By Mayo Clinic Staff
An electroencephalogram (EEG) is a test that detects electrical activity in your brain using small, flat metal discs (electrodes) attached to your scalp. Your brain cells communicate via electrical impulses and are active all the time, even when you're asleep. This activity shows up as wavy lines on an EEG recording.
An EEG is one of the main diagnostic tests for epilepsy. An EEG may also play a role in diagnosing other brain disorders.
An EEG can determine changes in brain activity that may be useful in diagnosing brain disorders, especially epilepsy. An EEG can't measure intelligence or detect mental illness. An EEG may be helpful for diagnosing or treating the following disorders:
- Epilepsy or other seizure disorder
- Brain tumor
- Head injury
- Brain dysfunction that may have a variety of causes (encephalopathy)
- Inflammation of the brain (encephalitis)
- Sleep disorders
An EEG may also be used to confirm brain death in someone in a persistent coma. A continuous EEG is used to help find the right level of anesthesia for someone in a medically induced coma.
EEGs are safe and painless. Sometimes seizures are intentionally triggered in people with epilepsy during the test, but appropriate medical care is provided if needed.
To prepare for an EEG:
- Wash your hair the night before or the day of the test, but don't use any conditioners, hair creams, sprays or styling gels. Hair products can make it harder for the sticky patches that hold the electrodes to adhere to your scalp.
- Avoid anything with caffeine on the day of the test, because caffeine can affect the test results.
- Take your usual medications unless instructed otherwise.
If you're supposed to sleep during your EEG test, your doctor may ask you to sleep less or even avoid sleep entirely the night before your EEG.
During the test
You'll feel little or no discomfort during an EEG. The electrodes don't transmit any sensations. They just record your brain waves.
Here are some things you can expect to happen during an EEG:
- A technician measures your head and marks your scalp with a special pencil, to indicate where to attach the electrodes. Those spots on your scalp may be scrubbed with a gritty cream to improve the quality of the recording.
A technician attaches flat metal discs (electrodes) to your scalp using a special adhesive. Sometimes, an elastic cap fitted with electrodes is used instead. The electrodes are connected with wires to an instrument that amplifies — makes bigger — the brain waves and records them on computer equipment.
Once the electrodes are in place, an EEG typically takes up to 60 minutes. If you need to sleep for the test, it may take up to three hours.
- You relax in a comfortable position with your eyes closed during the test. At various times, the technician may ask you to open and close your eyes, perform a few simple calculations, read a paragraph, look at a picture, breathe deeply (hyperventilate) for a few minutes, or look at a flashing light.
- Video is frequently recorded during the EEG. Your body motions are captured by a video camera while the EEG simultaneously records your brain waves. This combined recording may help your doctor diagnose and treat your condition.
After the test
After the test, the technician removes the electrodes or cap. If no sedative was given, you should feel no side effects after the procedure, and you can return to your normal routine.
If you used a sedative, it will take a little while for the medication to begin to wear off. Arrange to have someone drive you home. Once home, rest and don't drive for the remainder of the day.
Technicians conduct the test. Doctors trained to analyze EEGs interpret the recording, and the results are sent to the doctor who ordered the EEG. Your doctor may schedule an office appointment to discuss the results of the test.
If possible, bring along a family member or friend. It can be difficult to absorb all of the information provided to you during an appointment. The person who accompanies you may remember something that you forgot or missed.
Write down questions that you want to ask your doctor. Don't be afraid to ask questions or to speak up if you don't understand something your doctor says. Questions you may want to ask include:
- Based on the results, what are my next steps?
- What kind of follow-up, if any, do I need?
- Are there any factors that might have affected the results of this test in some way?
- Will I need to repeat the test at some point?
May 20, 2014
- Longo DL, et al. Harrison's Online. 18th ed. New York, N.Y.: The McGraw-Hill Companies; 2012. http://www.accessmedicine.com/resourceTOC.aspx?resourceID=4. Accessed Feb. 15, 2014.
- Neurological diagnostic tests and procedures. National Institute of Neurological Disorders and Stroke. http://www.ninds.nih.gov/disorders/misc/diagnostic_tests.htm. Accessed Feb. 16, 2014.
- EEG. The Epilepsy Foundation. http://www.epilepsyfoundation.org/aboutepilepsy/Diagnosis/examandtests/eeg.cfm. Accessed Feb. 16, 2014.
- Hirsch LJ, et al. Electroencephalography (EEG) in the diagnosis of seizures and epilepsy. http://www.uptodate.com/home. Accessed Feb. 15, 2014.
- Seizure disorders. The Merck Manuals: The Merck Manual for Health Care Professionals. http://www.merckmanuals.com/professional/neurologic_disorders/seizure_disorders/seizure_disorders.html?qt=eeg&alt=sh#v1037971. Accessed Feb. 16, 2014.