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EEG

Electroencephalogram; Brain wave test; Epilepsy - EEG; Seizure - EEG

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

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How the Test is Performed

The test is done by an electroencephalogram (EEG) technologist in your health care provider's office or at a hospital or laboratory.

The test is done in the following way:

If your provider 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 3 days. You will be able to go about your normal routine as the EEG is being recorded. Or, your provider may ask you to stay overnight in a special EEG monitoring unit where your brain activity will be monitored continuously. Video recording may be done at the same time to see what you are doing during the various electrical signals.

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 provider for special instructions.

Your 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 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

Brain cells communicate with each other by producing tiny electrical signals, called impulses. An EEG measures this 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 provider performing the EEG is trained to take care of you if this happens.

Related Information

Epilepsy
Confusion
Head injury - first aid
Sleep disorders
Decreased alertness
Seizures
Brain tumor - children
Brain abscess
Encephalitis
Narcolepsy
Cerebral arteriovenous malformation
Benign positional vertigo
Aneurysm in the brain
Delirium tremens
Creutzfeldt-Jakob disease
Delirium
Dementia
Dementia due to metabolic causes
Febrile seizures
Bilateral tonic-clonic seizure
Loss of brain function - liver disease
Hepatorenal syndrome
Labyrinthitis
Metastatic brain tumor
Optic glioma
Focal seizure
Frontotemporal dementia
Alzheimer disease
Multiple system atrophy - parkinsonian type
Syphilitic aseptic meningitis

References

Deluca GC, Griggs RC, Johnston C. Approach to the patient with neurologic disease. In: Goldman L, Cooney KA, eds. Goldman-Cecil Medicine. 27th ed. Philadelphia, PA: Elsevier; 2024:chap 366.

Hahn CD, Emerson RG. Electroencephalography and evoked potentials. In: Jankovic J, Mazziotta JC, Pomeroy SL, Newman NJ, eds. Bradley and Daroff's Neurology in Clinical Practice. 8th ed. Philadelphia, PA: Elsevier; 2022:chap 35.

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Review Date: 1/23/2023  

Reviewed By: Joseph V. Campellone, MD, Department of Neurology, Cooper Medical School of Rowan University, Camden, NJ. Review provided by VeriMed Healthcare Network. Internal review and update on 02/23/24 by David C. Dugdale, MD, Medical Director, Brenda Conaway, Editorial Director, and the A.D.A.M. Editorial team.

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