Stroke

I

Introduction

Stroke, brain damage caused by a lack of blood flow to part of the brain. In order to perform its many functions and direct activities throughout the body—from walking to seeing to reasoning—the brain requires a constant supply of energy, provided by the oxygen and nutrients that are delivered by the flowing blood. If blood flow is restricted or cut off at any point between the heart and the brain, portions of the brain relying on blood from the obstructed blood vessel become deprived of oxygen. Brain cells are extremely sensitive to such oxygen deprivation, and if they are deprived of oxygen and nutrients for more than several minutes, they, in effect, starve to death. A stroke results in permanent damage to the brain tissue—and in many cases, permanent disability for the patient. For example, a patient who has had a stroke may develop paralysis on one or both sides of the body; have difficulty with walking, eating, or other daily activities; or lose the ability to speak or understand speech.

Stroke is the third leading cause of death and a leading cause of serious, long-term disability in the United States. According to the American Heart Association, approximately 700,000 Americans suffer a stroke each year; about 25 percent of these strokes are fatal. Someone suffers a stroke every 45 seconds, and every 3.1 minutes someone dies of a stroke. Stroke is responsible for an estimated $40 billion in health-care costs and lost productivity each year.

II

Types of Stroke

Think of a blood vessel as a flexible, cylindrical tube, like a straw. The flow of liquid through a straw can be impeded in two different ways: by an obstruction within the straw, or by compression or pinching from outside the straw. The flow of blood through a blood vessel can also be blocked in these two ways. The two main types of stroke, ischemic stroke and hemorrhagic stroke, correspond to these two mechanisms of flow interruption.

A

Ischemic Strokes

Ischemic strokes, which account for about 80 percent of all strokes, are caused by an obstruction in an artery, generally one of the carotid arteries, the major arteries in the neck that carry oxygen-rich blood from the heart to the brain. The path to an ischemic stroke begins when atherosclerosis, in which fatty deposits build up on the inner wall of an artery, develops in one of the carotid arteries (see Arteriosclerosis). As the fatty deposit grows, it narrows the space through which blood can flow.

---

---

Atherosclerosis does not actually cause ischemic strokes, but it sets up the conditions that make them likely to occur. The actual obstruction that cuts off blood flow in an ischemic stroke is a blood clot. Often the obstruction develops by a process known as thrombosis, the formation of a clot inside a blood vessel. A clot is likely to form at the site of an atherosclerotic deposit because the deposit causes blood to flow in a turbulent, disorderly fashion. This turbulence can cause blood to clot just as it does in response to a wound. When the blood clot, or thrombus, develops at the site of an atherosclerotic deposit and cuts off blood flow to part of the brain, a stroke results.

An ischemic stroke can also be caused by a traveling clot, or embolus (see Embolism). In this case, the clot develops at some other location in the circulation, usually in one of the heart’s chambers. The clot then travels through the bloodstream until it encounters a vessel too small to let it pass through—often a vessel narrowed by atherosclerosis.

A transient ischemic attack (TIA) sometimes precedes an ischemic stroke. In a TIA, also known as a ministroke, strokelike symptoms develop but disappear within five minutes to 24 hours. TIAs can occur when a clot develops at the site of an atherosclerotic deposit but dissolves right away, or an embolism lodges in a narrowed vessel but is soon dislodged on its own. A TIA can also be caused by atherosclerosis alone when the narrowing of blood vessels by atherosclerosis restricts blood flow to part of the brain enough to cause strokelike symptoms. Regardless of the cause, the oxygen deprivation is not severe enough to kill brain cells, and the cells are able to bounce back from their injury. About 10 percent of ischemic strokes are preceded by TIAs.

B

Hemorrhagic Strokes

Hemorrhagic strokes account for the remaining 20 percent of all strokes. They occur when weakened blood vessels within the brain rupture and bleed into the surrounding tissue. The escaped blood can compress or pinch nearby blood vessels, cutting off blood flow and depriving the surrounding tissue of oxygen. Though hemorrhagic strokes occur less frequently than ischemic strokes, they tend to affect larger areas of the brain. Symptoms of a hemorrhagic stroke may be more sudden and more severe, and these strokes carry a greater risk of death than ischemic strokes.

Hemorrhagic strokes can result from an aneurysm, which develops when the wall of a blood vessel weakens and thins, ballooning outward. If left untreated, the aneurysm continues to expand and weaken, increasing the likelihood of rupture. Hemorrhagic strokes can also result from an arteriovenous malformation (AVM), a cluster of enlarged, structurally weak blood vessels that forms during fetal development or possibly at birth. These malformed blood vessels are susceptible to rupture from the normal forces exerted by the flowing blood.

	More from Encarta
	•  Important Notice: MSN Encarta to be discontinued. Learn more. •  Presidential Myths Quiz •  Coffee break: Recharge your brain