More than a half-million Americans every year have an ischemic stroke, the most common form, which disrupts blood flow to the brain. And at least a quarter of these cases have no apparent underlying cause.
Now two major studies suggest that many of these strokes of unknown origin — up to a 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 suffer 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, which was funded 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 with 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 a third of patients who had had strokes of unknown cause 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 its 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."
Dr. Gordon F. Tomaselli, the chief of cardiology at the Johns Hopkins University School of Medicine and a past president of the heart association, said the findings would "reinforce the notion that if you don't have a good reason for a stroke, you really need to take a pretty intensive look for atrial fibrillation."
Nationwide, about three 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 one in six 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 at all.
"What we are learning is that many patients have clinically silent atrial fibrillation," said Dr. Gladstone, who is also a scientist at Sunnybrook Research Institute in Ontario. "Often the first manifestation is when it leads to a stroke."
In the Embrace trial, which was carried out at 16 medical centers in the Canadian Stroke Consortium, Dr. Gladstone and his colleagues followed 572 people who had had either a stroke or a mini-stroke whose cause remained unclear after a battery of diagnostic tests.
About half of the patients wore a conventional device, known as a Holter monitor, which recorded their heart rhythms for an additional 24 hours. But the rest were assigned to wear a new chest electrode belt for 30 days after they went home.
Atrial fibrillation was detected in just 3 percent of the patients monitored short term, and in 16 percent of those who wore the new device for one month.
Among the patients monitored for 30 days was William Russell, 71, a retired businessman from Collingwood, Ontario. Mr. Russell suffered a major stroke two years ago during a ski trip with his family in Calgary.
"There was no prior warning — it just hit," he said. "My left side became completely paralyzed and my speech was slurred. Fortunately my daughter noticed it immediately and called 911."
At the hospital, doctors gave Mr. Russell a powerful clot-busting drug that reversed his stroke. But their next challenge was to find what had caused it. They took scans of his heart and his brain and did an electrocardiogram to measure the electrical activity of his heart, but the cause remained unknown.
Mr. Russell was enrolled in the study, and after a month of wearing the electrode belt day and night, his doctor was able to make a proper diagnosis.
"His heart monitoring revealed that he was having silent episodes of atrial fibrillation," Dr. Gladstone said. "As a result, we've been able to treat him with anticoagulant medication."
Mr. Russell said that he was doing well on his new medication and had returned to hiking, cycling and playing golf. Wearing the monitor for 30 days was "a bit of a pain," he said. "But it was worth it. Well worth it."
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