Electroencephalography (EEG) Introduction
The electroencephalogram (EEG) is a measure of brain waves. It is a readily available test that provides evidence of how the brain functions over time.
- The EEG is used in the evaluation of brain disorders. Most commonly it is used to show the type and location of the activity in the brain during a seizure. It also is used to evaluate people who are having problems associated with brain function. These problems might include confusion, coma, tumors, long-term difficulties with thinking or memory, or weakening of specific parts of the body (such as weakness associated with a stroke).
An EEG is also used to determine brain death. It may be used to prove that someone on life-support equipment has no chance of recovery.
Scientists first captured and recorded brain waves in dogs in 1912. By the 1950s the EEG was used commonly throughout the United States.
Very few risks are associated with an EEG. The patient may be asked not to take certain seizure or antidepressant medications 1 to 2 days before having an EEG. This may make the person more prone to having a seizure, which is exactly what the doctor would like to measure. During an EEG, the doctor may encourage the things that stimulate seizures, such as deep breathing or flashing lights, so that he or she can see what happens in the brain during the seizures.
The patient will be given instructions when the EEG is scheduled.
If the patient routinely takes seizure medication to prevent seizures, antidepressants, or stimulants, he or she may be asked to stop taking these medications 1 to 2 days before the test.
The patient may be told not to consume caffeine before the test.
The patient should avoid using hair styling products (hairspray or gel) on the day of the exam.
It is prudent to have someone take the patient to the EEG location, especially if he or she has been asked to stop taking seizure medications.
If the patient is having a sleep EEG, he or she may be asked to stay awake the night before the exam.
During the EEG Procedure
A standard noninvasive EEG takes about 1 hour. The patient will be positioned on a padded bed or table, or in a comfortable chair. To measure the electrical activity in various parts of the brain, a nurse or EEG technician will attach 16 to 20 electrodes to the scalp. The brain generates electrical impulses that these electrodes will pick up. To improve the conduction of these impulses to the electrodes, a gel will be applied to them. Then a temporary glue will be used to attach them to the skin. No pain will be involved.
The electrodes only gather the impulses given off by the brain and do not transmit any stimulus to the brain. The technician may tell the patient to breathe slowly or quickly and may use visual stimuli such as flashing lights to see what happens in the brain when the patient sees these things. The brain's electrical activity is recorded continuously throughout the exam on special EEG paper.
- Sleep EEG: During a specialized sleep EEG, the patient will be placed in a room that encourages relaxation and asked to fall asleep while the brain's electrical activity is recorded. The sleep EEG will last about 2 to 3 hours.
- Ambulatory EEG: During a specialized ambulatory (moving from place to place, walking) EEG, the electrodes are placed on the patient's scalp and attached to a portable cassette recorder. The patient will be allowed to go home and resume normal activities while the EEG continuously records. The ambulatory EEG typically lasts 24 hours.
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After the EEG Procedure
After the test is complete, the technician will remove the electrodes. The patient will be instructed when to resume any medications. The patient generally will be ready to go home immediately following the test. No recovery time is required. Because the patient may be at risk for a seizure (especially if their medication has been withheld prior to the EEG), they should have someone take them home, and they should not drive. The patient should avoid activities that may harm them if a seizure occurs, until they have resumed their seizure medication for an adequate length of time. These precautions do not necessarily apply to the person who was not on any seizure medication prior to the EEG.
The doctor or technician will tell the patient when and how they will learn the results of their EEG. Probably the patient will either get a telephone call or have an office visit. If the patient has not learned the test results within 2 to 3 weeks and doesn't yet have a follow up appointment, call the doctor.
When to Seek an EEG
If a patient's typical seizure pattern changes, he or she should notify the doctor. If the patient experiences any changes in mental status, or any new weakness, numbness, or tingling, he or she should tell the doctor immediately.
If the patient experiences new onset seizures, an increase in the usual number of seizures, an altered mental status, or a new loss of function, he or she should go to the nearest emergency department for treatment.
When the EEG is finished, the results are interpreted by a neurologist (a doctor who specializes in the nervous system). The EEG records the brain waves from various locations in the brain. Each area produces a different brain wave strip for the neurologist to interpret.
When examining the recordings, the neurologist looks for certain patterns that represent problems in a particular area of the brain.
For example, certain types of seizures have specific brain wave patterns that the trained neurologist recognizes.
Likewise, a normal brain has a specific brain wave pattern that the trained neurologist recognizes.
The neurologist must look at all recorded tracings, decide what is normal and what is not, and determine what the abnormal tracings represent.
The neurologist forwards the EEG results to the doctor who ordered the test, and the patient is then notified as arranged.
Reviewed on 11/17/2017
Medically reviewed by Avrom Simon, MD; Board Certified Preventative Medicine with Subspecialty in Occupational Medicine
Aminoff, Michael J. Electrodiagnosis in Clinical Neurology. 3rd ed. Harcourt Health Sciences Group, 1992.
Emerson, R. G. and T. A. Pedley. Neurology in Clinical Practice Vol. 1. Butterworth-Heinemann, 2000.
Luders, H. and S. Noachtar. Atlas and Classification of Electroencephalography. WB Saunders, 1999.
Niedermeyer, E. and F. L. Da Silva. Electroencephalography: Basic Principles, Clinical Applications and Related Fields. Lippincott Williams & Wilkins, 1999.