An ischemic brain attack occurs when a blood vessel in the brain or a vessel leading to the brain becomes blocked. Ischemia is caused by a lack of blood and oxygen in tissue. Ischemic brain attacks account for 80 to 85 percent of all brain attacks. The rest are due to hemorrhages. Two causes of ischemic brain attack have been identified, cerebral thrombosis and cerebral embolism.
Clots block the flow of blood to the brain or inside the brain in both types.
In a cerebral thrombosis, a blood clot forms in a vessel within the brain itself. In a cerebral embolism, the clot travels to the brain from another part of the body, usually from diseased areas of the heart. These types of clots often form due to cardiac arrhythmia. Atrial fibrillation is a common type of arrhythmia associated with a cerebral embolism. During fibrillation blood is not pumped completely out of the atria.