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Strokes occur when blood vessels carrying oxygen and other nutrients to the brain become blocked or suddenly burst. Although doctors can sometimes intervene to reduce stroke damage, in many cases the brain cells served by these blood vessels become starved and begin to die off. After the stroke, the resulting damage may impair behavior or body functions controlled by the affected parts of the brain.

On average, someone in the United States has a stroke about once every 45 seconds, for a total of between 700,000 and 750,000 people a year. Strokes are the third-leading cause of death in the United States behind heart disease and cancer, killing about 275,000 people a year. About 5.4 million stroke survivors live in the United States today.

Strokes are closely linked with cardiovascular diseases like atherosclerosis, heart rhythm disorders, heart attacks, heart valve disorders, and especially high blood pressure. While the number of strokes increased nearly 7 percent from 1992 to 2002, the death rate fell nearly 14 percent during that same period. This increased survival rate is due, in part, to advances in diagnostic techniques and new treatments that allow physicians to intervene with less risk to the patient.

This section has more on:

  • Anatomy
  • Causes
  • Risk factors
  • Need-to-know anatomy

    The brain is divided up into three main parts: the cerebrum, the cerebellum, and the brain stem. In humans, the cerebrum is the largest part of the brain and is responsible for most higher-level brain functions, including language, consciousness, emotions, integration of sensory stimuli, and some motor functions. The second-largest structure in the brain is the cerebellum. This portion of the brain governs balance and some other aspects of movement. Finally, the brain stem, the smallest part, is the most "primitive" part of the human brain and controls many of the body functions that occur automatically, including heartbeat, blood pressure, and wakefulness. Each part of the brain breaks down into smaller regions that control various body functions and behaviors.

    A complex system of arteries, capillaries, and veins provides the brain with blood. Two vertebral arteries run alongside the brainstem and join at the basilar artery. The basilar artery then branches off to the cerebellum and the cerebrum. Branching off the heart, the aortic arch feeds another major pair of arteries, the left and right carotids. At the base of the brain, the vertebrobasilar arteries and the carotid arteries form a structure called the circle of Willis, which gives rise to many of the arteries serving the brain.

    Since specific parts of the brain govern specific behaviors and bodily functions, doctors can usually estimate where the brain damage from a stroke occurred by examining the patient. For example, if a stroke victim can't move his or her left arm, a doctor might assume that the stroke damaged the section of the cerebral cortex governing motor function in the right hemisphere (the right side of the brain controls the left half of the body and vice versa).


    Strokes result from an interruption of blood flow to a portion of the brain and the subsequent death of those brain cells. This interruption of blood flow can occur in two different ways: by a blood clot—a thrombus—that gets lodged in blood vessels or by blood vessels that rupture.

    Those caused by blocked blood vessels, or ischemic strokes, account for roughly 80 percent of all strokes and can arise in one of two ways: a blockage caused by a blood clot that forms inside the brain or in arteries of the neck (a thrombotic stroke) or a blockage resulting from clots that form in another part of the body and travel to the brain (an embolic stroke).

    Thrombotic strokes form most often in arteries damaged by atherosclerosis, a disease in which rough, fatty deposits build up in the walls of the arteries. These deposits gradually narrow the artery, slowing down or even blocking (occluding) the blood flow.

    Embolic stroke occurs when a clot breaks loose from the heart or one of the arteries and travels to the brain. The arteries that the clot is moving through are continuously branching off into smaller vessels, so the clot reaches a point where it can go no farther. It then plugs the vessel, blocking off the blood supply.

    Hemorrhagic stroke occurs when a blood vessel in or around the brain ruptures. Not only does the ruptured blood vessel deprive the brain of oxygen, but the accumulated blood from the ruptured artery also clots, displacing normal brain tissue and disrupting function. Hemorrhagic strokes occur within the brain itself (intracerebral hemorrhage) or in the space between the brain and the skull (subarachnoid hemorrhage).

    One of the causes of hemorrhagic strokes is an aneurysm, an abnormal blood vessel in the brain that contains a weak spot that bulges and finally bursts. Sometimes this weak spot is stretched out over several years by high blood pressure and then bursts suddenly. These cerebral vascular time bombs don't usually cause symptoms before the stroke.

    Another cause of hemorrhagic strokes is a cluster of abnormal blood vessels called a cerebral vascular malformation. One type of vascular malformation, an arteriovenous malformation, occurs when a tangle of blood vessels in or around the brain bypasses the normal brain tissue and directly diverts blood from the arteries to the veins.

    Risk factors

    Some risk factors for stroke, like age, cannot obviously be changed. For more information on these, see below. For information on factors that can be changed, such as smoking, see the prevention section.

    As with most diseases of the circulatory system, your chances of having a stroke increase with age. Gender, race, and family history are also important factors.

    Age: Two thirds of strokes occur in people over the age of 65. In adults over 55, the lifetime risk for stroke is greater than 1 in 6. The prevalence of most types of stroke increases every five to 10 years for people 55 to 85.

    Gender: In most age groups, men are more likely to have strokes than women. However, about 40,000 more women than men have a stroke each year—most likely because women tend to live longer.

    Race: African-American males have the highest risk of stroke. African-Americans have almost twice the risk of a first-ever stroke that Caucasians have. Studies also indicate that Mexican-Americans have a greater incidence of stroke than non-Hispanic whites. Death rates from stroke are highest for African-Americans, followed by Caucasians, Asian and Pacific Islanders, Hispanics, and American Indians.

    Family history: Your risk of stroke is greater if a parent, sibling, or grandparent has had a stroke.

    Personal history: Your risk of stroke is greater if you have already had a stroke.

    Last reviewed on 09/15/2005

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