An estimated one in four adults has a small defect of the heart-an incomplete closure of the wall separating the two upper chambers, or atria. In the vast majority of cases, the condition, known as patent foramen ovale (PFO), will never cause a problem. But among people under 55 who suffer a stroke with no obvious cause (called a cryptogenic stroke), a PFO is often considered the prime suspect. For older adults, a PFO has been largely ignored as a contributor to strokes because seniors often have other stroke risks as well, such as high blood pressure and cholesterol, diabetes, and atrial fibrillation, according to Cleveland Clinic cardiologist Richard Krasuski, MD. "The thinking was that if you had an unexplained stroke and you were older with a patent foramen ovale, its more likely that it [PFO] was an innocent bystander," Dr. Krasuski says.