Cerebral embolism occurs when a wandering clot (an embolus) or some other particle forms in a blood vessel away from the brain, usually in the heart. The clot is carried by the bloodstream until it lodges in an artery leading to or in the brain, blocking the flow of blood.

The most common cause of these emboli is blood clots that form during atrial fibrillation , a disorder found in some 2-3 million Americans. In atrial fibrillation the two small upper chambers of the heart, the atria, quiver instead of beating effectively. Blood isn't pumped completely out of them when the heart beats, so it pools and clots. Stroke occurs when the blood clot enters the circulation and becomes lodged in a narrowed artery of the brain.