(Reuters Health) - Patients with the irregular heart rhythm atrial fibrillation may overestimate both their risk of having a stroke and their risk of bleeding as a side effect of medication prescribed to prevent strokes, a recent study suggests.

Researchers surveyed 227 patients with atrial fibrillation about their perceived risk of stroke or bleeding. Many of these patients had an annual estimated stroke risk of less than five percent - and most had an estimated risk of less than 20 percent.

But one-third of these patients thought they had an annual stroke risk of 21 to 50 percent, and one in five of them thought their annual stroke risk was even higher, researchers report in Mayo Clinic Proceedings.

And nearly all of the patients had an estimated annual bleeding risk of less than 10 percent - often much less. But more than half of them thought they had more than a 54 percent annual risk of bleeding.

“Patients tend to overestimate both stroke risk and bleeding risk,” said Dr. Craig January of the University of Wisconsin School of Medicine and Public Health in Madison.

In atrial fibrillation, electrical impulses in the upper chambers of the heart are chaotic, causing that part of the heart muscle to quiver rather than contracting normally. As a result, blood doesn’t move as well to the heart’s lower chambers. This can lead to the formation of clots that can travel through the arteries to the brain and cause a stroke. Atrial fibrillation patients have several times the stroke risk of people without the disorder.

The bleeding risk with atrial fibrillation can depend on what type of anticoagulant, or blood thinner, patients take to help prevent strokes.

Long-term use of an older blood thinner, warfarin, can increase the risk of severe bleeding, and it’s inconvenient because it requires frequent lab tests and check-ups to ensure that patients have the right dose.

Newer anticoagulants are associated with fewer serious bleeding complications than warfarin but can have serious complications like severe heartburn, nausea or vomiting.

“Many patients ask, `Am I going to have a stroke?” as though it is inevitable,” January, who wasn’t involved in the study, said by email.

“Particularly when warfarin (Coumadin) was the only anticoagulant, patients often viewed it as `rat poison’ (which it is),” January added. “Patients know less about the newer non-vitamin K oral anticoagulants (NOACs).”

Overall, 173 patients in the study, or 76 percent, were taking blood thinners.

Among the 54 patients who were not taking blood thinners, fear of bleeding was the main reason, the study also found.

One limitation of the study is that researchers excluded survey participants who were unaware that there was any stroke risk at all associated with atrial fibrillation. Out of 287 patients surveyed, 60 people were excluded because they were unaware of any stroke risk.

More research is also needed to determine why patients perceptions of stroke and bleeding risk are so mismatched with reality, Dr. Mohamad Alkouli of West Virginia University School of Medicine in Morgantown and colleagues write. Alkhouli didn’t respond to requests for comment.

It’s possible that patients need to believe there’s a bigger reduction in the risk of stroke to justify taking blood thinners, and this might partly explain why they tend to overestimate their stroke risk, said Dr. Tom Marshall, a researcher at the University of Birmingham in the U.K. who wasn’t involved in the study.

And patients may come away with an outside perception of risk based on what they hear from their doctors, Marshall said by email.

“Clinicians tend to emphasize the risks because we want to persuade people to take treatment to reduce their risk,” Marshall said.

SOURCE: mayocl.in/2PErUkz Mayo Clinic Proceedings, online March 29, 2019.