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More than 500,000 Americans every year have an ischemic stroke, the most common form, which disrupts blood flow to the brain. At least one-quarter of these cases have no apparent underlying cause.

Two major studies suggest that many of these strokes of unknown origin — up to one-third — may stem from atrial fibrillation, a common irregularity of the heartbeat that often goes unrecognized.

The findings are likely to encourage doctors to look more aggressively for signs of atrial fibrillation in patients who experience strokes of unknown cause.

After such strokes, doctors usually prescribe a mild blood thinner such as low-dose aspirin. But aspirin alone may not be enough to prevent additional strokes in patients with underlying atrial fibrillation. These patients generally require more powerful anticoagulant medications to prevent clotting that can lead to additional strokes.

Stroke patients are generally screened with electrocardiographic monitoring for 24 hours to rule out atrial fibrillation. But the new studies, published Wednesday in The New England Journal of Medicine, suggest that some patients may need their hearts monitored much longer to detect abnormal rhythms.

One of the studies, paid for by the Canadian Stroke Network and known as the Embrace trial, found that atrial fibrillation was diagnosed in five times as many patients who wore special heart monitors for 30 days compared to those who underwent conventional 24-hour testing.

The second study, led by researchers in Italy and carried out at clinics in the United States and other countries, found that one-third of patients who had strokes of unknown causes and were followed for up to three years experienced at least one episode of atrial fibrillation — and in most cases, there were no obvious symptoms.

In both studies, the longer monitoring periods resulted in significantly more patients being prescribed anticoagulants to lower their risk of another stroke.

“If more patients with atrial fibrillation can be detected, then more patients can receive appropriate stroke-prevention therapy, and the hope is that more strokes, deaths, disability and dementia can be avoided,” said Dr. David Gladstone, an associate professor in the department of medicine at the University of Toronto and the lead author of the Embrace trial.

Some medical centers monitor patients beyond the usual 24 hours, and in May the American Heart Association updated guidelines to say it was “reasonable” for patients with unexplained strokes to be monitored for 30 days.

But longer monitoring “hasn’t been the standard of care,” said Dr. Hooman Kamel, a neurologist at the Brain and Mind Research Institute at Weill Cornell Medical College in New York, who was not involved in the new research. “I think these two studies are really what was needed to put it on very firm footing and to make it more widespread.”

Nationwide, about 3 million Americans have a diagnosis of atrial fibrillation, which occurs when erratic electrical signals cause the heart’s upper chambers, the atria, to contract abnormally. The fluttering can cause blood to pool in the atria, forming clots that can then travel to the brain.

At least 1 in 6 strokes are attributed to atrial fibrillation, and they are often more debilitating and deadly than strokes stemming from other causes.

But atrial fibrillation can be difficult to detect. The episodes are typically sporadic, coming and going unpredictably and lasting minutes or days at a time. Some people experience heart palpitations, shortness of breath and dizziness. But many experience no symptoms.

“What we are learning is that many patients have clinically silent atrial fibrillation,” said Gladstone, who is also a scientist at Sunnybrook Research Institute in Ontario. “Often the first manifestation is when it leads to a stroke.”