MONDAY, March 7 (HealthDay News) -- People who suffer a stroke and also have an irregular heart rhythm called atrial fibrillation may be at greater risk of developing dementia than stroke survivors without the heart condition, British researchers report.

The likelihood of atrial fibrillation increases with age, and it is a significant risk factor for stroke. More than 2 million Americans have the condition, according to the study, published in the March 8 issue of Neurology.

"We know that atrial fibrillation is a common arrhythmia in older patients, but it has been unclear whether the arrhythmia is a major risk factor for dementia," said researcher Dr. Yoon K. Loke, a senior lecturer in clinical pharmacology at the University of East Anglia in Norwich, U.K.

"In the stroke population, atrial fibrillation appears to have a major role in contributing to dementia, and clinicians should concentrate their efforts on tackling this, in addition to any associated cardiovascular risk factors," he added.

For the study, Loke and colleagues gathered data on 46,637 people, average age 72, who took part in 15 separate studies.

This is a method known as a meta-analysis in which researchers pull out certain data from studies not necessarily designed to evaluate the specific outcomes these researchers are interested in. The goal is to identify any significant trends.

In this case, the pooled data showed that people who survive a stroke and who also have atrial fibrillation are 2.4 times more likely to develop dementia, compared with stroke survivors without this irregular heart beat.

In all, about 25 percent of patients with stroke and atrial fibrillation developed dementia during follow-up, the researchers noted.

Strategies are needed to reduce this excess dementia risk in stroke patients, Loke said. "This may include steps such as better control of the arrhythmia and more effective prevention of clots."

The researchers were unable to determine whether people with atrial fibrillation but no stroke history are at a greater risk for dementia. "In wider populations that involve patients who do not have stroke, atrial fibrillation does not seem to be a major contributor to the risk of dementia," Loke said.

Many factors other than atrial fibrillation probably contribute to dementia, he said. "A targeted or focused approach on management of atrial fibrillation may not help to reduce the burden of dementia," he added.

Commenting on the study, Dr. Larry B. Goldstein, a professor of medicine and director of the Duke Stroke Center at Duke University Medical Center, said that "there was considerable variability among the included studies."

The relationship between atrial fibrillation and dementia in those with stroke is not unexpected, Goldstein said. Strokes that result from heart problems tend to be larger and more frequently involve the left middle cerebral artery, leading to aphasia (damage to the brain area that controls language), which can complicate cognitive testing, he explained.

"The meta-analysis did not control for these factors, although two of the studies excluded those with aphasia," Goldstein said.

Dr. Richard B. Libman, chief of the division of vascular neurology at Long Island Jewish Medical Center in New Hyde Park, N.Y., added that this study is not conclusive, but appears to suggest a connection.

"If it turns out that atrial fibrillation is associated with dementia only because atrial fibrillation is a cause of stroke, then we do whatever we can to prevent strokes in people who have atrial fibrillation," he said.

But it's possible that atrial fibrillation by itself could play a role in dementia. "That's a little trickier," Libman noted. The goal of treatment then would be to control the arrhythmia through drugs or medical procedures, he said.

Libman said a meta-analysis has limitations, because the various studies use different methods. "You can't tease out relationships always by combining data," he said. "All you can do is get a general idea that perhaps will guide further research."

More information

For more information on stroke, visit the U.S. National Library of Medicine.