A stroke is damage to part of the brain caused by interruption to its blood supply or leakage of blood outside the vessel walls. Sensation, movement, or function controlled by the damaged area is impaired. Strokes are fatal in about one third of cases and are a leading cause of death in developed countries.

Certain factors increase the risk of having a stroke. The two most important are high blood pressure, which weakens the walls of arteries, and atherosclerosis, which narrows arteries.

Other factors that increase the risk of a stroke include an irregular heart beat, a damaged heart valve or a recent heart attack. All these conditions can cause blood clots in the heart that may break off and migrate to the brain.

A raised level of red cells in the blood, a high level of fatty substances in the blood, diabetes mellitus, and smoking also increase the risk of stroke by increasing the risk of hypertension and/or atherosclerosis. Oral contraceptives increase the risk of stroke in women under 50.