A new study suggests that depression may be a risk factor for, rather than a symptom of, Alzheimer's disease.

The research, published Tuesday in the journal Neurology, concluded that those who suffer from depression were 2.5 times more likely to develop Alzheimer's. Those who had depression before age 60 were four times more likely to develop the disease.

The study followed almost 500 adults between the ages of 60 and 90 for about six years.

The study's authors, from the Erasmus University Medical Centre in Rotterdam, the Netherlands, concluded that more research needs to be done to determine the exact relationship between depression and Alzheimer's.

"We don't know yet whether depression contributes to the development of Alzheimer's disease or whether another unknown factor causes both depression and dementia," study author Dr. Monique M.B. Breteler said in a statement.

The release of this study coincided with that of another study, conducted by researchers at the Rush University Medical Centre in Chicago, Ill. and published in the April issue of the Archives of General Psychiatry. They analyzed data from a study that followed more than 900 Catholic clergy to determine how the symptoms of depression change before and after an Alzheimer's diagnosis.

During this study, 190 of the subjects developed Alzheimer's. The subjects who had more symptoms of depression at the onset of the study had an increased risk of developing Alzheimer's.

Alzheimer's is the most common form of dementia, according to the Alzheimer Society of Canada. It is a progressive, degenerative disorder that causes severe memory and thought impairment, which then eventually leads to limited physical abilities. It has no known cause or cure, and affects about 300,000 Canadians over age 65.

Abstract:

History of depression, depressive symptoms, and medial temporal lobe atrophy and the risk of Alzheimer disease

M. I. Geerlings, PhD, T. den Heijer, MD, PhD, P. J. Koudstaal, MD, PhD, A. Hofman, MD, PhD and M.M.B. Breteler, MD, PhD

Background: Depression may increase risk for Alzheimer disease (AD), but it is not clear whether this risk is mediated by structural brain changes. We determined whether history of depressive episodes and presence of depressive symptoms were associated with smaller hippocampal and amygdalar volumes and with increased risk for incident AD.

Methods: Within the Rotterdam Scan Study 503 persons, aged 60-90 years at baseline and without dementia, reported their history of depressive episodes. Depressive symptoms were assessed with the Center for Epidemiologic Studies Depression Scale. Volumetric assessment of the hippocampus and amygdala was performed using three-dimensional MRI. All subjects were followed for an average of 6 years for development of AD, diagnosed according to National Institute of Neurological and Communicative Disorders and Stroke-Alzheimer's Disease and Related Disorders Association criteria.

Results: A total of 134 subjects (26.6%) reported a history of depression (88 reported an onset <60 years and 46 a late onset). Multiple linear regression analyses did not reveal a significant association with hippocampal or amygdalar volume for any of the depression parameters. During follow-up, 33 persons developed AD. Cox regression analyses showed that subjects with early onset depression had an increased risk for AD (HR 3.76; 95% CI 1.41 to 10.06), independent of hippocampal and amygdalar volume, whereas this risk was 2.34 (95% CI 0.82 to 6.69) in subjects with a late-onset depression. Depressive symptoms at baseline were not associated with increased risk for AD.

Conclusion: History of depression, and particularly an early onset, but not presence of depressive symptoms increased the risk for Alzheimer disease. This risk was not mediated by smaller hippocampal or amygdalar volumes.

Abbreviations: AD = Alzheimer disease; CAMDEX = Cambridge Examination for Mental Disorders of the Elderly; CES-D = Center for Epidemiologic Studies Depression Scale; HASTE = half-Fourier acquisition single-shot turbo spin-echo; HPA = hypothalamic-pituitary-adrenal; ICV = intracranial volume; MMSE = Mini-Mental State Examination.

Link to Full Study(registration required)