Health Encyclopedia

EEG

Definition

An electroencephalogram (EEG) is a test to detect problems in the electrical activity of the brain.

Alternative Names

Electroencephalogram; Brain wave test

How the test is performed

Brain cells communicate with each other by producing tiny electrical impulses. In an EEG, this faint electrical activity is measured by putting electrodes on the scalp.

The test is performed by an EEG technician in your health care provider's office, at a hospital, or at an independent laboratory. You will be asked to lie on your back on a bed or in a reclining chair.

The technician will apply between 16 and 25 flat metal discs (electrodes) in different positions on your scalp. The discs are held in place with a sticky paste. The electrodes are connected by wires to an amplifier and a recording machine.

The recording machine converts the electrical impulses into patterns that can be seen on a computer screen, as well as stored on a computer disk. Before computers, the activity was printed on paper. In either case, the electical activity looks like a series of wavy lines. You will need to lie still with your eyes closed because any movement can alter the results.

You may be asked to do certain things during the recording, such as breathe deeply and rapidly for several minutes or look at a bright flashing light.

How to prepare for the test

You will need to wash your hair the night before the test. Do not use any oils, sprays, or conditioner on your hair before this test.

Your health care provider may want you to stop taking certain medications before the test. Do not change or stop medications without first talking to your health care provider.

You should avoid all foods containing caffeine for 8 hours before the test.

Sometimes it is necessary to sleep during the test, so you may be asked to reduce your sleep time the night before. If you're asked to sleep as little as possible before the test, don't consume any caffeine, energy drinks, or other products that help you stay awake.

How the test will feel

This test causes no discomfort. Although having electrodes pasted onto your skin may feel strange, they only record activity and do not produce any sensation. No significant electricity passes from the electrode into your skin.

Why the test is performed

EEG is used to help diagnose if you're having seizures and if so, what type. An EEG is also used to find the causes of confusion, and to evaluate head injuries, tumors, infections, degenerative diseases such as Alzheimer's disease, and abnormal changes in body chemistry that affect the brain.

It is also used to evaluate problems with sleep (see: sleep disorders) and to investigate periods of unconsciousness. The EEG may be done to show that the brain has no activity, in the case of someone in a deep coma. It can be helpful when trying to decide if someone's brain is dead.

EEG canNOT be used to measure intelligence.

Normal Values

Brain electical activity has certain frequencies (the number of waves per second) that are normal for different levels of consciousness. For example, brain waves are faster when you are awake, and slower when you're sleeping. There are also normal patterns to these waves. These frequencies and patterns are what the EEG reader looks for.

What abnormal results mean

Abnormal results on an EEG test may mean:

An abnormal structure in the brain (such as a brain tumor)
Attention problems
Cerebral infarction (tissue that is dead because of a blockage of the blood supply)
Drug or alcohol abuse
Encephalitis (inflammation of the brain)
Head injury
Hemorrhage (abnormal bleeding caused by a ruptured blood vessel)
Migraines (in some cases)
Seizure disorder (such as epilepsy or convulsions)
Sleep disorder (such as narcolepsy)

Note: a normal EEG does not mean that a seizure did not occur.

What the risks are

The procedure is very safe. If you have a seizure disorder, a seizure may be triggered by flashing lights or by fast breathing ( hyperventilation) that is done during the test. The health care provider performing the EEG is trained to take care of you if this happens. It may be difficult to get the paste out of your hair, but it should come out after a few washings with regular shampoo.

Review Date:5/29/2008
Reviewed By:Daniel B. Hoch, PhD, MD, Assistant Professor of Neurology, Harvard Medical School, Department of Neurology, Massachusetts General Hospital. Also reviewed by David Zieve, MD, MHA, Medical Director, A.D.A.M., Inc.

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