Electroencephalogram (EEG): About This Test

What is it?

An electroencephalogram (EEG) lets a doctor see the electrical activity of your brain.

You will have small pads or patches attached to different places on your head. These are called electrodes. Wires connect the electrodes to a computer.

The computer records the activity of the brain. This looks like wavy lines on the computer screen or on paper.

Why is this test done?

The test is often used to diagnose epilepsy. It helps a doctor know what types of seizures are happening. An EEG can also check brain activity in people with sleep disorders. It can also help a doctor know why a person passed out (lost consciousness).

How do you prepare for the test?

• Tell your doctor ALL the medicines, vitamins, supplements, and herbal remedies you take. Some may increase the risk of problems during your test. Your doctor will tell you if you should stop taking any of them before the test and how soon to do it.
• Do not eat or drink anything with caffeine in it for 12 hours before the test. This includes cola, energy drinks, and chocolate.
• 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 you wash your hair.
• Your doctor may ask you not to sleep the night before the test or to sleep for only about 4 or 5 hours. This is because some types of brain activity can only be seen if you are asleep. If your doctor asks you to get less sleep than normal, plan to have someone drive you to and from the test.

How is the test done?

• You will lie on your back on a bed or table. Or you might relax in a chair with your eyes closed.
• A technologist will attach the electrodes to different places on your head. Or you might get a cap with fixed electrodes on it.
• You will lie still with your eyes closed. The technologist will tell you not to talk unless you need to.
• The technologist may ask you to:
  • Breathe deeply and rapidly. This is called hyperventilating.
  • Look at a bright, flashing light called a strobe.
  • Go to sleep. If you can't fall asleep, you may get medicine to help you.

How long does the test take?

The test will take about 1 to 2 hours.

What are the risks of an electroencephalogram (EEG)?

An EEG is a very safe test. The electrical activity of your brain is recorded. But no electrical current is put into your body. An EEG is not the same as electroshock (electroconvulsive) therapy.

If you have a seizure disorder such as epilepsy, the flashing lights may trigger a seizure. Or a seizure may happen if you hyperventilate. If it happens, the technologist is trained to take care of you during the seizure.

What happens after the test?

• You will probably be able to go home right away. It depends on the reason for the test.
• You can go back to your usual activities right away.
• If paste was used to hold the electrodes in place, it may stick in your hair after the test. You will have to wash your hair to get it out.

When should you call for help?

Watch closely for changes in your health, and be sure to contact your doctor if:

• You have any problems that you think may be from the test.
• You have any questions about the test or have not received your results.