Health A-Z

Medical Content Created by the Faculty of the
Harvard Medical School

Seizure

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

Brain tumors

• Infections, especially meningitis and encephalitis

• Genetic conditions, including tuberous sclerosis

• Structural abnormalities in the brain's blood vessels

• Eating or drinking toxic substances

• Metabolic problems

Epilepsy is a condition in which seizures continue to occur without any trigger. It is the second most common neurological disease in the United States, affecting approximately 1% of Americans. Each year, about 200,000 people are diagnosed with epilepsy in the United States.