Researchers at Rush University Medical Center, Chicago, have found that depression is a risk factor for Alzheimer's disease rather than an early sign of its underlying pathology.
In the study, researchers found no evidence of an increase in depressive symptoms during the prodromal phase before the clinical diagnosis of Alzheimer's disease.
Previous studies have found higher levels of depressive symptoms in old age to be associated with increased incidence of Alzheimer's disease and mild cognitive impairment.
However, those studies have not been able to answer as to whether depressive symptoms actually boost the development of dementia or are a consequence of the disease.
To find out, Robert S. Wilson, Ph.D., a neuropsychologist of Rush University Medical Center, and colleagues examined 917 older Catholic clergy without dementia at study onset, to look at the change in depressive symptoms of Alzheimer's disease before and after the emergence of the cognitive symptoms of the disease.
The study was conducted for up to 13 years, in which the participants underwent annual clinical evaluations that included assessment of depressive symptoms, cognitive testing, and clinical classification of mild cognitive impairment (MCI) and Alzheimer's disease.
During the study, 190 participants developed Alzheimer's disease.
Researchers found that having more depressive symptoms at baseline was associated with increased incidence of Alzheimer's disease and MCI.
However, they also found that those who developed Alzheimer's disease showed no increase in depressive symptoms before clinical diagnosis.
Researchers were able to observe patients during a mean of approximately four years before the onset of dementia.
Also, researchers found no increase in depression during the three to four years preceding the onset of MCI, which antedates the onset of dementia by several years.
"If depressive symptoms are a consequence of dementia or a reaction to declining function, depressive symptoms would likely increase at some point before dementia is clinically evident. We observed no such increase," Wilson said.
Furthermore, researchers found that even after the diagnosis of Alzheimer's disease was made there was no general increase in depression, but rather an increase that was confined to individuals with certain personality traits.
"Depressive symptoms may be associated with distinctive changes in the brain that somehow reduce neural reserve, which is the brain's ability to tolerate the pathology associated with Alzheimer's disease," Wilson said.
"Understanding how depression contributes to the development of Alzheimer's disease may suggest new approaches to disease prevention," he added.
The study is published in the April issue of the Archives of General Psychiatry.