella, PhD, unpublished data, 2005; Slide 8). Hyperlipidemia, diabetes, and hypothyroidism were present in 20% to 35% of patients. These medical conditions are common in the elderly population in general, but are much more prevalent in the depressed elderly.
According to standard epidemiology, when two things are associated either A causes B, B causes A, or C causes both A and B. It is thought that all three are the case when it comes to the association between depression and medical illness. Medical conditions increase the risk for depression through several routes. First, neurodegenerative diseases (eg, stroke, Parkinson’s, Alzheimer’s) can cause direct disruption of neurotransmitters or neural circuits involved in mood regulation. Second, cardiac surgery, serious cardiopulmonary event, or hip fracture, can cause severe physiologic stress, which can lead to depression. Finally, the functional disability that occurs with these medical illnesses is a major risk factor for late-life depression. Conversely, depression can increases the risk of several medical conditions in elderly people. Depression can increase the risk for cardiovascular disease, falls and fractures, and possibly cancer. Finally, some theories suggest that there is some shared, perhaps genetic, vulnerability to both depression and medical conditions.