Usually your brain waves will be recorded for about 20 to 40 minutes. It takes about the same amount of time to prepare for the recording, so the entire procedure usually takes 1 to 1½ hours. You can help by washing your hair the night before or the day of the test. Don't use any conditioners, hair creams, sprays, or styling gels.

The test is performed by an EEG technologist. First the technologist will measure your head so that the electrodes can be placed in the correct position. A wax crayon, which can be easily washed off later, is used to mark the points on your scalp where the electrodes should be placed.

Usually the electrodes are held in place by a paste that can be washed off easily when the test is over. The technologist will probably scrub each position on your scalp with a mildly abrasive cream before applying the electrodes. This will help improve the quality of the recording.

The EEG is recorded in a quiet room, which often is dimly lit. The technologist may ask you to open and close your eyes several times, may shine flashing lights into your eyes, or may ask you to breath rapidly or deeply. If you have any symptoms of a seizure during the test, tell the technologist.

If you fall asleep briefly during the session, that's fine. It may even be helpful, because an EEG obtained during while you are asleep may provide extra information. Sometimes doctors ask patients to stay up the entire night before an EEG is performed because sleep deprivation can increase the likelihood that waves indicating epilepsy will be recorded.

After the EEG recording is done, the technologist will remove the electrodes from your scalp, and you can go home and wash the paste out of your hair. The doctor usually reads the EEG after you have left and will tell you the results later.

What can the EEG show?
The EEG shows patterns of normal or abnormal brain electrical activity. Some abnormal patterns may occur with a number of different conditions, not just seizures. For example, certain types of waves may be seen after head trauma, stroke, brain tumor, or seizures. A common example of this type is called "slowing," in which the rhythm of the brain waves is slower than would be expected for the patient's age and level of alertness.

Certain other patterns indicate a tendency toward seizures. Your doctor may refer to these waves as "epileptiform abnormalities" or "epilepsy waves." These include spikes, sharp waves, and spike-and-wave discharges. Spikes and sharp waves in a local area of the brain, such as the left temporal lobe, indicate that partial seizures are beginning in that area. Primary generalized epilepsy, on the other hand, is suggested by spike-and-wave discharges that are widely spread over both hemispheres of the brain, especially if they begin in both hemispheres at the same time.

Topic Editor: Steven C. Schachter, M.D.,
Last Reviewed: 12/15/06