A history of depression has been associated with an increased risk of Alzheimer’s disease and dementia in several studies [1,2].  Some studies, however, have failed to replicate this finding.

In a study of 1,953 people with Alzheimer’s disease, depressive symptoms before dementia onset were twice as common as in controls [3]. It may be that early awareness of cognitive decline contributes to depression, but it seems that this does not fully explain the association.  A review of all epidemiological studies also reported an association between depression and Alzheimer’s disease even when those episodes of depression had been more than 10 years before the onset of Alzheimer’s disease [4].

It is well known that people with depression, especially older adults, have reduced cognitive performance and that many people with dementia also have depression.  It is therefore difficult to determine whether depression is a risk factor for dementia or whether it is a prodromal symptom [1,2].

Depression is associated with elevated cortisol production, which may directly damage the hippocampus and increase the risk of dementia.  Conversely, depression may arise secondary to dementia due to frontostriatal damage from cerebrovascualr or neurodegenerative pathology [1,2].  Recent studies have also suggested that people with depression have enhanced deposition of beta amyloid plaques [1], and depression has been associated with vascular changes in the brain, which may mediate the association [5].

The treatment of depression seems to improve cognitive function, but it may not return cognition to normal levels even when the depression is in remission [1,2].  Whether the treatment of depression decreases the risk of dementia among people with depressive symptoms has not yet been studied, but it is clearly important to identify and treat depression.  Preventing new episodes of depression may be a useful preventative approach to dementia [6].

References

1. Middleton LE, Yaffe K. Promising strategies for the prevention of dementia. Arch Neurol, 2009, 66:1210-1215.
2. Hughes TF, Ganguli M. Modifiable midlife risk factors for late-life cognitive impairment and dementia. Curr Psychiatry Rev, 2009, 5:73-92.
3. Green R, et al. Depression as a risk factor for Alzheimer’s disease. The MIRAGE study. Arch Neurol, 2003, 60:753-759.
4. Jorm A, et al. Psychiatric history and related exposures as risk factors for Alzheimer’s disease: a collaborative re-analysis of case-control studies. EURODEM risk factors research group. Int J Epidemiol, 1991, 20:S43-S47.
5. Woodward M, et al.  Dementia risk reduction: the evidence. 2007, Alzheimer's Australia, Canberra.
6. Scalco MZ, van Reekum R. Prevention of Alzheimer disease. Encouraging evidence. Can Fam Physician, 2006, 52:200-207.