WEDNESDAY, Feb. 1 (HealthDay News) -- Sleep apnea, the disorder marked by abnormal pauses in breathing during sleep, is already known to boost the risk of stroke. Now, a new study links sleep apnea to so-called silent strokes, in which there is tissue death in the brain without symptoms.

In another new study, researchers found that rapid memory loss before a stroke boosts the risk of the stroke being fatal.

Both studies are slated for presentation Wednesday at the American Stroke Association's International Stroke Conference in New Orleans.

Stroke affects 795,000 Americans annually, according to the association.

In one study, Dr. Jessica Kepplinger, a fellow at the University of Technology in Dresden, Germany, and her colleagues evaluated 56 patients who had a stroke. They knew that silent strokes had been linked to an increased risk of strokes. However, "there are barely any studies that have investigated the relationship between sleep apnea and the so-called clinically silent strokes," she said.

To look at the relationship, they first gave patients in-hospital testing for apnea. "We found an overall high frequency of sleep apnea, 91 percent, in our study population of acute stroke patients, which underlines the importance of this stroke risk factor," Kepplinger said.

The team also performed brain-imaging studies. Those with sleep apnea were more likely to have the silent strokes, as evidenced on the brain scans, the researchers found. Having more than five episodes a night was linked with having silent strokes. The higher the severity of the apnea, the more likely these silent strokes were found on brain imaging.

The more severe the apnea, the less favorable the outcome when the patient was discharged.

The patients were on average 67 years old, and just over half of them were women, the study authors noted.

While the study found an association between sleep apnea and stroke, it did not prove a cause-and-effect relationship.

In the second study, Qianyi Wang, a graduate student at the Harvard University School of Public Health, and colleagues evaluated nearly 12,000 men and women, all above age 50, enrolled in the U.S. Health and Retirement Study.

All were stroke-free at the start. The men and women were given memory tests every two years for up to 10 years.

Over time, 1,820 strokes were reported, including 364 people who died after the stroke.

The others were stroke-free for the entire follow-up period, the study authors noted.

The research looked at the memory declines over time. Those who later survived a stroke "had memory decline that is nearly twice as fast as stroke-free individuals, even before their stroke," Wang said.

"For people who do not survive stroke, this difference is even more striking," said M. Maria Glymour, an assistant professor of society, human development and health at Harvard and a study co-author. "Prior to stroke, people who later died shortly after stroke were declining three times as fast as the stroke-free."

The study was funded by the U.S. National Institute on Aging and the American Heart Association.

"Our study is the first national picture of how memory changes over the long-term before and after stroke onset, compared to individuals who do not have a stroke," Glymour said.

Both studies provide some valuable information, said Dr. Ralph Sacco, chair of neurology at the University of Miami Miller School of Medicine and past president of the American Heart Association. He reviewed the findings.

"It's been mainly in smaller studies that sleep apnea has been shown to be a risk factor for stroke," Sacco said. The new research, he noted, goes even further by linking sleep apnea with the milder "silent" strokes.

"There are many reasons to treat sleep apnea, including reducing the risk for clinical and now silent stroke," Sacco said.

The memory-loss study, he said, "is telling us that those who have the worst memory loss may have a greater death rate when they have the stroke." Those with more memory loss in the study may also have had more risk factors for stroke, Sacco added.

Even so, he said, the message seems to be that taking care of brain health may help us in several ways. "What is good for our memory may also be good for surviving a stroke," Sacco said.

Because these studies were presented at a medical meeting, the data and conclusions should be viewed as preliminary until published in a peer-reviewed journal.

More information

Visit the American Stroke Association to learn more about stroke.