Ischemic stroke is caused by a blockage in an artery carrying blood to the brain. The blocked artery cuts off the blood supply to part of the brain. Usually, a blood clot causes the blockage. Sometimes the blockage occurs when an artery becomes too narrow for enough blood to pass through it. This narrowing is called stenosis. Stenosis is caused by a buildup of blood clots and plaque—a mixture of fatty substances, including cholesterol—on the inner walls of the artery.
Problems with blood clotting become more frequent as people age. Certain medications, such as hormonal birth control pills, also can increase the risk of blood clots. Stenosis is caused most commonly by a blood vessel disease called atherosclerosis. In atherosclerosis, plaque builds up on the inner walls of arteries. This causes the artery walls to become thick, hard, and less flexible, and decreases blood flow. The presence of plaque also leads to the formation of blood clots. Eating foods high in saturated fat and cholesterol can make the amount of cholesterol in the body too high. Having high cholesterol can lead to its buildup in plaque in the artery walls. High blood pressure also contributes to atherosclerosis.
Hemorrhagic stroke is caused by a burst artery that spills blood into part of the brain. In a healthy brain, the blood remains in the arteries and does not come into contact with neurons. When blood bursts into the brain during a hemorrhagic stroke, the normal flow of blood to the brain is upset, and the blood interferes with the normal chemical balance that neurons need to function.
High blood pressure can weaken artery walls and make them more likely to break or burst. Poor blood clotting also can increase the risk of bleeding in a hemorrhagic stroke. Poor blood clotting may be caused by blood disorders or medications that decrease clotting. Another cause of hemorrhagic stroke is abnormalities in the structure of blood vessels that form during brain development.1,2