Health Encyclopedia:


Electroencephalogram (EEG)

What is an EEG?

An EEG is a procedure that detects abnormalities in your brain waves, or in the electrical activity of your brain. During the procedure, electrodes consisting of small metal discs with thin wires are pasted onto your scalp. The electrodes detect tiny electrical charges that result from the activity of your brain cells. The charges are amplified and appear as a graph on a computer screen, or as a recording that may be printed out on paper. Your doctor then interprets the reading.


During an EEG, your doctor typically evaluates about 100 pages, or computer screens, of activity. He or she pays special attention to the basic wave form, but also examines brief bursts of energy and responses to stimuli, such as light.

Evoked potential studies are related procedures that also may be performed. These studies measure electrical activity in your brain in response to stimulation of sight, sound, or touch.

Why might I need an EEG?

The EEG is used to evaluate several types of brain disorders. When epilepsy is present, seizure activity will appear as rapid spiking waves on the EEG.


Patients with lesions of their brain, which can result from tumors or stroke, may have unusually slow EEG waves, depending on the size and the location of the lesion.

The test can also be used to diagnose other disorders that influence brain activity, such as Alzheimer's disease, certain psychoses, and a sleep disorder called narcolepsy.

The EEG may also be used to determine the overall electrical activity of the brain (for example, to evaluate trauma, drug intoxication, or extent of brain damage in comatose patients). The EEG may also be used to monitor blood flow in the brain during surgical procedures.

There may be other reasons for your doctor to recommend an EEG.

What are the risks of an EEG?

The EEG has been used for many years and is considered a safe procedure. The test causes no discomfort. The electrodes record activity. They do not produce any sensation. In addition, there is no risk of getting an electric shock.

In rare instances, an EEG can cause seizures in a person with a seizure disorder. This is due to the flashing lights or the deep breathing that may be involved during the test. If you do get a seizure, your healthcare provider will treat it immediately.

Other risks may be present, depending on your specific medical condition. Be sure to discuss any concerns with your healthcare provider before the procedure.

Certain factors or conditions may interfere with the reading of an EEG test. These include, but are not limited to, the following:

  • Low blood sugar (hypoglycemia) caused by fasting
  • Body or eye movement during the tests (but this will rarely, if ever, significantly interfere with the interpretation of the test)
  • Lights, especially bright or flashing ones
  • Certain medications, such as sedatives
  • Drinks containing caffeine, such as coffee, cola, and tea (while these drinks can occasionally alter the EEG results, this almost never interferes significantly with the interpretation of the test)
  • Oily hair or the presence of hair spray

How do I get ready for an EEG?

Ask your healthcare provider to tell you what you should do before your test. Below is a list of common steps that you may be asked to do.

  • Your healthcare provider will explain the procedure to you and offer you the opportunity to ask any questions that you might have about the procedure.
  • You will be asked to sign a consent form that gives your permission to do the procedure. Read the form carefully and ask questions if something is not clear.
  • Wash your hair with shampoo, but do not use a conditioner the night before the test. Do not use any hair care products, such as hairspray or gels.
  • Notify your healthcare provider of all medications (prescription and over-the-counter) and herbal supplements that you are taking.
  • Discontinue using medications that may interfere with the test if your healthcare provider has directed you to do so. Do not stop using medications without first consulting your healthcare provider.
  • Avoid consuming any food or drinks containing caffeine for 8 to 12 hours before the test.
  • Follow any directions your healthcare provider gives you about reducing your sleep the night before the test. Some EEG tests require that you sleep through the procedure, and some do not. If the EEG is to be performed during sleep, adults may not be allowed to sleep more than 4 or 5 hours the night before the test. Children may not be allowed to sleep for more than 5 to 7 hours the night before.
  • Avoid fasting the night before or the day of the procedure. Low blood sugar may influence the results.
  • Based on your medical condition, your healthcare provider may request other specific preparations.

What happens during an EEG?

An EEG may be performed on an outpatient basis, or as part of your stay in a hospital. Procedures may vary depending on your condition and your healthcare provider's practices. Talk with your healthcare provider about what you will experience during your test.

Generally, an EEG procedure follows this process:

  1. You will be asked to relax in a reclining chair or lie on a bed.
  2. Between 16 and 25 electrodes will be attached to your scalp with a special paste, or a cap containing the electrodes will be used.
  3. You will be asked to close your eyes, relax, and be still.
  4. Once the recording begins, you will need to remain still throughout the test. Your healthcare provider may monitor you through a window in an adjoining room to observe any movements that can cause an inaccurate reading, such as swallowing or blinking. The recording may be stopped periodically to let you rest or reposition yourself.
  5. After your healthcare provider does the initial recording while you are at rest, he or she may test you with various stimuli to produce brain wave activity that does not show up while you are resting. For example, you may be asked to breathe deeply and rapidly for three minutes, or you may be exposed to a bright light.
  6. This study is generally performed by an EEG technician and may take approximately 45 minutes to 2  hours.
  7. If you are being evaluated for a sleep disorder, the EEG may be performed while you are asleep.

What happens after an EEG?

Once the test is completed, the electrodes will be removed and the electrode paste will be washed off with warm water, acetone, or witch hazel. In some cases, you may need to wash your hair again at home.

If you took any sedatives for the test, you may be required to rest until the sedatives have worn off. You will need to have someone drive you home.

Skin irritation or redness may be present at the locations where the electrodes were placed, but this will wear off in a few hours.

Your healthcare provider will inform you when you may resume any medications you stopped taking before the test.

Your healthcare provider may give you additional or alternate instructions after the procedure, depending on your particular situation.

Next steps

Before you agree to the test or the procedure, make sure you know:

  • The name of the test or procedure
  • The reason you are having the test or procedure
  • The risks and benefits of the test or procedure
  • When and where you are to have the test or procedure and who will do it
  • When and how will you get the results
  • How much will you have to pay for the test or procedure
Online Medical Reviewer: Fincannon, Joy, RN, MN
Online Medical Reviewer: Weisbart, Ed, M.D.
Last Review Date: 8/29/2013
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