Health A-Z

Medical Content Created by the Faculty of the Harvard Medical School


What Is It?

A seizure is a sudden change in the brain's normal electrical activity. During a seizure, brain cells "fire" uncontrollably at up to four times their normal rate, temporarily affecting the way a person behaves, moves, thinks or feels.

There are two major types of seizures:

  • Primary generalized seizures The seizure affects the entire cerebral cortex, the outer portion of the brain that contains the majority of brain cells. In this type of seizure, the abnormal firing of brain cells occurs on both sides of the brain at about the same time.

  • Partial (focal) seizure The abnormal firing of brain cells begins in one region of the brain and remains in that one region.

Many conditions can affect the brain and trigger a seizure, including:

  • Brain injury, either before or after birth

  • Infections, especially meningitis and encephalitis

  • Eating or drinking toxic substances

  • Metabolic problems

  • High fever (in children)

  • Genetic conditions, including tuberous sclerosis

  • Structural abnormalities in the brain's blood vessels

Seizures are common. A person may have only one seizure without a recurrence. Epilepsy is a condition in which seizures continue to recur.

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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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