There is a well-established association between chronic medical illness and depression in the general population. In one study, for example, the 6-month prevalence of affective disorder was 6% in medically well individuals,

9% in respondents with a chronic medical illness, and 12% in those who were being treated for a medical condition (Wells, Golding, & Burnam, 1988).

A recent review concluded that disability is the only significant health-related risk factor for depression in individuals age 50 years or older. Some studies find that poor health status and the onset of a new medical illness predict the onset of depression, but other studies do not (Cole & Dendukuri, 2003). This suggests that the disabling effects of chronic medical illnesses are more reliably depressogenic than are other aspects of these conditions. There is also a robust association between physical inactivity and depression, regardless of health status (Goodwin, 2003). These findings provide valuable clues as to which health-related problems are likely to be fruitful targets of treatment in CT for depression in medically ill patients.

Many studies have examined comorbid depression in patients with specific chronic medical illnesses. Prevalence estimates are consistently higher when depression is defined by self-report questionnaires rather than by structured interviews and diagnostic criteria. The estimated prevalence of major depression also varies within and between medical illnesses. Studies of various cardiac patient populations consistently find that about 15-20% of patients meet the DSM-IV criteria for a major depressive episode (Rudisch & Nemeroff, 2003), but very different prevalence estimates have been found in some subgroups. In patients with congestive heart failure (CHF), the prevalence ranges from 2% in older adult patients with mild CHF to 67% in younger patients with severe CHF (Freedland et al., 2003). Although depression is common in patients with cancer, the prevalence of major depression varies widely among different types of cancer (Massie, 2004). This literature has been summarized in a number of recent review articles (e.g., Anderson, Freedland, Clouse, & Lustman, 2001; Evans et al., 2005; Massie, 2004; Rudisch & Nemeroff, 2003).

Severity ofillness is a complex construct, and there are multiple ways to measure it. Furthermore, many patients have more than one illness, so no measure of the severity of any single condition captures the total burden of medical illness. Across conditions, the severity of depression correlates more strongly with measures of the clinical or functional severity of illness than with physiological indicators. For example, the number of diseased coronary arteries is a very weak correlate of depression in patients with coronary heart disease (CHD),but the severity of functional limitations and of symptoms such as angina are strong correlates (Spertus, McDonell, Woodman, & Fihn, 2000; Sullivan, La Croix, Russo, & Walker, 2001). Here again, it is evident that the most depressogenic aspects of chronic medical illnesses tend to be the ones that adversely affect how the patient feels and functions in daily life, as opposed to physiological factors that may be more important determinants of the patient's prognosis or of the treatability of the medical condition.

Much of the current interest in comorbid depression revolves around the discovery that it has prognostic importance in certain medical illnesses, especially in heart disease. Depression predicts cardiac morbidity and mortality in patients with stable CHD (Carney et al., 1988), a recent acute myocardial infarction (MI; Frasure-Smith, Lesperance, & Talajic, 1993), recent coronary artery bypass surgery (Connerney, Shapiro, McLaughlin, Bagiella, & Sloan, 2001), and CHF (Freedland et al., 1991). There is also considerable interest in the prognostic importance of depression in other major illnesses, such as diabetes and cancer (Evans et al., 2005). Depression is associated with poor glycemic control and an increased risk of serious complications in patients with diabetes (De Groot, Anderson, Freedland, Clouse, & Lustman, 2001; Lustman et al., 2000). There is very limited evidence linking depression to the incidence of cancer (Penninx et al., 1998), but it may be a risk factor for mortality in patients who already have cancer (e.g., Herrmann et al., 1998). Little is known about whether treatment of depression can improve the medical outcomes of any of these conditions.

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