How a PFO (Hole in the Heart) Causes Strokes & Migraines

To understand how a small hole in the heart can cause neurological issues like a stroke or a migraine, it helps to think of a patent foramen ovale (PFO) as a “back door” to the brain ^[1]. It is important to know that PFOs are very common—about 1 in 4 people have one, and the vast majority never experience any complications ^[2].

Under normal circumstances, all blood returning from your body’s veins is pumped through your lungs before it goes anywhere else. Your lungs act as a massive filter, trapping small blood clots and breaking down certain chemicals. However, a PFO is a flap-like opening between the upper chambers of the heart (the right atrium and the left atrium). When pressure in the right atrium is high, this flap can push open, allowing blood to cross directly from the right side to the left side ^[3]. This allows venous blood to completely bypass the lungs and be pumped directly out to the body—and straight up to the brain.

The Mechanics of a “Paradoxical” Stroke

When a stroke is caused by a PFO, it is called a paradoxical embolism ^[4]. In the medical world, an “embolism” is a blood clot that travels through the bloodstream and gets stuck, blocking blood flow. It is called “paradoxical” because clots that form in the veins (like in the legs) are supposed to get safely trapped in the vast network of tiny blood vessels in the lungs, where they slowly dissolve without causing harm.

Because of the PFO, a clot can take a shortcut ^[3]. It travels up a vein, enters the right atrium of the heart, and slips through the PFO into the left atrium ^[4]. From there, the heart pumps the clot out into the arteries. If that clot travels up the arteries leading to the neck and brain, it can lodge in a small brain vessel, cutting off oxygen and causing an ischemic stroke ^[5].

This right-to-left shortcut usually happens when the pressure in the right side of your heart temporarily spikes ^[6]. This spike in pressure can be triggered by a Valsalva maneuver—a physical action that involves bearing down, such as coughing hard, lifting a heavy object, straining during a bowel movement, or holding your breath ^[7]. Note: These are just mechanical explanations for how a shortcut can happen. You do not need to restrict normal daily activities or live in fear of coughing or lifting, as the vast majority of these actions never cause an issue. Always follow your doctor’s specific guidance on activity.

Migraines with Aura: The Chemical Bypass

The connection between PFOs and migraines—specifically migraines with aura (migraines preceded by visual or sensory changes)—relies on a similar concept: escaping the lung’s filtration system.

Instead of a large clot causing a stroke, migraines may be triggered by much smaller particles or unfiltered chemicals reaching the brain ^[1]. Researchers believe that serotonin, a chemical messenger that regulates blood vessels, plays a major role ^[8]. Usually, the lungs break down excess serotonin in the blood. If venous blood bypasses the lungs through a PFO, high concentrations of serotonin can reach the brain ^[1].

When these high levels of serotonin hit the blood vessels in the brain, they can cause sudden narrowing (vasoconstriction) and trigger a temporary, reversible wave of altered electrical activity known as cortical spreading depression ^[1]. While this sounds intimidating, it does not cause permanent brain damage; it is simply the physical mechanism that causes the temporary visual or sensory changes you experience during a migraine aura ^[1]. Some studies also suggest that tiny, microscopic blood clots (micro-emboli) or slightly lower oxygen levels in the blood sneaking through the PFO might also irritate the brain and trigger these severe headaches ^[1].

What Makes a PFO Higher Risk?

Not all PFOs are the same, and having a PFO does not guarantee you will ever have a stroke or a migraine. Certain anatomical features make the “back door” more likely to swing open and allow larger amounts of blood to bypass the lungs:

• Large Defect Size: A wider opening simply allows more blood—and potentially larger clots—to pass through ^[9].
• Atrial Septal Aneurysm (ASA): This is a condition where the tissue separating the two upper chambers of the heart is unusually floppy or hyper-mobile, bulging back and forth. Having both an ASA and a PFO significantly increases the likelihood of a paradoxical stroke ^[10][11].
• Eustachian Valve Prominence: This is a small ridge of tissue in the right atrium that can direct the flow of blood coming from the body straight toward the PFO, making it easier for clots or chemicals to cross over ^[9].

Understanding these anatomical features is essential because they help your care team determine whether your specific PFO is a bystander or the true cause of your symptoms.