Stroke is a disease that affects the blood vessels that supply blood to the brain.

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

There are two main types of stroke. One (ischemic stroke) is caused by blockage of a blood vessel; the other (hemorrhagic stroke) is caused by bleeding. Bleeding strokes have a much higher fatality rate than strokes caused by clots.

What is ischemic stroke?

Ischemic stroke is the most common type. It accounts for about 88 percent of all strokes. 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 fatty buildups, called atherosclerosis.

When the blood clot forms within an artery of the brain, it's called cerebral thrombotic stroke. These often occur at night or first thing in the morning. Another distinguishing feature is that very often they're preceded by a transient ischemic attack. This is also called a TIA or "warning stroke."

What is a cerebral embolism?

A wandering clot (an embolus) or some other particle that forms away from the brain, usually in the heart, may also cause an ischemic stroke. This is called cerebral embolism. 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 (AF). AF is a disorder found in about 2.2 million Americans. It's responsible for 15–20 percent of all strokes. In AF, the heart's two small upper chambers (the atria) quiver instead of beating effectively. Some blood isn't pumped completely out of them when the heart beats, so it pools and clots. When a blood clot enters the circulation and lodges in a narrowed artery of the brain, a stroke occurs.

What is hemorrhagic stroke?

A subarachnoid hemorrhage occurs when a blood vessel on the brain's surface ruptures and bleeds into the space between the brain and the skull (but not into the brain itself).

A cerebral hemorrhage occurs when a defective artery in the brain bursts, flooding the surrounding tissue with blood. 

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 made worse by high blood pressure. Aneurysms aren't always dangerous, but if one bursts in the brain, they cause a hemorrhagic stroke.

When a cerebral or subarachnoid hemorrhage occurs, the loss of a constant blood supply means some brain cells no longer can work. Accumulated blood from the burst artery also may put pressure on surrounding brain tissue and interfere with how the brain works. Severe or mild symptoms can result, depending on the amount of pressure.

The amount of bleeding determines the severity of cerebral hemorrhages. In many cases, people with cerebral hemorrhages die of increased pressure on their brains. But those who live tend to recover much more than people who've had strokes caused by a clot. That's because 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 goes away. Then the brain may regain some of its former function.