Depression and dementia are common in older people and their association is very complex. Major and minor depression occur often in patients with dementia and can be associated with deterioration in cognitive functioning. Many clinicians have difficulty determining whether dementia, depression or both are the underlying disease for their patients' apathy, psychomotor retardation, concentration deficit and short-term memory impairment. Moreover, depression in dementia brings additional disability to patients who are demented and their caregivers, and a previous history of depression may be associated with an increased risk for the subsequent development of a dementing illness.
In this article, we review the literature to address the interaction between depression and dementia and provide clinicians with information to improve the care of their patients who are demented.
The aging of the U.S. population has been accompanied by a dramatic rise in the prevalence of both depression and dementia. Among community-dwelling older adults, 3% to 11% have dementia (Boustani et al., 2003) and 2% to 14% have depression (Beekman et al., 1999). In long-term care settings, 44% to 53% of the residents have dementia (Magaziner et al., 2000) and 9% to 30% have depression (Parmelee et al., 1989; Payne et al., 2002; Rovner et al., 1991; Watson et al., 2003).
Depending on the threshold used to define depression, the different scales and methods implemented to measure depressive symptoms, and various clinical settings, numerous studies in dementia estimate that depression occurs in 30% to 50% of patients (Olin et al., 2002). Approximately 24% of community-dwelling older adults with dementia (Lyketsos et al., 2000), 24% of residents in assisted living with dementia and 27% of those in nursing homes suffer from depression (Gruber-Baldini et al., 2003).
Dementia leads to a high burden of suffering for patients, their families and for the entire society, with an annual cost of approximately $100 billion (Boustani et al., 2003). Furthermore, depression is independently related to poor outcomes, including greater medical morbidity, increased health care services use, functional decline and death (Charney et al., 2003). Having both dementia and depression increases the risk of mortality and disability and leads to higher health care utilization and costs (Kopetz et al., 2000; Lyketsos et al., 1999, 1997).
Does depression increase the risk for the future development of dementia? History of depression in early, mid- or late life has been associated with an increased risk for the future development of dementia. In a meta-analysis of seven case-controlled and six prospective cohort studies, Jorm (2001) found that history of depression approximately doubled the risk of developing dementia. More recently, a retrospective cohort study found a significant association between dementia and a history of depression symptoms that first reported within one year or more than 25 years before the onset of dementia (Green et al., 2003). Another longitudinal study found that 43% to 89% of elderly patients with depression who presented with cognitive impairment developed dementia within the following three to eight years (Table 1) (Schweitzer et al., 2002).
Numerous hypotheses have been suggested to explain the previous association between depression and the increased risk for developing dementia. These hypotheses included that: 1) pharmacological treatment of depression might be a risk factor for dementia; 2) depression and dementia share common risk factors such as cerebrovascular disease; 3) depression is an early stage of dementia; 4) depression occurs as a reaction to the early cognitive decline of dementia; 5) depression hastens the clinical manifestation of dementia; and 6) depression-related glucocorticoid effects lead to hippocampal injuries and damage (Green et al., 2003; Jorm, 2001; Wilson et al., 2003). Nevertheless, recent data suggest that the association of depressive symptoms with Alzheimer's disease (AD) and cognitive impairment appears to be independent of cortical plaques and tangles and, therefore, depression may be a risk factor for dementia more than being early sign of its symptoms (Wilson et al., 2003).
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