user: What is (are) Stroke ?

assistant: Stroke -- A Serious Event A stroke is serious, just like a heart attack. Each year in the United States, approximately 795,000 people have a stroke. About 610,000 of these are first or new strokes. On average, one American dies from stroke every four minutes. Stroke is the fourth leading cause of death in the United States, and causes more serious long-term disabilities than any other disease. Nearly three-quarters of all strokes occur in people over the age of 65. And the risk of having a stroke more than doubles each decade between the ages of 55 and 85. Stroke occurs in all age groups, in both sexes, and in all races in every country. It can even occur before birth, when the fetus is still in the womb. Learning about stroke can help you act in time to save a relative, neighbor, or friend. And making changes in your lifestyle can help you prevent stroke. What Is Stroke? A stroke is sometimes called a "brain attack." Most often, stroke occurs when blood flow to the brain stops because it is blocked by a clot. When this happens, the brain cells in the immediate area begin to die. Some brain cells die because they stop getting the oxygen and nutrients they need to function. Other brain cells die because they are damaged by sudden bleeding into or around the brain. The brain cells that don't die immediately remain at risk for death. These cells can linger in a compromised or weakened state for several hours. With timely treatment, these cells can be saved. New treatments are available that greatly reduce the damage caused by a stroke. But you need to arrive at the hospital as soon as possible after symptoms start to prevent disability and to greatly improve your chances for recovery. Knowing stroke symptoms, calling 911 immediately, and getting to a hospital as quickly as possible are critical. Ischemic Stroke There are two kinds of stroke. The most common kind of stroke is called ischemic stroke. It accounts for approximately 80 percent of all strokes. An ischemic stroke is caused by a blood clot that blocks or plugs a blood vessel supplying blood to the brain. Blockages that cause ischemic strokes stem from three conditions: - the formation of a clot within a blood vessel of the brain or neck, called thrombosis  - the movement of a clot from another part of the body, such as from the heart to the neck or brain, called an embolism  - a severe narrowing of an artery (stenosis) in or leading to the brain, due to fatty deposits lining the blood vessel walls. the formation of a clot within a blood vessel of the brain or neck, called thrombosis the movement of a clot from another part of the body, such as from the heart to the neck or brain, called an embolism a severe narrowing of an artery (stenosis) in or leading to the brain, due to fatty deposits lining the blood vessel walls. Hemorrhagic Stroke The other kind of stroke is called hemorrhagic stroke. A hemorrhagic stroke is caused by a blood vessel that breaks and bleeds into the brain. One common cause of a hemorrhagic stroke is a bleeding aneurysm. An aneurysm is a weak or thin spot on an artery wall. Over time, these weak spots stretch or balloon out due to high blood pressure. The thin walls of these ballooning aneurysms can rupture and spill blood into the space surrounding brain cells. Artery walls can also break open because they become encrusted, or covered with fatty deposits called plaque, eventually lose their elasticity and become brittle, thin, and prone to cracking. Hypertension, or high blood pressure, increases the risk that a brittle artery wall will give way and release blood into the surrounding brain tissue.