An electroencephalogram (EEG) is a test to measure the electrical activity of the brain.
Electroencephalogram; Brain wave test; Epilepsy - EEG; Seizure - EEG
How the Test is Performed
The test is done by an electroencephalogram technologist in your doctor's office or at a hospital or laboratory.
The test is done in the following way:
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 3 days. You will be able to go about your normal routine as the EEG is being recorded. Or, your doctor may ask you to stay overnight in a special EEG monitoring unit where your brain activity will be monitored continuously.
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 health care 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.
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:
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.
Deluca GC, Griggs RC. Approach to the patient with neurologic disease. In: Goldman L, Schafer AI, eds. Goldman-Cecil Medicine. 26th ed. Philadelphia, PA: Elsevier; 2020:chap 368.
Hahn CD, Emerson RG. Electroencephalography and evoked potentials. In: Daroff RB, Jankovic J, Mazziotta JC, Pomeroy SL, eds. Bradley's Neurology in Clinical Practice. 7th ed. Philadelphia, PA: Elsevier; 2016:chap 34.
Review Date: 2/4/2020
Reviewed By: Amit M. Shelat, DO, FACP, FAAN, Attending Neurologist and Assistant Professor of Clinical Neurology, Stony Brook University School of Medicine, Stony Brook, NY. Review provided by VeriMed Healthcare Network. Also reviewed by David Zieve, MD, MHA, Medical Director, Brenda Conaway, Editorial Director, and the A.D.A.M. Editorial team.
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