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.
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.
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