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What Causes a Stroke?

Ischemic Stroke and Transient Ischemic Attack

An ischemic stroke or transient ischemic attack (TIA) occurs if an artery that supplies oxygen-rich blood to the brain becomes blocked. Many medical conditions can increase the risk of ischemic stroke or TIA.

For example, atherosclerosis (ath-er-o-skler-O-sis) is a disease in which a fatty substance called plaque builds up on the inner walls of the arteries. Plaque hardens and narrows the arteries, which limits the flow of blood to tissues and organs (such as the heart and brain).

Plaque in an artery can crack or rupture (break open). Blood platelets (PLATE-lets), which are disc-shaped cell fragments, stick to the site of the plaque injury and clump together to form blood clots. These clots can partly or fully block an artery.

Plaque can build up in any artery in the body, including arteries in the heart, brain, and neck. The two main arteries on each side of the neck are called the carotid (ka-ROT-id) arteries. These arteries supply oxygen-rich blood to the brain, face, scalp, and neck.

When plaque builds up in the carotid arteries, the condition is called carotid artery disease. Carotid artery disease causes many of the ischemic strokes and TIAs that occur in the United States.

An embolic stroke (a type of ischemic stroke) or TIA also can occur if a blood clot or piece of plaque breaks away from the wall of an artery. The clot or plaque can travel through the bloodstream and get stuck in one of the brain’s arteries. This stops blood flow through the artery and damages brain cells.

Heart conditions and blood disorders also can cause blood clots that can lead to a stroke or TIA. For example, atrial fibrillation (A-tre-al fi-bri-LA-shun), or AF, is a common cause of embolic stroke.

In AF, the upper chambers of the heart contract in a very fast and irregular way. As a result, some blood pools in the heart. The pooling increases the risk of blood clots forming in the heart chambers.

An ischemic stroke or TIA also can occur because of lesions caused by atherosclerosis. These lesions may form in the small arteries of the brain, and they can block blood flow to the brain.

Hemorrhagic Stroke

Sudden bleeding in the brain can cause a hemorrhagic stroke. The bleeding causes swelling of the brain and increased pressure in the skull. The swelling and pressure damage brain cells and tissues.

Examples of conditions that can cause a hemorrhagic stroke include high blood pressure, aneurysms, and arteriovenous (ar-TEER-e-o-VE-nus) malformations (AVMs).

"Blood pressure" is the force of blood pushing against the walls of the arteries as the heart pumps blood. If blood pressure rises and stays high over time, it can damage the body in many ways.

Aneurysms are balloon-like bulges in an artery that can stretch and burst. AVMs are tangles of faulty arteries and veins that can rupture within the brain. High blood pressure can increase the risk of hemorrhagic stroke in people who have aneurysms or AVMs.

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Stroke Clinical Trials

Clinical trials are research studies that explore whether a medical strategy, treatment, or device is safe and effective for humans. To find clinical trials that are currently underway for Stroke, visit

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March 26, 2014 Last Updated Icon

The NHLBI updates Health Topics articles on a biennial cycle based on a thorough review of research findings and new literature. The articles also are updated as needed if important new research is published. The date on each Health Topics article reflects when the content was originally posted or last revised.