Health A-Z

Medical Content Created by the Faculty of the Harvard Medical School


What Is It?

In a thrombotic stroke, a blood clot (thrombus) forms inside one of the brain's arteries. The clot blocks blood flow to a part of the brain. This causes brain cells in that area to stop functioning and die quickly.

The blood clot that triggers a thrombotic stroke usually forms inside an artery that already has been narrowed by atherosclerosis. This is a condition in which fatty deposits (plaques) build up inside blood vessels.

Thrombotic strokes can affect large or small arteries in the brain. Strokes that affect large arteries block flow to greater portions of the brain. These strokes tend to cause the most disability

When a thrombotic stroke occurs in a small artery, the artery is usually one that is deep within the brain. This stroke is more specifically named a lacunar stroke. Lacunar strokes often have minimal symptoms because only a small part of the brain is affected.

Another type of stroke embolic stroke is also caused by a blood clot. However, in an embolic stroke, the blood clot forms somewhere else in the body. It then travels through the bloodstream to the brain artery. The blood clot usually comes from the heart.

At first, it can be impossible for a doctor to determine which type of stroke a person is having. That is because the symptoms can be identical.

A much less common cause of thrombotic stroke is migraine headache. In especially severe cases, a migraine headache can cause a brain artery to go into spasm for a long time. This can allow a blood clot to form.

About half to two-thirds of all strokes are thrombotic strokes. The factors that increase your risk of having a thrombotic stroke are:

  • A family history of thrombotic stroke

  • Diabetes

  • High blood pressure (hypertension)

  • High cholesterol

  • Smoking

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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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