By Lisa Rapaport

(Reuters Health) - Chronic depression may more than double the risk of stroke for older adults, and the danger remains high even when mental health improves, a large U.S. study suggests.

While previous research has linked depression to greater odds of having a stroke, the current study offers fresh evidence that mood-improving treatments like counseling or medications may not completely address the stroke risk tied to psychological problems.

"When symptoms of depression go away, risk of stroke remains higher, at least for two years," lead study author Paola Gilsanz, a researcher at Harvard T.H. Chan School of Public Health in Boston, said by email. Scientists still don't know if this holds true for people of all ages or whether it matters how or why depression gets better, she added.

Strokes, a leading cause of death worldwide, occur when blood flow to the brain is interrupted. Usually, this happens because a blood vessel bursts or gets blocked by a clot, depriving the brain of oxygen and damaging tissue.

Depression, while less deadly, affects an estimated 350 million people globally, according to the World Health Organization. More than half of them never receive treatment because they aren't diagnosed or can't afford drugs.

To understand the connection between depression and stroke, Gilsanz and colleagues reviewed 12 years of data from health interviews for 16,178 U.S. adults age 50 and older. With interviews every two years, and an average follow-up of almost nine years, researchers were able to monitor strokes and changes in mental health over time.

Participants were typically around 66 years old and most of them had few, if any, symptoms of depression when they started the study.

During the study period, 1,192 strokes occurred. Soon after developing depression, participants had a slightly higher risk of stroke, though the increase might be due only to chance.

But if depression lingered over four years, the stroke risk over the following two years was more than doubled, compared with people who hadn't shown many signs of depression.

For people whose mood improved, the stroke risk over two years remained similar to those who remained chronically depressed.

One limitation of the study is its reliance on participants to report strokes and recall depression symptoms, the authors acknowledge in the Journal of the American Heart Association. Researchers also didn't have data on the type or severity of strokes or details on any psychiatric medications participants might have taken.

Moreover, the study doesn't prove depression causes strokes, Gilsanz said. It's possible that depression leads people to smoke, drink excessively, eat poorly and exercise less – all unhealthy behaviors that could contribute to some of the increase in stroke risk.

Depression might also cause biological changes such as inflammation that can elevate stroke risk, Gilsanz said.

At the same time, it's possible that depression produces changes in the nervous system that lead to an overproduction of the stress hormone cortisol, said Dr. Olajide Williams, director of acute stroke services at Columbia University Medical Center in New York City. Excess cortisol can narrow the blood vessels and raise blood pressure.

When depression is diagnosed, patients can make lifestyle changes and get treatment for inflammation or high blood pressure that might minimize their risk of stroke, said Williams, who is also a spokesman for the American Heart Association.

In particular, exercise can help ease depression and also improve cardiovascular health, he said.

"Depression is under-detected and under-treated," Williams said. "This study highlights the need to focus more attention on diagnosis because beyond the mental debilitation there may also be a very serious physical danger."

SOURCE: http://bit.ly/1K6u05P Journal of the American Heart Association, online May 13, 2015.