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Causes
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.
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