THE EEG TEST

by Epilepsy Association of Scotland

An EEG, short for electroencephalogram, is a test which records the patterns of electrical changes (both normal and abnormal), from the millions of nerve cells in the brain. It is used only as an aid in diagnosing epilepsy and other conditions.

A doctor or consultant refers a patient for an EEG test, which is then performed by an EEG technician, in the EEG department of a hospital or special centre. A doctor who is a specialist in assessing EEG tests reports on the findings. Medication should never be altered before or after an EEG test, unless directed by a doctor.

The test monitors any seizures that may occur. The EEG recording helps the doctor to determine whether the seizure is epileptic or not. The test may also be carried out to detect abnormal activity going on in the brain over a prolonged period.

The test

The patient is first asked to lie down, and the technician explains what is going to happen and tries to ensure that the patient feels relaxed. The technician then places small discs, called electrodes, on to the patient's head, either by attaching them with a glue-like substance to the scalp, or by inserting them under a rubber cap placed on the patient's head. The electrodes are then connected by wires to the EEG machine, which is then switched on.

The patient is asked to keep very still, since movement can actually hide what is happening, and is then asked to do simple tasks, for example:

• to open and close his/her eyes several times
• to take deep breaths for approximately three minutes
• to look at a flashing fight, called a stroboscope

The cap on the head may be a little uncomfortable, but the patient does not experience any pain during, or as a consequence of, the test. The EEG machine cannot "read" minds or give an electric shock.

Approximately one hour is required for a routine EEG test. This allows time for the technician to explain the procedure, for the patient to feel reassured, and for the technician to complete the recording. Some tests last longer than one hour.

The EEG machine

The EEG machine is really an amplifier. The electrical changes from the brain are very tiny indeed, and the EEG machine magnifies them sufficiently to produce enough power to move recording pens up and down with each change. The movements are shown as waves on the recording paper. The pattern of these waves changes according to the brain activity, thus showing up any irregularities.

Test frequency

The doctor will decide if and when such a test is needed. There are no hard and fast rules about testing. It may be that after the initial EEG test, the patient may not require to have an EEG test carried out for a long time.

On the other hand, the doctor may wish to monitor the effects of the drugs he prescribes on the EEG machine. If seizures become more frequent or more severe, it may be desirable to carry out an EEG. EEG tests are sometimes carried out on patients who have conditions other than epilepsy.

Ambulatory EEG

"Ambulatory" means to be capable of walking, and an ambulatory EEG test allows the patient to walk around and carry out normal duties while recordings are being made. It is another way of recording electrical changes from the brain. These changes are relayed to a tape recorder strapped to the waist. The recording may last for a minimum of 24 hours, or for several days, thus allowing a considerable amount of information to be collected.

Video telemetry

A video telemetry test involves both EEG and video recordings being made consecutively. It is available at some hospitals and specialist centres.

The purpose of the test is to try and record a seizure on video as well as an EEG trace on paper. Electrodes are attached to the scalp as before, and the person is usually able to walk about the room freely, within view of the video camera.

Results

No test is absolutely foolproof, so that if no unusual activity shows up on an EEG it does not necessarily mean that the patient does not have epilepsy. The test results are considered along with the results of other tests, and with the patient's clinical history, to help the doctor in his diagnosis. It is important to remember that the EEG test is but one aid in the diagnosis of epilepsy.