When the blood supply to the brain is interrupted or blocked for any reason, the consequences are usually dramatic. Control over movement, perception, speech, or other mental or bodily functions is impaired, and consciousness itself may be lost. Disruptions of blood circulation to the brain may result in a stroke -- a disorder that occurs in two basic forms, both potentially life-threatening.
Clots near the brain. About three-quarters of all strokes are due to blockage of the oxygen-rich blood flowing to the brain. Called ischemic strokes, they are triggered by either a thrombus (a stationary clot that forms in a blood vessel) or an embolus (a clot that travels through the bloodstream and becomes lodged in a vessel).
This type of stroke may be preceded by a brief transient ischemic attack, or TIA -- an episodes of inadequate blood flow that may produce these symptoms:
Sudden numbness or weakness on one side of the body
An inability to talk
Double or blurred vision in one eye
Sudden dizziness or falling
A TIA usually lasts 15 minutes or less. Because these may be signs of an impending stroke, take them seriously and see your doctor immediately.
With a TIA, circulation and the vital oxygen supply are quickly restored and lasting brain damage is usually avoided. With any stroke, however, if the interruption of blood flow lasts long enough to kill brain cells, it can produce irreversible damage.
Bleeding in the brain. The second basic type of stroke is a cerebral hemorrhage, or bleeding in the brain. It occurs when a brain aneurysm ruptures or when a weakened or inflamed blood vessel in the brain starts to leak. An aneurysm is a pouch that balloons out from a weakened spot on the wall of an artery. As blood flows into the brain, the buildup of pressure may either kill the tissue directly or destroy cells by impeding normal circulation to the affected region. This typically produces an excruciating headache, sometimes followed by loss of consciousness.
In contrast to ischemic strokes, which are generally survived, massive bleeding strokes are fatal about 40% of the time within the first month.