July 6, 2010 — Older people who suffer from depression have nearly double the risk for developing Alzheimer’s disease and dementia, a new study finds.

Researchers followed elderly participants in the ongoing Framingham Heart Study for up to 17 years to explore late-life depression and dementia.

They found depression to be a significant risk factor for dementia, even after other suspected contributors to dementia and Alzheimer’s disease had been considered.

But it is not clear if depression is a risk factor for dementia or if vulnerability to depression also makes people more vulnerable to Alzheimer’s disease.

Previous studies examining the impact of late-life depression on dementia have been mixed, possibly because participants were not followed long enough, study researcher Jane Saczynski, PhD, of the University of Massachusetts Medical School tells WebMD.

“A major criticism of many of the earlier studies was that the interval between the measurement of depression and dementia was not long enough,” she says. “In our study, people were followed for up to 17 years and the assessment of dementia was very, very rigorous.”

Depression, Dementia Common

As many as 6 million Americans ages 65 and older suffer from depression, but only about one in 10 receives treatment because depression is often not recognized or is wrongly considered a normal part of aging.

Depression can also lead to memory and other cognitive impairments in older people, complicating the diagnosis of both disorders.

The newly published study included 947 longtime participants in the Framingham Heart Study, which has followed residents of Framingham, Mass., since the late 1940s.

All were elderly but showed no signs of dementia when enrolled in the study. Their average age at enrollment was 79, and 125 (13%) were classified as having depression at the start of the study.

By the end of follow-up, 164 people had developed dementia, including 136 with a specific diagnosis of Alzheimer’s disease. Those with a diagnosis of depression at the start of follow-up had a 70% greater risk for developing dementia.

Roughly one in three people with depressive symptoms at the start of the study developed dementia compared to one in five people without a diagnosis of depression.

3 Studies Show Depression, Dementia Link

The study was one of three published in the July 6 issue of Neurology, suggesting a link between late-life depression and dementia.

In a separate study that included just over 1,200 older participants, having two or more episodes of depression late in life doubled the risk of dementia, but not a lesser form of cognitive decline known as mild cognitive impairment.

In a third study, symptoms of depression showed little change during the development and progression of Alzheimer’s disease.

“The association between depression and dementia has been a major topic for more than two decades, and it is increasingly clear this association is real,” Alzheimer’s researcher Yonas E. Geda, MD, tells WebMD.

Geda is an associate professor of neurology and psychiatry at the Mayo Clinic College of Medicine in Rochester, Minn.

In an editorial published with the studies, Geda suggested several possible explanations for the observed link between late-life depression and dementia, including:

• Major depression may directly damage the part of the brain associated with learning and memory via inflammation or the release of stress hormones.
• Depression may be in response to early, but medically unrecognized, memory declines.
• Depression may act synergistically with biological factors that have been linked to dementia to cause cognitive decline.
• The same biological factors that lead to depression late in life also lead to dementia.

A major brain imaging study known as the Alzheimer’s Disease Neuroimaging Initiative may lead to new insights into the association between depression and dementia, Geda says.

If depression is a direct risk factor for dementia, treating more elderly people for depression could have a big impact, Saczynski adds.