A patent foramen ovale, or PFO, is a small flap-like opening between the two upper chambers of your heart that never fully closed after birth. About 25% of all adults have one, making it one of the most common heart variations in the general population. Most people never know it’s there, but in certain situations, a PFO can allow blood clots to cross from one side of the heart to the other and travel to the brain, causing a stroke.
Why the Opening Exists in the First Place
Before birth, a baby’s lungs aren’t yet in use. Oxygen comes from the mother through the placenta, so fetal blood doesn’t need to pass through the lungs to pick up oxygen. Instead, a natural opening called the foramen ovale allows oxygen-rich blood to flow directly from the right upper chamber of the heart (right atrium) to the left upper chamber (left atrium), essentially bypassing the lungs entirely.
This works because pressure in the right atrium is higher than in the left during fetal life, pushing the blood through. At birth, the baby takes its first breaths, the lungs expand, and the pressure dynamics flip. The left atrium’s pressure rises above the right, pressing the flap shut like a one-way door closing. Over time, the two tissue layers (called the septum primum and septum secundum) fuse together permanently in most people. In roughly one in four people, that fusion never happens. The flap stays closed most of the time because of the pressure difference, but it can still open under certain conditions. That’s a PFO.
How a PFO Can Cause a Stroke
The primary concern with a PFO is something called paradoxical embolism. Normally, if a small blood clot forms in a vein (say, in your leg), it travels to the right side of the heart and into the lungs, where it either gets filtered out or causes a pulmonary issue. With a PFO, that clot can take a detour. If pressure in the right atrium temporarily rises, for instance during straining, coughing, or bearing down, the flap can open and allow the clot to cross directly into the left atrium. From there, it enters the general circulation and can travel to the brain, blocking an artery and causing a stroke.
This is particularly relevant in younger and middle-aged patients who have a stroke with no obvious explanation, a category doctors call cryptogenic stroke. PFO is a common cause of stroke in this group. Patients with larger PFOs or with an atrial septal aneurysm (extra-floppy tissue in the wall between the chambers) face a higher risk of stroke recurrence.
The Connection to Migraines
People with PFOs, especially those who experience migraines with aura, show a striking overlap. Among migraine patients who have aura (visual disturbances, tingling, or other sensory changes before the headache), the rate of PFO ranges from 46% to 88%, compared to 16% to 35% in migraine patients without aura. The relationship appears to follow a dose-response pattern: larger PFOs and those with more significant blood shunting are associated with more frequent migraine attacks.
The leading theory is that certain chemicals in the blood, including serotonin, normally get filtered or broken down in the lungs. When blood bypasses the lungs through a PFO, these substances reach the brain directly, potentially triggering migraine episodes. Tiny clots crossing through the opening may also cause brief disruptions in brain blood flow that set off a migraine.
Whether closing the PFO helps migraines remains an open question. Observational studies have reported headache resolution in 14% to 85% of patients after closure, with the strongest improvements in those who had migraine with aura. However, the major randomized controlled trials, including the MIST, PRIMA, and PREMIUM trials, failed to reach their primary goals of showing a statistically significant reduction in headache episodes. One bright spot: the PREMIUM trial did find that frequent migraineurs with aura were significantly more likely to benefit from closure, and 15.4% of patients in the procedure group experienced complete headache cessation. PFO closure is not currently recommended as a migraine treatment outside of clinical trials.
How a PFO Is Diagnosed
The standard screening test is a “bubble study,” a type of echocardiogram where a saline solution is vigorously shaken to create tiny microbubbles and then injected into a vein. You’ll be asked to perform a Valsalva maneuver (bearing down as if straining) during the injection to temporarily raise right-sided heart pressure. If bubbles appear in the left atrium within a few heartbeats, that confirms blood is crossing through an opening. The threshold varies slightly between studies, but generally, the appearance of at least one to three bubbles in the left atrium counts as a positive result.
This can be done with a standard ultrasound probe on the chest (transthoracic echocardiography) or with a probe placed down the esophagus for a closer view (transesophageal echocardiography). The chest-based approach using modern imaging techniques detects about 82% of PFOs with a specificity of 95%, meaning false positives are rare. Performing the Valsalva maneuver during the test increases sensitivity by roughly 40%. The esophageal approach provides even more detailed images and is often used when the chest ultrasound is inconclusive or when closure is being planned.
PFO Closure: What the Procedure Looks Like
For patients who’ve had a cryptogenic stroke and have a PFO that’s moderate to large, closure is generally recommended over long-term medication alone. The procedure is minimally invasive. A catheter is threaded through a vein in the groin up to the heart, and a small device (the AMPLATZER PFO Occluder is one FDA-approved option) is deployed across the opening. The device sits on both sides of the flap like a double-disc sandwich, and over time, heart tissue grows over it, permanently sealing the gap.
Most patients go home within 24 hours. Strenuous activity is typically off-limits for about a month. After the procedure, you’ll take blood-thinning medications for several months to prevent clots from forming on the device while tissue covers it.
Who Qualifies for Closure
Not every PFO needs to be closed. The decision hinges on a combination of factors: the size of the PFO, the presence of an atrial septal aneurysm, and the likelihood that the PFO actually caused the stroke rather than being an incidental finding. Clinicians use risk stratification tools like the PASCAL system, which weighs PFO features against a scoring system called RoPE. Patients with high-risk PFO features and a RoPE score of 7 or higher are considered strong candidates for closure. If your PFO was discovered incidentally and you’ve never had a stroke or other concerning event, closure is generally not recommended.
When Medication Is the Primary Approach
For patients who aren’t candidates for closure, or whose PFO is small and low-risk, the standard approach is blood-thinning medication to reduce the chance of clot formation. This typically involves antiplatelet drugs (which prevent blood cells from clumping together) taken daily on an ongoing basis. In some cases, doctors choose anticoagulant therapy instead, which works on a different part of the clotting process. The choice between these options is individualized based on stroke risk, bleeding risk, and other medical factors.
PFO and Scuba Diving
If you’re a recreational diver, a PFO carries a specific and well-documented risk. During ascent, dissolved nitrogen in the blood forms tiny bubbles. Normally these get filtered in the lungs. With a PFO, nitrogen bubbles can cross into the arterial circulation, reaching the brain, spinal cord, or inner ear and causing decompression sickness. A prospective study found that divers with a right-to-left shunt had a 3.02 times higher incidence of confirmed decompression sickness compared to divers without one. That translates to about 5.5 decompression events per 10,000 dives versus 1.8 per 10,000 dives.
This doesn’t mean you can never dive with a PFO, but it changes the risk calculation. Many diving medicine specialists recommend conservative dive profiles, slower ascent rates, and longer safety stops. Some divers who’ve experienced decompression sickness and are found to have a PFO choose to pursue closure before returning to diving.