In recent years, patients who have echocardiogramsare often told they have a condition called patent foramen ovale, or PFO. The advice they receive after being given this diagnosis will vary wildly. Some doctors will want to treat them with Coumadin or aspirin to try to prevent blood clots. Others will recommend an invasive procedure to install a special device to close the PFO. Still others will tell the patient that a PFO has no real significance at all, and that no therapy is required.
In this review I will describe what is known about PFO, and will try to place the current controversy regarding its treatment into perspective.
What is a PFO?
In the developing fetus, the foramen ovale is a hole that is normally present in the atrial septum (the thin structure that separates the right atrium from the left atrium), which allows blood to flow freely from the right atrium directly to the left atrium. This flow of blood from the right atrium to the left atrium allows the blood to bypass the developing lungs in the fetus, and is necessary for normal fetal development. At birth, when the baby begins to breathe, the pressure in the left atrium increases, causing a flap of tissue to impose itself over the foramen ovale, effectively closing it. At this point, blood no longer is able to flow from the right to the left atrium. In most individuals, this flap of tissue becomes "tacked down," so the foramen ovale is completely sealed off. However, in about 1 out of 4 normal adults (25%), the tissue flap is not completely tacked down, and relies on the higher pressure in the left atrium to keep the foramen ovale closed. When the pressure in the right atrium becomes intermittently higher than in the left atrium (as can occur, for instance, when coughing), for those moments the foramen ovale is open, and blood can again flow from the right to the left atrium. These patients are said to have a patent foramen ovale, PFO.
PFO is diagnosed by echocardiography. There are different degrees of PFO. In a few cases the PFO is fairly obvious and would be noticed by almost any echocardiographer. More often, special maneuvers are necessary to identify a PFO, including trans-esophageal echocardiography, injecting contrast material into the bloodstream, and even applying positive pressure to the airway through a special breathing apparatus. The harder one tries to identify a PFO the more likely one is to see a PFO (and the higher the incidence of PFO in the studied population.)
In some patients, the flap of tissue that covers the foramen ovale can develop a balloon-like bulge, which is called an atrial septal aneurysm (ASA.) In most patients who have an ASA, a PFO is also present, so these two conditions are generally associated with one another. The ASA and PFO are thus very similar, and it is probably not incorrect to think of an ASA as a slightly exaggerated (and perhaps somewhat more significant) instance of a PFO.
What is the possible significance of a PFO?
The concern with a PFO is that, during those transient episodes when the right atrial pressure is higher than the left atrial pressure, blood can flow from the right atrium to the left atrium. If a blood clot happens to be traveling through the right atrium at that moment, it too can enter the left atrium. From the left atrium the clot can then flow through the left ventricle and onward to any part of the body - if it goes to the brain, it could cause a stroke. Thus, the chief concern regarding a PFO is that it might lead to an increased risk of stroke.
Some investigators have also reported an increased incidence of PFO in patients with migraine headache. There are no plausible physiologic theories as to how a PFO might cause migraines, but the lack of such a theory has not prevented some doctors from recommending PFO closure devices in these patients.
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