STROKE/ISCHEMIA BACKGROUND
A stroke occurs when the blood supply to part of the brain is suddenly interrupted or when a blood vessel in the brain bursts, spilling blood into the spaces surrounding brain cells. Brain cells die when they no longer receive oxygen and nutrients from the blood or there is sudden bleeding into or around the brain. Stroke is a medical emergency. Prompt treatment of a stroke could mean the difference between life and death. Early treatment can also minimize damage to the brain and potential disability.
In the United States, stroke is a leading cause of adult disability and the third-leading cause of death; only heart disease and cancer cause more deaths annually. Over 160,000 people die each year from stroke in the United States. And stroke is a leading cause of serious long-term disability. About 700,000 strokes occur in the United States each year. About 500,000 of these are first or new strokes. About 200,000 occur in people who have previously had a stroke. Nearly three-quarters of all strokes occur in people over the age of 65. The risk of having a stroke more than doubles each decade after the age of 55. Stroke death rates are higher for African Americans than for whites, even at younger ages. According to the American Heart Association, $57.9 billion will be spent in 2006 in the U.S. on both direct and indirect costs to treat the immediate and long-term consequences of stroke.
About 80 percent of strokes are ischemic strokes. They occur when blood clots or other particles block arteries to the brain and cause severely reduced blood flow (ischemia). This deprives the brain cells of oxygen and nutrients, and cells may begin to die within minutes. The most common ischemic strokes are:
- Thrombotic stroke. This type of stroke occurs when a blood clot (thrombus) forms in one of the arteries that supply blood to the brain. A clot usually forms in areas damaged by atherosclerosis - a disease in which the arteries are clogged by an accumulation of cholesterol-containing fatty deposits (plaques). This process can occur within one of the two carotid arteries in the neck that carry blood to the brain, as well as in other arteries. An ischemic stroke may also be caused by plaques that completely clog or markedly narrow an artery. This narrowing is called stenosis.
- Embolic stroke. An embolic stroke occurs when a blood clot or other particle forms in a blood vessel away from the brain - commonly in the heart - and is swept through the bloodstream to lodge in narrower brain arteries. This type of blood clot is called an embolus. It is often caused by a cardiac arrhythmia such as atrial fibrillation. This abnormal heart rhythm can lead to poor blood flow and the formation of a blood clot.
"Hemorrhage" is the medical word for bleeding. Hemorrhagic stroke occurs when a blood vessel in the brain leaks or ruptures. Hemorrhages can result from a number of conditions that affect blood vessels, including uncontrolled high blood pressure (hypertension) and weak spots in blood vessel walls (aneurysms). A less common cause of hemorrhage is the rupture of an arteriovenous malformation (AVM) - a malformed tangle of thin-walled blood vessels, present at birth. There are two types of hemorrhagic stroke:
- Intracerebral hemorrhage. In this type of stroke, a blood vessel in the brain bursts and spills into the surrounding brain tissue, damaging cells. Brain cells beyond the leak are deprived of blood and are also damaged. High blood pressure is the most common cause of this type of hemorrhagic stroke. High blood pressure can cause small arteries inside the brain to become brittle and susceptible to cracking and rupture.
- Subarachnoid hemorrhage. In this type of stroke, bleeding starts in a large artery on or near the membrane surrounding the brain and spills into the space between the surface of the brain and skull. A subarachnoid hemorrhage is often signaled by a sudden, severe "thunderclap" headache. This type of stroke is commonly caused by the rupture of an aneurysm, which can develop with age or result from a genetic predisposition. After a subarachnoid hemorrhage, vessels may go into vasospasm, a condition in which arteries near the hemorrhage constrict erratically, causing brain cell damage by further restricting or blocking blood flow to portions of the brain.
STROKE IS A NEURODEGENERATIVE DISEASE
The initial acute effects of a stroke occur primarily during the period of vessel occlusion or rupture in which there is a lack of blood flow and oxygen delivery to a localized region of the brain. However, a considerable amount of neuronal damage continues to occur over a much larger area of the brain even after resolution of the vessel occlusion. This neuronal damage and resultant cellular death is due to a neurodegenerative signaling cascade initiated by the reduction in oxygen and nutrient supply. Once started, this cascade may continue for a period of up to three days after the resolution of the stroke.
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