Two new studies may have just found the culprit for up to a third of strokes of unknown cause and it’s called atrial fibrillation (in which abnormal contractions in the heart’s upper chambers result in clots that reach the brain).

Strokes attributed to atrial fibrillations are more debilitating and life-threatening than other kinds, and unfortunately are hard to recognize. In both studies, what enabled doctors to detect atrial fibrillations was extended heart monitoring.

To rule out atrial fibrillation, the traditional practice has been to monitor patients heart activity for 24 hours. However, the two studies suggest this may not be enough time to detect abnormal rhythms.

In the Embrace trial, half of the patients wore Holter monitors for 24 hours and the other half went home wearing a chest electrode belt for 30 days. Atrial fibrillation was diagnosed in 16 percent of patients who used the belt and in only 3 percent of the group that was monitored for 24 hours.

In the second study, patients who had unexplained strokes were followed for three years, during which a third of them suffered at least one episode of atrial fibrillation, in most cases without any apparent symptoms.

Through longer monitoring periods, more patients were prescribed with the anticoagulants they needed to avoid a subsequent stroke. Ultimately, this lowers the risk of disability and deaths due to strokes.

A couple of medical centers have already extended monitoring periods. And the guidelines of the American Heart Association now say that 30 days of monitoring for patients with unexplained strokes is a “reasonable” course of action.

While extended heart monitoring isn’t the standard of care yet, the findings of the studies may prompt doctors to be more aggressive in finding ways to detect atrial fibrillation and prevent strokes.