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EEG

Electroencephalogram; Brain wave test

Last reviewed: February 10, 2014.

An electroencephalogram (EEG) is a test to measure the electrical activity of the brain.

How the Test is Performed

Brain cells communicate with each other by producing tiny electrical signals, called impulses.

An EEG measures this activity. The test is done by an EEG technologist in your doctor's office or at a hospital or laboratory.

The test is done in the following way:

  • You lie on your back on a bed or in a reclining chair.
  • Flat metal disks called electrodes are placed all over your scalp. The disks are held in place with a sticky paste. The electrodes are connected by wires to a recording machine. The machine changes the electrical signals into patterns that can be seen on a monitor or drawn on paper. It looks like wavy lines.
  • You need to lie still during the test with your eyes closed. This is because movement can change the results. You may be asked to do certain things during the test, such as breathe fast and deeply for several minutes or look at a bright flashing light.

If your doctor needs to monitor your brain activity for a longer period, an ambulatory EEG will be ordered. In addition to the electrodes, you will wear or carry a special recorder for up to three days. You will be able to go about your normal routine as the EEG is being recorded.

How to Prepare for the Test

Wash your hair the night before the test. Do not use conditioner, oils, sprays, or gel on your hair. If you have a hair weave, ask your doctor or nurse for special instructions.

Your health care provider may want you to stop taking certain medicines before the test. Do not change or stop taking any medicines without first talking to your health care provider. Bring a list of your medicines with you.

Avoid all food and drinks containing caffeine for 8 hours before the test.

You may need to sleep during the test. If so, you may be asked to reduce your sleep time the night before. If you are asked to sleep as little as possible before the test, do not eat or drink any caffeine, energy drinks, or other products that help you stay awake.

Follow any other specific instructions you are given.

How the Test will Feel

The electrodes may feel sticky and strange on your scalp, but should not cause any other discomfort. You should not feel any discomfort during the test.

Why the Test is Performed

EEG is used to look at your brain activity. It can be used to diagnose or monitor the following health conditions:

EEG is also used to:

An EEG may be done to show that the brain has no activity, in the case of someone who is in a deep coma. It can be helpful when trying to decide if a person is brain dead.

EEG cannot be used to measure intelligence.

Normal Results

Brain electrical activity has a certain number of waves per second (frequencies) that are normal for different levels of alertness. For example, brain waves are faster when you are awake, and slower in certain stages of sleep.

There are also normal patterns to these waves.

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

What Abnormal Results Mean

Abnormal results on an EEG test may be due to:

Risks

An EEG test is very safe. The flashing lights or fast breathing (hyperventilation) required during the test may trigger seizures in those with seizure disorders. The health care provider performing the EEG is trained to take care of you if this happens.

References

  1. Emerson RG, Pedley TA. Clinical neurophysiology: electroencephalography and evoked potentials. In: Daroff RB, Fenichel GM, Jankovic J, Mazziotta JC, eds. Bradley's Neurology in Clinical Practice. 6th ed. Philadelphia, Pa: Elsevier Saunders; 2012:chap 32A.

Review Date: 2/10/2014.

Reviewed by: Joseph V. Campellone, M.D., Division of Neurology, Cooper University Hospital, Camden, NJ. Review provided by VeriMed Healthcare Network. Also reviewed by David Zieve, MD, MHA, Isla Ogilvie, PhD, and the A.D.A.M. Editorial team.

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Copyright © 2013, A.D.A.M., Inc.

What works?

  • EEG for children with complex febrile seizuresEEG for children with complex febrile seizures
    Febrile seizures can be classified as simple or complex. Complex febrile seizures are associated with fever that lasts longer than 15 minutes, occur more than once within 24 hours and are confined to one side of the child's body. It is common in some countries for doctors to recommend an electroencephalograph (EEG) on children with complex febrile seizures. Limited evidence is available to prove or refute the use of an EEG and its timing after complex febrile seizures in children; therefore the authors decided to undertake this systematic review. We attempted to search all possible sources (searches carried out on 17 October 2013) but were unable to find even a single randomised controlled trial (regarded as a gold standard for trial design) to address the issue. We concluded that no base of high‐quality evidence can be found to support or refute the use of an EEG and its timing after complex febrile seizures in children. Therefore well‐designed randomised trials are required. We intend to update this review regularly with the hope that new randomised studies will be reported in the future.
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