Electroencephalogram (EEG)

General

An electroencephalogram (EEG) is a test that measures and records the electrical activity of your brain. Special sensors (electrodes ) are attached to your head and hooked by wires to a computer. The computer records your brain's electrical activity on the screen or on paper as wavy lines. Certain conditions, such as seizures, can be seen by the changes in the normal pattern of the brain's electrical activity.

Why It Is Done

An electroencephalogram (EEG) may be done to:

  • Diagnose epilepsy and see what type of seizures are occurring. EEG is the most useful and important test in confirming a diagnosis of epilepsy.
  • Check for problems with loss of consciousness or dementia.
  • Help find out a person's chance of recovery after a change in consciousness.
  • Find out if a person who is in a coma is brain-dead.
  • Study sleep disorders, such as narcolepsy.
  • Watch brain activity while a person is receiving general anesthesia during brain surgery.
  • Help find out if a person has a physical problem (problems in the brain, spinal cord, or nervous system) or a mental health problem.

How To Prepare

Before the day of the electroencephalogram (EEG) test, tell your doctor if you are taking any medicines. Your doctor may ask you to stop taking certain medicines (such as sedatives and tranquilizers, muscle relaxants, sleeping aids, or medicines used to treat seizures) before the test. These medicines can affect your brain's usual electrical activity and cause abnormal test results.

Do not eat or drink foods that have caffeine (such as coffee, tea, cola, and chocolate) for 8 hours before the test.

Since the electrodes are attached to your scalp, it is important that your hair be clean and free of sprays, oils, creams, and lotions. Shampoo your hair and rinse with clear water the evening before or the morning of the test. Do not put any hair conditioner or oil on after shampooing.

To find certain types of abnormal electrical activity in the brain, you may have to be asleep during the recording. You may be asked not to sleep at all the night before the test or to sleep less (about 4 or 5 hours) by going to bed later and getting up earlier than usual. If your child is going to be tested, try to keep him or her from taking naps just before the test. If you know that you are going to have a sleep-deprived EEG, plan to have someone drive you to and from the test.

How It Is Done

The electroencephalogram (EEG) will be performed by an EEG technologist. The EEG record is read by a doctor who is specially trained to diagnose and treat disorders affecting the nervous system (neurologist).

You will be asked to lie on your back on a bed or table or relax in a chair with your eyes closed. The EEG technologist will attach 16 to 25 flat metal discs (electrodes) to different places on your head, using a sticky paste to hold the electrodes in place.

The electrodes are hooked by wires to a computer that records the electrical activity inside the brain. A machine can show the activity as a series of wavy lines drawn by a row of pens on a moving piece of paper or as an image on the computer screen.

An EEG takes 1 to 2 hours. After the test, you may do your normal activities. But if you were sleep-deprived or given a sleep medicine, have someone drive you home after the test.

How It Feels

There is no pain with an electroencephalogram (EEG).

Paste is used to hold the electrodes, some paste may stay in your hair after the test, so you will have to wash your hair to remove it. Tape is used to hold some electrodes and the tape removal may slightly irritate the skin.

Risks

An electroencephalogram (EEG) is a very safe test. The electrical activity of your brain is recorded, but at no time is any electrical current put into your body. An EEG should not be confused with electroshock (electroconvulsive) therapy.

If you have a seizure disorder such as epilepsy, a seizure may be triggered by flashing lights or by hyperventilation. If this occurs, the technologist is trained to take care of you during the seizure.

What Affects the Test

Reasons you may not be able to have the test or why the results may not be helpful include:

  • Moving too much.
  • Taking some medicines, such as those used to treat seizures (antiepileptic medicines) or sedatives, tranquilizers, and barbiturates.
  • Being unconscious from severe drug poisoning or a very low body temperature (hypothermia).
  • Having hair that is dirty, oily, or covered with hairspray or other hair preparations. This can cause a problem with the placement of the electrodes.