A stroke is a "brain attack," a sudden interruption of bloodflow to the brain that causes brain damage and loss of function. Stroke is a leading cause of disability and death. Stroke develops suddenly, usually in a matter of minutes, and causes symptoms such as paralysis, numbness or weakness that often affects just one side of the body, confusion, dizziness, speech problems, and loss of vision. The blood supply interruption kills brain cells by depriving them of oxygen and other nutrients found in the blood. Brain cells also become damaged if bleeding occurs in or around the brain, which happens in certain types of strokes. As brain cells die, function is lost in the areas of the brain that they control, such as memory, speech, or movement. The specific functions that will be affected depend on which areas of the brain were damaged and the severity of the stroke Figure 01.
There are two main types of strokes: ischemic and hemorrhagic. An ischemic stroke is by far the more common type, and occurs when a blood clot or other material blocks an artery supplying blood to the brain. A hemorrhagic stroke occurs when an artery in the brain bursts, causing blood to flow into the surrounding tissue. The mortality rate is higher for hemorrhagic stroke than for ischemic stroke, with most deaths occurring within the first 48 hours.
A transient ischemic attack (TIA) is a type of stroke that usually lasts only a few minutes. TIAs are sometimes considered to be "mini-strokes." While TIAs cause no long–term damage, having a TIA puts you at increased risk of acute stroke.
Figure 01. Sites in the Brain where Stroke Can Affect Function
All forms of stroke are caused by a loss of circulation to the brain, disrupting the neurological functions that correspond to the affected area. Blood reaches the brain through two arteries in the front of the neck called the carotid arteries, and two in the back of the neck called the vertebral arteries. Once in the brain, the arteries branch off to different areas that control different neurological functions such as speech, memory, limb movement, eye movement, and coordination. When blood circulation in this network of arteries is diminished or stopped for even a few seconds, brain cells are destroyed by the lack of oxygen and nutrients that the blood normally supplies. This destruction can lead to permanent selective brain damage or death.
Ischemic strokes account for up to 80% of all strokes, and are usually caused by a blood clot that blocks bloodflow to the brain. This blockage can occur in two ways. The first way involves a clot from another part of the body (often the heart) that travels through the blood vessels to the brain artery. This type of clot is called an embolus, and when it becomes stuck in the brain artery it causes what is known as an embolic stroke.
The second type of ischemic stroke is called a thrombotic stroke. In this type of stroke, the blood clot remains attached to the artery wall, and grows until it becomes large enough to interfere with bloodflow.
A buildup of plaque—a mixture of cholesterol and other fatty substances that accumulate in the inner walls of the arteries—can also cause an ischemic stroke. Plaque buildup is known as atherosclerosis, or hardening of the arteries. As artery walls thicken and lose elasticity, blood flow gradually becomes diminished, which can lead to a stroke. However, this hardening does not necessarily mean that a stroke will occur. The arteries can still carry an adequate supply of blood, even when up to 75% of their area has become obstructed. In addition, small connections between the arteries may expand to route blood around the obstruction, thus compensating for the blocked main artery.
Hemorrhagic strokes, classified as an intracerebral hemorrhage or subarachnoid hemorrhage, occur when a vessel ruptures inside the brain. In an intracerebral hemorrhage, the more common type of hemorrhagic stroke, a vessel inside the brain ruptures, and blood leaks into the surrounding brain tissue. In a subarachnoid hemorrhage, bleeding spreads into the area surrounding the brain. The most common cause of the bleeding is a ruptured bulging of the artery wall (aneurysm). Although hemorrhagic strokes account for only about 20% of all strokes, they are responsible for up to 80% of the deaths from stroke.
In addition to atherosclerosis, other health–related conditions or trauma can lead to stroke. High blood pressure, diabetes mellitus, elevated blood cholesterol, carotid artery disease, heart disease, heavy alcohol abuse, cigarette smoking, drug use, and sickle cell disease can all lead to stroke. Head or neck injuries can also produce bleeding similar to that of a hemorrhagic stroke. Neck injuries that put pressure on the carotid or vertebral arteries can trigger a stroke as well.
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