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Causes  By Mayo Clinic staff

A stroke disrupts the flow of blood through your brain and damages brain tissue. There are two chief types of stroke. The most common type — ischemic stroke — results from blockage in an artery. The other type — hemorrhagic stroke — occurs when a blood vessel leaks or bursts. A transient ischemic attack (TIA) — sometimes called a ministroke — temporarily disrupts blood flow through your brain.

Ischemic stroke

Almost 90 percent of strokes are ischemic strokes. They occur when the arteries to your brain are narrowed or blocked, causing severely reduced blood flow (ischemia). Lack of blood flow deprives your brain cells of oxygen and nutrients, and cells may begin to die within minutes. The most common ischemic strokes are:

Thrombotic stroke.

This type of stroke occurs when a blood clot (thrombus) forms in one of the arteries that supply blood to your brain. A clot usually forms in areas damaged by atherosclerosis — a disease in which the arteries are clogged by fatty deposits (plaques). This process can occur within one of the two carotid (kuh-ROT-id) arteries of your neck that carry blood to your brain, as well as in other arteries of the neck or brain.

Embolic stroke.

An embolic stroke occurs when a blood clot or other debris forms in a blood vessel away from your brain — commonly in your heart — and is swept through your bloodstream to lodge in narrower brain arteries. This type of blood clot is called an embolus. It's often caused by irregular beating in the heart's two upper chambers (atrial fibrillation). This abnormal heart rhythm can lead to pooling of blood in the heart and the formation of blood clots that travel elsewhere in the body.

Hemorrhagic stroke
Hemorrhage is the medical term for bleeding. Hemorrhagic stroke occurs when a blood vessel in your brain leaks or ruptures. Brain hemorrhages can result from a number of conditions that affect your blood vessels, including uncontrolled high blood pressure (hypertension) and weak spots in your blood vessel walls (aneurysms). A less common cause of hemorrhage is the rupture of an arteriovenous malformation (AVM) — an abnormal tangle of thin-walled blood vessels, present at birth. There are two types of hemorrhagic stroke:

Intracerebral hemorrhage.

In this type of stroke, a blood vessel in the brain bursts and spills into the surrounding brain tissue, damaging cells. Brain cells beyond the leak are deprived of blood and are also damaged. High blood pressure is the most common cause of this type of hemorrhagic stroke. Over time, high blood pressure can cause small arteries inside your brain to become brittle and susceptible to cracking and rupture.

Subarachnoid hemorrhage.

In this type of stroke, bleeding starts in an artery on or near the surface of the brain and spills into the space between the surface of your brain and your skull. This bleeding is often signaled by a sudden, severe "thunderclap" headache. This type of stroke is commonly caused by the rupture of an aneurysm, which can develop with age or be present from birth. After the hemorrhage, the blood vessels in your brain may widen and narrow erratically (vasospasm), causing brain cell damage by further limiting blood flow to parts of your brain.

Transient ischemic attack (TIA)

A transient ischemic attack (TIA) — sometimes called a ministroke — is a brief episode of symptoms similar to those you'd have in a stroke. The cause of a transient ischemic attack is a temporary decrease in blood supply to part of your brain. Many TIAs last less than five minutes.

Like an ischemic stroke, a TIA occurs when a clot or debris blocks blood flow to part of your brain. But unlike a stroke, which involves a more prolonged lack of blood supply and causes permanent tissue damage, a TIA doesn't leave lasting effects because the blockage is temporary.

Seek emergency care even if your symptoms seem to clear up. If you've had a TIA, it means there's likely a partially blocked or narrowed artery leading to your brain, putting you at a greater risk of a full-blown stroke that could cause permanent damage later. And it's not possible to tell if you're having a stroke or a TIA based only on your symptoms. Up to half of those whose symptoms appear to go away are actually having a stroke that's causing brain damage.

Blood vessels rupturing in the brain.

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