The fossa ovalis is a shallow, oval-shaped depression on the wall between the two upper chambers of your heart. It sits on the right side of the interatrial septum, the tissue dividing the right atrium from the left atrium. In most people it measures roughly 15 mm across and 13 mm top to bottom, though size and shape vary. What makes it interesting is its origin: the fossa ovalis is the sealed remnant of an opening that every human heart relies on before birth.
How It Forms Before Birth
During fetal development, a baby’s lungs are filled with fluid and do very little work. Oxygen arrives through the umbilical cord and placenta instead. To route that oxygen-rich blood efficiently, the fetal heart has a shortcut called the foramen ovale, a one-way flap between the right and left atria. Most blood entering the right atrium passes directly through this opening into the left atrium, skipping the lungs entirely and heading straight out to the body.
The moment changes at birth. A newborn’s first breaths expand the lungs and dramatically increase blood flow through them. All that extra blood returning from the lungs raises pressure inside the left atrium. At the same time, pressure in the right atrium drops because the umbilical cord is clamped and less blood flows in from the body. This pressure shift pushes the thin flap of the foramen ovale against the thicker surrounding rim, sealing it shut. Over weeks to months, tissue grows across the seal and fuses permanently. The smooth, slightly depressed patch it leaves behind is the fossa ovalis.
When the Seal Doesn’t Fully Close
In about 25% of the general population, the flap never fuses completely. The result is a patent foramen ovale, or PFO. A small gap persists in the fossa ovalis, covered by a flap that can open under certain pressure conditions, like coughing, straining, or bearing down. Most people with a PFO never know they have one and never develop symptoms. It is usually discovered incidentally during heart imaging or after a medical event that prompts a closer look.
The concern with a PFO is something called paradoxical embolism. Normally, if a small blood clot forms in a vein, it travels to the right side of the heart and gets filtered by the tiny blood vessels in the lungs. With a PFO, a clot can bypass the lungs entirely by slipping through the gap in the fossa ovalis and entering the left atrium. From there it has a direct path to the brain, potentially causing a stroke. Because the average PFO opening is about 10 mm wide, clots passing through it are often large enough to block a major brain artery or its branches.
Blood clots can also form within the PFO tunnel itself, not just travel through it from elsewhere. Strokes linked to PFO tend to be classified as “cryptogenic,” meaning no obvious cause like an irregular heart rhythm or blocked neck artery can be identified. Certain features raise the risk: a large shunt (detected when more than 20 to 30 tiny bubbles cross to the left atrium during testing), a floppy or aneurysmal septum that bulges 10 mm or more, or the presence of a prominent tissue flap near the opening that directs blood flow toward the fossa ovalis.
The Diving Connection
PFO carries a specific risk for scuba divers. During a dive, nitrogen dissolves into the blood under pressure. As a diver ascends, that nitrogen comes out of solution and forms tiny gas bubbles in the veins. The lungs normally filter these harmless venous bubbles before they reach the arterial side. A PFO, however, can let bubbles cross directly into arterial blood, where they may lodge in the brain, spinal cord, inner ear, or skin, causing decompression sickness.
Divers with a PFO have roughly 2.5 times the overall risk of decompression sickness and four times the risk of the neurological form compared to divers without one. The absolute numbers are still low: about 4.7 neurological decompression sickness cases per 10,000 dives among divers with a PFO. Inner ear symptoms (vertigo, hearing loss) are a particular red flag. In cases where divers present with isolated inner ear decompression sickness, about 80% turn out to have a large PFO.
How Doctors Evaluate the Fossa Ovalis
The standard way to check for a PFO is an echocardiogram combined with a “bubble study.” A provider injects a small amount of agitated saline (which creates microbubbles) into a vein while imaging the heart. If bubbles appear in the left atrium within a few heartbeats, it suggests they crossed through a gap in the fossa ovalis rather than passing through the lungs.
A transesophageal echocardiogram, where a small ultrasound probe is guided into the esophagus behind the heart, provides a closer view. Patients are sometimes asked to perform a Valsalva maneuver (bearing down) during the test to temporarily raise right atrial pressure and coax a PFO open. This can be difficult when a patient is sedated, which limits sensitivity. Newer three-dimensional versions of this imaging can visualize the entire fossa ovalis and surrounding structures in a single view, making it possible to watch bubbles crossing the septum in real time. This technique helps distinguish a true PFO from other types of shunts, like abnormal connections involving the pulmonary veins, and can even reveal additional small holes (fenestrations) in the fossa ovalis that two-dimensional imaging misses.
PFO Closure Procedures
Most PFOs require no treatment. Closure is typically considered after a person has had a stroke attributed to paradoxical embolism, particularly when the PFO is large or accompanied by an aneurysmal septum. It may also be recommended for someone judged to be at elevated risk of a future clot who cannot take blood-thinning medication long-term.
The procedure is catheter-based, meaning it does not require open-heart surgery. A thin tube is threaded through a vein in the leg up to the heart, and a small device is deployed across the gap in the fossa ovalis. The device acts like a double-sided patch, with one disc on each side of the septum. Over time, heart tissue grows over the device and permanently seals the opening. Implantation is successful in about 96% of cases without serious complications.
Physical Features of the Fossa Ovalis
If you could look at the inside of a right atrium, the fossa ovalis appears as a smooth, slightly sunken area surrounded by a raised muscular ridge called the limbus (or annulus). In about 90% of hearts, this rim is visibly elevated. In roughly one in five hearts, a small recess tucks underneath the edge of the limbus. The floor of the fossa is the thinnest part of the entire atrial septum, which is why it serves as the preferred crossing point for catheters during procedures that need to access the left atrium, not just PFO closures but also catheter ablations for certain heart rhythm disorders.
The fossa is oval-shaped in about 82% of hearts, though it can appear round or irregular. In some people, the floor of the fossa is redundant and billowy, bulging back and forth between the atria with each heartbeat. When this bulging exceeds 10 mm, it is classified as an atrial septal aneurysm, one of the features that increases stroke risk when a PFO is also present.