Electroencephalogram (EEG)

Published by Bupa's Health Information Team, May 2010.

This factsheet is for people who are having an electroencephalogram (EEG), or who would like information about it.

This factsheet is relevant for both adults who are having an EEG and parents of children who are having the procedure. However, for simplicity we refer to 'you' throughout.

An EEG is a test which measures the electrical activity of the surface of the brain. It is used by doctors to help diagnose different brain conditions.

You will meet the EEG technician who will be carrying out your procedure to discuss your care. It may differ from what is described here as it will be designed to meet your individual needs.

About EEG

An EEG is used to help diagnose conditions affecting the brain. It's particularly used to help diagnose different types of epilepsy.

Conditions such as epilepsy affect certain parts of the brain. An EEG is a way of measuring the electrical impulses passing between the nerve cells in the surface of the brain, known as the cortex. It shows where the impulses come from and if they have changed in any way from what is usually expected.

A series of small electrodes are placed at different points on your scalp to measure the electrical activity naturally occurring in your brain. More electrodes may be added to one area of your scalp to record the readings more accurately in that part of the brain. The electrodes are attached to wires, which are linked to a computer so your neurologist can read the activity.

This test should not be confused with a treatment called electroconvulsive therapy that involves electricity being passed down wires to the brain. This can't and doesn't happen during an electroencephalogram.

An EEG is a test to monitor brain activity and isn't painful.

Getting advice about an EEG

This test is used to help your neurologist (a medical professional who specialises in conditions affecting the brain and nervous system) to diagnose your condition. It is very safe.

Preparing for an EEG

Your neurologist will explain how to prepare for your EEG. You should not use hair products or lotions on your scalp before the test. You can use shampoo to wash your hair before the test but don't use conditioner.

Don't stop taking any medication before the test unless you're advised to do so.

An EEG is generally done as an out-patient appointment in a hospital, which means you will go to hospital but don't need to stay overnight. The main part of the test usually lasts about 30 minutes and it takes about 15 minutes for the electrodes to be placed on the scalp. You may need to stay in hospital if you're having an ambulatory or video telemetry EEG. This may be for a few hours, days or weeks.

You can ask your neurologist for advice.

What happens during an EEG

You will sit or lie down and small electrodes are placed on your head. The electrodes may be stuck onto the skin of your scalp using a gel, or a cap may be put over the electrodes. This is to put the electrodes as close to the skin as possible to get an accurate reading.

The electrodes record and increase the size of (amplify) the electrical signals in your brain. These signals are passed down the wires to the electronic equipment that provides a reading.

You will need to keep still during the EEG. The technician will ask you to do certain things such as breathe deeply or open and close your eyes and he or she will monitor the results.

Photosensitivity EEG

You may need to look at a flickering light during the EEG to see if you are sensitive to this type of lighting (photosensitive). It's turned off quickly so there is little risk of having a seizure.

Sleep-deprived and sleep EEG

For this type of EEG, you may be asked to stay awake and not sleep the night before the test so you can sleep during the test. When you fall asleep your brain activity changes, which may increase the likelihood of detecting unusual EEG activity. A mild sedative may be given before the test so you fall asleep more quickly.

Ambulatory EEG

If you are having an ambulatory EEG, you may be asked to wear an EEG recording device for a few hours or even a few days, particularly if the events that you're experiencing are frequent and there is a chance of picking them up on the recording. The recording device is put around your waist and you will need to record all meals, drinks, exercise, sleep and any seizures you have while you wear it.

Video telemetry EEG

You may be admitted to a hospital for a test called video telemetry. This enables the neurologists to observe what happens to you while you're asleep or having a fit by videoing you while wearing EEG electrodes. This is particularly useful to help diagnose unusual attacks as it shows whether your brain impulses change during a seizure.

Sometimes an EEG doesn't help clarify if you have a condition, and it can sometimes record irregular signals when you may not have a condition. You may need more than one EEG. All the results of these tests will be used alongside other information to diagnose any condition.

What to expect afterwards

Arrange for someone to drive you home if you have been given a sedative or were advised not to sleep for the test. Otherwise, you should expect to feel the same as usual after the test.

Recovering from an EEG

You will not feel any pain and don't need to do anything specific after an EEG. If you have taken a sedative it can temporarily affect your co-ordination and reasoning skills, so you must not drive, drink alcohol, operate machinery or sign legal documents for 24 hours afterwards. If you're in any doubt about driving, contact your motor insurer so that you're aware of their recommendations, and always follow the advice of the EEG technician.

Getting enough sleep will reduce any feelings of tiredness.

Getting your results

The results of your EEG will usually be discussed with you at your next appointment with your neurologist.

What are the risks?

There are no risks.

Side-effects

There are no side-effects after having an EEG. You may find some of the glue is still on your scalp once the electrodes have been removed; it will wash away with shampoo.

Complications

There is a small risk of a seizure during the EEG if you have epilepsy and are asked to hyperventilate (breathe rapidly) or if strobe lighting is switched on and you have photosensitive epilepsy. Your neurologist can explain how this risk applies to you.

For answers to frequently asked questions on this topic, see Common questions.

For sources and links to further information, see Resources.