The relationship between old age and the decline in mental capabilities, possibly leading to Alzheimer's disease, has always presented something of a dilemma to clinicians. Higher levels of depressive symptoms have long been associated with mental impairments but it is the association that presents the puzzle. Current thinking suggests that depression does not represent a risk factor but they are a consequence of Alzheimer's. Of course the alternative explanation might be that depressive symptoms increase at some point early in the disease process, which may act as a symptom or reaction to the changes in brain function.
The question as to how depression changes during the development of Alzheimer's has recently been investigated by Robert S Wilson, Ph.D., and reported in the April 2008 edition of the Archives of General Psychiatry. Wilson and his team set out to test the hypothesis that depression increases during the ‘prodromal' (pre-symptomatic) phase of Alzheimer's.
Using data from the Rush Religious Orders Study, which began in 1994 and continues today, the team tracked the time line and course of depressive symptoms before a diagnosis of Alzheimer's disease was confirmed. People who did not develop dementia were used as a comparison group and any changes after diagnosis were also examined in order to establish as full a picture as possible of the natural history of depression.
Results from the study, which involved 917 participants from the outset, found a strong association between depression and dementia. The report however points out that, "in a cohort [sample] of more than 900 older persons observed for up to 13 years, we found no evidence of the increase in depressive symptoms during a mean [average] of approximately 4 years of observation before the onset of dementia in Alzheimer's disease." No increase in depression was noted during the 4 years building towards the onset of mild cognitive impairment which may develop into Alzheimer's.
If depressive symptoms are a risk factor for Alzheimer's rather than a sign of its development, the question as to how these symptoms contribute to risk still remains. Wilson points out that major depression is associated with some structural changes in areas of the brain. Neurological degeneration has also been identified in animals subjected to chronic stress. Wilson speculates that new approaches to delaying dementia onset might be developed if we can gain an understanding of the mechanisms linking depression with dementia.
Wilson, R.S., Arnold, S.E., Todd, L., Beck, M.S., Bienias, J.L., Bennett, D.A. (2008) Changes in Depressive Symptoms During the Prodromal Phase of Alzheimer's Disease. Archives of General Psychiatry, 65. (4) 439-446.