An embolism is not the same thing as a stroke, but it is one of the most common causes of one. When a blood clot or other debris travels through the bloodstream and lodges in a brain artery, it blocks blood flow and triggers what doctors call an embolic stroke, a specific type of ischemic stroke. Not all embolisms cause strokes (they can block arteries in the lungs, legs, or other organs), and not all strokes are caused by embolisms. But the overlap between the two is significant enough that the terms often get tangled together.
How an Embolism Causes a Stroke
A stroke happens when part of the brain loses its blood supply. About 87% of strokes are ischemic, meaning a blockage cuts off blood flow. That blockage can form in two ways. In a thrombotic stroke, a fatty plaque inside a brain artery ruptures, triggering a clot that grows right at that spot. In an embolic stroke, the clot forms somewhere else in the body, breaks loose, and travels until it gets stuck in a narrower brain artery.
The distinction matters because it changes both the treatment and the prevention strategy. A thrombotic stroke points to diseased arteries in or near the brain. An embolic stroke points to a problem upstream, often in the heart, meaning doctors need to find and treat the original source to prevent it from happening again.
Where Emboli Come From
The heart is the most common source. Cardioembolic strokes account for roughly 20% to 30% of all ischemic strokes, and atrial fibrillation is the leading culprit. In atrial fibrillation, the upper chambers of the heart quiver instead of contracting fully. Blood pools in those chambers, clots form, and pieces eventually break free and travel to the brain.
But clots can also originate in arteries outside the brain. Plaque buildup in the carotid arteries (the large vessels running up each side of the neck) can shed fragments that travel upward into brain arteries. Less commonly, clots form in leg veins and reach the brain through a structural heart defect, a route called paradoxical embolism.
Paradoxical Embolism
About one in four people have a small opening between the two upper chambers of the heart called a patent foramen ovale, or PFO. It’s a remnant from fetal development that usually stays closed. But certain actions that briefly raise pressure on the right side of the heart, like coughing, straining, or bearing down, can push the opening apart. If a blood clot from the legs happens to be passing through at that moment, it can cross into the left side of the heart and travel directly to the brain. Up to 14% of patients with a related defect called an atrial septal defect experience this kind of crossover event.
Not All Emboli Are Blood Clots
Most embolic strokes involve blood clots, but other materials can block a brain artery too. Fat globules released after a bone fracture, air bubbles introduced during surgery or trauma, and (rarely) fragments of tumor tissue can all act as emboli. One extreme example is amniotic fluid embolism during childbirth, where fluid enters the mother’s circulation and can cause widespread obstruction. As many as 85% of survivors of amniotic fluid embolism have lasting neurological damage, primarily from oxygen deprivation rather than a traditional clot.
Embolic Strokes vs. Other Stroke Types
Embolic strokes tend to come on suddenly and without warning, because the clot arrives in the brain all at once rather than building gradually. Thrombotic strokes sometimes announce themselves with brief episodes of symptoms (called transient ischemic attacks) in the days or weeks beforehand, as a growing plaque intermittently restricts flow.
Cardioembolic strokes also tend to be more severe. Research comparing stroke subtypes found that patients with cardioembolic strokes had larger areas of brain damage and were more likely to have blockages in major brain arteries (about 51% of cardioembolic patients had a large vessel occlusion, compared to 27% of patients whose embolic source was unknown). Only about 41% of cardioembolic stroke patients achieved a good functional recovery, compared to 69% of those with embolic strokes from an undetermined source, who tended to be younger and have smaller clots.
Finding the Source After a Stroke
When someone has an ischemic stroke, one of the first priorities is figuring out whether it was embolic and, if so, where the embolus came from. The workup typically includes brain imaging with CT or MRI to confirm the stroke, imaging of the neck and brain arteries with ultrasound or angiography to look for diseased vessels, an echocardiogram to examine the heart’s chambers and valves, and an electrocardiogram to check for atrial fibrillation.
Despite this testing, roughly one third of ischemic strokes remain unexplained after the initial evaluation. These are called cryptogenic strokes, and a substantial portion are suspected to be embolic. For these patients, doctors often recommend extended heart rhythm monitoring, sometimes with a small implantable device worn for weeks or months, to catch intermittent episodes of atrial fibrillation that a standard test might miss.
Treatment in the First Hours
Embolic strokes are treated the same way as other ischemic strokes in the emergency phase: the goal is to restore blood flow as quickly as possible. For larger clots blocking major brain arteries, a procedure called mechanical thrombectomy can physically retrieve the clot. Current guidelines support this procedure up to 24 hours after symptoms begin in selected patients, though outcomes are best when treatment happens within the first six hours.
The long-term treatment strategy diverges based on the source. If atrial fibrillation caused the embolism, blood thinners that target the clotting process are the standard prevention strategy. If a diseased carotid artery was the source, the focus shifts to managing that blockage. If a PFO is identified in a younger patient, closure of the opening with a small device may be considered. Identifying the source isn’t just an academic exercise. It directly determines what keeps the next stroke from happening.