The diagnosis of depression in dementia is not an easy task. Although the majority of patients with dementia do not develop major depression (Olin et al., 2002), more than half suffer from one or more depressive symptoms such as anxiousness, sadness, irritability, agitation or psychomotor retardation, sleep problems, diminished social activity, or loss of interest (Gruber-Baldini et al., 2003). There is an overlap between dementia and depressive symptoms and behaviors, and it is difficult to determine whether the dementing illness or depression is the underlying pathology. Although apathy is often associated with depression, approximately 60% of patients with dementia with apathy have no depression and 44% of patients with dementia with depression have no apathy (Lyketsos et al., 2001).
Diminished social activity, loss of interest and apathy may be responses to a cognitive deficit, or they may be manifestations of major depression. Moreover, memory problems in early stages of dementia might compromise patients' self-report of depressive symptoms. Language and communication difficulties in later stages of dementia might change the expression of depressive symptoms. These atypical presentations often lead to lower rates of clinician recognition and diagnosis of depression (Gallo and Rabins, 1999).
In one study that used the DSM-III criteria to diagnose major depression among patients with dementia, 14% of the sample met the criteria for major depression based on patient interviews, and 50% met the diagnostic criteria on the basis of caregiver interviews (Mackenzie et al., 1989). This discrepancy might be due to the inability of the caregivers to distinguish between depressive symptoms and apathy, or to the caregivers' own depression.
Adding to the complexity of depression diagnosis in dementia, 60% to 70% of depressive symptoms in community-dwelling patients with dementia disappear within six months and 85% recur within one year (Lee and Lyketsos, 2003). Furthermore, patients with dementia might suffer from depressive disorders that are not related to dementia per se, such as adjustment disorder with depressed mood, recurrence of early and midlife major and minor depression, late-onset depression, and vascular depression (Lee and Lyketsos, 2003).
Due to the previous complexity and the absence of consensus on how to diagnose depression in patients who are demented, a panel of 21 experts in dementia and depression were assembled by the National Institute of Mental Health and suggested specific criteria for diagnosing depression in AD (Table 2) (Olin et al., 2002).