Electroencephalogram (EEG)

An electroencephalogram (EEG) is a test that measures and records the electrical activity of the brain. An EEG uses sensors or electrodes that are attached to the head and connected by wires to a computer. An EEG is a painless test. No electricity is put into your body.

Why an EEG is done

An EEG is done to:

  • detect and locate abnormal electrical activity in the brain
  • identify the location of a suspected brain tumour, inflammation, infection, bleeding or head injury
  • diagnose and monitor diseases such as epilepsy (a seizure disorder), narcolepsy (a sleep disorder) and swelling of the brain
  • monitor the brain during brain surgery

How an EEG is done

An EEG may be done in a hospital, medical clinic or doctor’s office. The appointment takes about an hour with the EEG usually taking 20 to 40 minutes. It may be longer if you need to sleep during the test.

Before the test, you can usually eat and drink, with some limits, and continue to take your medicines as normal. Your doctor will tell you more. Avoid foods that contain caffeine, such as cola and chocolate, for at least 8 hours before the test.

Because electrodes will be attached to your scalp, your hair should be clean, with no hair product of any kind in it. Shampoo and rinse your hair with water the night before or the morning of the test. If you have a hair weave, ask about special instructions.

You will lie on a bed or examination table or relax in a chair with your eyes closed. You need to be still and not speak during the test.

Your scalp will be cleaned and about 20 metal discs, called electrodes or sensors, will be attached to different places on your head. Your head will not need to be shaved. A sticky paste holds the electrodes in place. Some people may wear a cap with fixed electrodes. The electrodes are connected by wires to a machine that amplifies and records the electrical activity inside the brain. The machine records the electrical activity as a series of wavy lines drawn by a row of pens on a moving piece of paper or as an image on a computer screen.

The recording may be stopped from time to time to allow you to take a break to stretch and get back in position.

You may be asked to do certain tasks to observe how the brain responds to different forms of stimulation. These tasks may include breathing deeply and rapidly, looking at a bright, flashing light and sleeping.

You can usually go home and return to normal activities after the EEG. Some paste may need to be washed out of your hair after the test.

What happens if the result is abnormal?

Depending on the result, your doctor will decide if you need more tests, any treatment or follow-up care.

Special considerations for children

Preparing children before a test or procedure can lower anxiety, increase cooperation and help them develop coping skills. Preparation includes explaining to children what will happen during the test, including what they will see, feel, hear, taste or smell.

Preparing a child for an EEG depends on the age and experience of the child. Find out more about helping your child cope with tests and treatments.

Expert review and references