Researchers don't yet know whether there is a causal relationship between microvascular dysfunction and the kind of plaque accumulation that spreads evenly throughout the artery wall, although that is a leading hypothesis.

"The two travel together," explains Dr, Bairey Merz, who chairs the WISE study. "We think there may be as many as 2 to 3 million women in the U.S. with this condition."

As reports of WISE findings begin to circulate, Dr. Bairey Merz is encouraged to see patients taking action. "Women are going in now to their physicians' offices and saying, 'Look at this article!'"

At least with proper diagnosis, there is hope of receiving appropriate treatment. Women who have atherosclerotic plaque may require preventive therapies, such as baby aspirin and cholesterol-lowering statins. Patients with microvascular dysfunction are often treated with angiotensin-converting enzyme (ACE) inhibitors. Some small studies also suggest that supplementation with over-the-counter L-arginine, an amino acid that improves blood flow in the coronary arteries, can be an effective treatment.

"I would say that in about 60 to 70 percent of patients in my personal experience that I put on L-arginine get a benefit from it," says Oscar C. Marroquin, MD, assistant professor of medicine and assistant director of the Ladies Hospital Aid Society Women's Heart Center at the University of Pittsburgh Medical Center.

Still, researchers haven't identified the exact factors that might make a woman prone to experiencing problems with the small heart vessels. Microvascular dysfunction has been observed among women with traditional risk factors for heart disease, such as advanced age, cigarette smoking, hypertension and high cholesterol. But it is also seen in women without those risk factors.