What causes a stroke (or brain attack)?
Stroke is a cardiovascular disease that affects the blood vessels supplying blood to the brain. It is also sometimes called brain attack.

A stroke occurs when a blood vessel bringing oxygen and nutrients to the brain bursts or is clogged by a blood clot or some other particle. Because of this rupture or blockage, part of the brain doesn't get the flow of blood it needs. Deprived of oxygen, nerve cells in the affected area of the brain can't function and die within minutes. And when nerve cells can't function, the part of the body controlled by these cells can't function either. The devastating effects of stroke are often permanent because dead brain cells aren't replaced.

There are four main types of stroke: two caused by blood clots or other particles, and two by hemorrhage . Cerebral thrombosis and cerebral embolism are by far the most common, accounting for about 70-80 percent of all strokes. They're caused by clots or particles that plug an artery. Cerebral and subarachnoid hemorrhages are caused by ruptured blood vessels. They have a much higher fatality rate than strokes caused by clots.

What is a cerebral thrombosis?
Cerebral thrombosis is the most common type of stroke . It occurs when a blood clot (thrombus) forms and blocks blood flow in an artery bringing blood to part of the brain. Blood clots usually form in arteries damaged by atherosclerosis .

One identifying feature of cerebral thrombotic strokes is that they often occur at night or first thing in the morning, when blood pressure is low. Another is that very often they're preceded by a transient ischemic attack, also called a T.I.A. or "mini-stroke."

What is a cerebral embolism?
Cerebral embolism accounts for from five to 14 percent of all strokes. This type of stroke occurs when a wandering clot (an embolus - EM'bo-lus) or some other particle forms in a blood vessel away from the brain, usually in the heart. The clot is carried by the bloodstream until it lodges in an artery leading to or in the brain, blocking the flow of blood.

The most common cause of these emboli is blood clots that form during atrial fibrillation a disorder found in close to 2 million Americans. In atrial fibrillation the two small upper chambers of the heart, the atria, quiver instead of beating effectively. Blood isn't pumped completely out of them when the heart beats, allowing the blood to pool and clot. About 15 percent of strokes occur in people with atrial fibrillation.

What is a hemorrhage?
A subarachnoid hemorrhage occurs when a blood vessel on the surface of the brain ruptures and bleeds into the space between the brain and the skull (but not into the brain itself). Subarachnoid hemorrhages account for about seven percent of all strokes.

Another type of stroke occurs when a defective artery in the brain bursts, flooding the surrounding tissue with blood. This is a cerebral hemorrhage. About 10 percent of all strokes result from cerebral hemorrhages.

Hemorrhage, or bleeding, from an artery in the brain can be caused by a head injury or a burst aneurysm . Aneurysms are blood-filled pouches that balloon out from weak spots in the artery wall. They're often caused or aggravated by high blood pressure. Aneurysms aren't always dangerous, but if one bursts in the brain, a stroke results.

When a cerebral or subarachnoid hemorrhage occurs, the loss of a constant blood supply means some brain cells can no longer function. Another problem is that accumulated blood from the burst artery may put pressure on surrounding brain tissue and interfere with how the brain functions. Severe or mild symptoms can result, depending on the amount of pressure.

The amount of bleeding determines the severity of cerebral hemorrhages. In 50 percent of the cases, people with cerebral hemorrhages die of increased pressure on their brains. Those who live, however, tend to recover much more than those who've had strokes caused by a clot. The reason is that when a blood vessel is blocked, part of the brain dies - and the brain doesn't regenerate. But when a blood vessel in the brain bursts, pressure from the blood compresses part of the brain. If the person survives, gradually the pressure diminishes and the brain may regain some of its former function.