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Electroencephalogram (EEG)

What is an EEG?

An EEG is a test that detects abnormalities in your brain waves, or in the electrical activity of your brain. During an EEG, electrodes are pasted onto your scalp. These are small metal disks with thin wires. They 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 the recording may be printed out on paper. Your healthcare provider then interprets the reading.

During an EEG, your provider typically looks at about 100 pages, or computer screens, of activity. They pay special attention to the basic waveform. But your provider also looks at brief bursts of energy and responses to stimuli, such as flashing lights.

You may also have tests called evoked potential studies. These studies measure electrical activity in your brain in response to stimulation of sight, sound, or touch.

Why might I need an EEG?

An 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.

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People with lesions on their brain, which can result from tumors or stroke, may have very slow EEG waves. It depends 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 disease, certain psychoses, and a sleep disorder called narcolepsy.

An EEG may also be used to determine the overall electrical activity of the brain. For example, it may be used to evaluate trauma, drug intoxication, or the extent of brain damage in a person who is in a coma. Depending on where the injury is, an EEG is one test of many to help decide brain death in critically ill patients. An EEG may also be used to monitor blood flow in the brain or neck's blood vessels during surgery.

There may be other reasons for your provider to advise an EEG.

What are the risks of an EEG?

An EEG has been used for many years. It's considered a safe procedure. The test causes no discomfort. The electrodes record activity. They don't produce any sensation. There is also no risk of getting an electric shock.

In rare cases, 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 right away.

Other risks may be present, depending on your specific health condition. Talk about any concerns with your provider before the procedure.

Certain factors or conditions may interfere with the reading of an EEG test. These include:

  • Low blood sugar (hypoglycemia) caused by fasting

  • Body or eye movement during the test. This will rarely, if ever, majorly interfere with the interpretation of the test.

  • Lights, especially bright or flashing ones

  • Certain medicines, such as sedatives

  • Drinks containing caffeine, such as coffee, cola, and tea. While these drinks can sometimes change the EEG results, they almost never interfere significantly with the interpretation of the test.

  • Oily hair or the presence of hair spray

How do I get ready for an EEG?

Your healthcare provider will explain the procedure to you, and you can ask questions. Ask your provider what you should do before your test. Below is a list of common steps that you may be asked to do:

  • You'll be asked to sign a consent form that gives your permission to do the procedure. Read the form carefully and ask questions if something isn't clear.

  • Wash your hair with shampoo, but don't use a conditioner the night before the test. Don't use any hair styling products, such as hairspray or gel.

  • Tell your provider about all prescription and over-the-counter medicines and any herbal supplements that you're taking.

  • Stop taking medicines that may interfere with the test if your provider has told you to do so. Don't stop using medicines without first talking your provider.

  • Don't consume any food or drinks containing caffeine for 8 to 12 hours before the test.

  • Follow any directions your provider gives you about reducing your sleep the night before the test. Some EEG tests require that you sleep through the procedure, and some don't. If the EEG is to be done 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.

  • Don't fast the night before or the day of the procedure. Low blood sugar may influence the results.

  • Based on your health condition, your provider may request other specific preparations.

What happens during an EEG?

An EEG may be done 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 provider about what to expect.

Generally, an EEG goes like this:

  1. You'll 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 with the electrodes will be used.

  3. You'll be asked to close your eyes, relax, and be still.

  4. Once the recording begins, you'll need to remain still throughout the test. Your provider may monitor you through a window in an adjoining room to watch for 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 provider does the initial recording while you're at rest, they may test you with various stimuli to make brain wave activity that doesn't show up while you're resting. For example, you may be asked to breathe deeply and rapidly for 3 minutes. Or you may be exposed to a bright flashing light.

  6. This study is generally done by an EEG technician. It may take 45 minutes to 2 hours.

  7. If you're being evaluated for a sleep disorder, the EEG may be done while you are asleep.

  8. If you need to be monitored for a longer period of time, you may also be admitted to the hospital for prolonged EEG (24-hour EEG) monitoring.

  9. In cases where prolonged inpatient monitoring isn't possible, your provider may consider doing an ambulatory EEG.

What happens after an EEG?

Once the test is done, 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 need to rest until the sedatives have worn off. You'll 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 medicines you stopped taking before the test.

Your 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

  • What results to expect and what they mean

  • The risks and benefits of the test or procedure

  • What the possible side effects or complications are

  • When and where you are to have the test or procedure

  • Who will do the test or procedure and what that person’s qualifications are

  • What would happen if you did not have the test or procedure

  • Any alternative tests or procedures to think about

  • When and how you will get the results

  • Who to call after the test or procedure if you have questions or problems

  • How much you will have to pay for the test or procedure

Online Medical Reviewer: Luc Jasmin MD
Online Medical Reviewer: Marianne Fraser MSN RN
Online Medical Reviewer: Raymond Kent Turley BSN MSN RN
Date Last Reviewed: 5/1/2021
© 2000-2021 The StayWell Company, LLC. All rights reserved. This information is not intended as a substitute for professional medical care. Always follow your healthcare professional's instructions.