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Health A-Z

Medical Content Created by the Faculty of the
Harvard Medical School

What Is It?

A stroke is a brain injury that occurs because the brain's blood supply is interrupted.

The brain's blood supply can be disrupted for different reasons. Doctors generally classify strokes into three categories, depending on the cause:

  • Hemorrhagic stroke Bleeding (hemorrhage) causes this type of stroke. Bleeding can occur within the brain or between the brain and the skull. When bleeding occurs, small blood vessels near the hemorrhage tighten in a spasm. As a result, some brain areas get too little blood flow.

  • A hemorrhagic stroke that occurs within the brain is called an intracerebral hemorrhage. It often is linked to high blood pressure, old age, heavy alcohol use, or the use of cocaine or methamphetamines. A stroke that occurs between the brain and the skull is called a subarachnoid hemorrhage.

  • Hemorrhagic strokes are much less common than strokes caused by clots.

  • Thrombotic stroke A blood clot (thrombus) forms inside one of the brain's arteries. It blocks blood flow. This usually happens inside an artery that has been narrowed by atherosclerosis. Atherosclerosis is the buildup of fatty deposits along the walls of blood vessels.

  • Thrombotic strokes are the most common type of stroke. They account for almost half of all strokes. Thrombotic strokes can affect large or small arteries in the brain. When a thrombotic stroke occurs in a small artery deep within the brain, the stroke is called a lacunar stroke.

  • Embolic strokes In an embolic stroke, a blood clot or other solid mass of debris travels to the brain, where it blocks a brain artery. In many cases a floating blood clot, called an embolus, originates inside the heart. In another type of embolic stroke, the floating debris is a clump of bacteria and inflammatory cells. This type of embolus can form if there is a bacterial infection on the valves of the heart.

In some cases, the type of stroke cannot be clearly determined.

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From Health A-Z, Harvard Health Publications. Copyright 2007 by the President and Fellows of Harvard College. All rights reserved. Written permission is required to reproduce, in any manner, in whole or in part, the material contained herein. To make a reprint request, contact Harvard Health Publications. Used with permission of StayWell.

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