Before birth, the fetal heart has an opening called the foramen ovale between the right and left atria. This allows blood to bypass the lungs and be directed straight to the left side of the circulation, supplying blood to the brain and body before it returns to the placenta. The foramen ovale usually closes spontaneously after birth; however, in approximately 1 in 4 people the foramen ovale remains fully or partially open into adulthood. This is then known as patent foramen ovale.
Most people with patent foramen ovale have no ill effects. However, patent foramen ovale increases the risk of blood clots (for example from deep vein thrombosis in the legs) crossing from the right side into the left side of the heart, and from there into the arterial system where they may block blood vessels. If arteries in the brain become blocked, then a stroke or a transient ischaemic attack occurs. This passage of material from the right of the circulation to the left is called paradoxical embolism.
The optimal treatment for patent foramen ovale in patients who have had a thromboembolic event remains undefined. Medical management with anticoagulation (usually warfarin) or antiplatelet therapy (for example aspirin) is commonly used to reduce the risk of further paradoxical thrombus emboli. Surgical closure of patent foramen ovale is sometimes performed as an adjunct to other open-heart surgery, but is rarely done on its own because of associated morbidity.