If a person is believed to be having seizures, they may be referred to hospital for an EEG.
An EEG is a simple test that records the tiny electrical signals or impulses produced by the brain while it is sending and receiving messages to and from the rest of the body.
These electrical impulses are amplified by the Electroencephalogram machine and recorded on paper or computer as an Electroencephalograph. The graph gives an indication of how the brain is working by showing whether the nerve cells in the brain are giving off signals in a normal, regular way or in an abnormal, irregular manner. This information may be used in order to contribute to the classification or guide treatment choices.
At the beginning of the test a number of electrodes, which are either small metal discs or padded metal rods will be placed on the surface of the scalp. The electrodes are held in place with gel or by a rubber cap. The procedure is painless and takes up to 15 minutes. During this time the patient may ask any questions they have about the EEG.
The person is then connected to the recording machine. During the test itself, which usually lasts for 30 minutes, it is important to relax and keep still so that signals from the muscles in the body don’t obscure the brain impulses. Deep breathing for a few minutes, and opening and closing the eyes a few times will be used to help any abnormalities in the brain to show up more clearly.
As a small number of people have photosensitive epilepsy, meaning that flickering lights may trigger their seizures, a strobe light test may be given at the end of the EEG. If the flashing light produces an abnormal pattern on the EEG, the light is switched off immediately to prevent a seizure from occurring. At the end of the test, the electrodes are removed from the hair; any remains of gel can be washed away later with shampoo. Babies and children will not be expected to do all of the above: the test is adapted for them. Sometimes more detailed EEG recordings may be taken, with the aim of highlighting any abnormalities not picked up in the routine EEG.
This EEG allows the activity of the brain to be recorded for several hours, days or even weeks. It allows the person to move around and continue with normal daily activities while recording takes place. The electrodes used are stick-on electrodes, which are hidden under the hair and held in place by glue. The wires are connected to a small recording machine, which is worn on a belt round the waist. The person will be asked to keep a diary with times of meals, exercise, sleep and any seizures; these are then related to the patterns seen on the EEG.
This test involves both EEG and video recordings being made at the same time. The test is carried out in hospital and may last a day or up to a week; the purpose is to try and record a seizure on video as well as the corresponding EEG.
The electrodes are attached to the scalp as in the ambulatory EEG, and the person is able to move freely around the room within view of the video camera. The diagnosis of epilepsy is a clinical one, which means that the most important information is a description of what has happened during the person’s seizures. The results of the EEG may support a diagnosis, but will not give a definite indication one way or another.
The EEG only gives information about the electrical activity of the brain during the period of recording. Many people, including those who go on to have a diagnosis of epilepsy, have a normal result from a routine EEG between seizures. This is because their brain shows no patterns characteristic of epilepsy when they are not having seizures. Therefore a negative EEG does not necessarily rule out epilepsy. However in some people the brain may show continuous activity characteristic of epilepsy, even between seizures. A small number of people who do not have epilepsy do show irregular electrical activity in the EEG, and so a positive finding does not always imply that a person has epilepsy. The EEG results will be considered alongside other information to provide a fuller picture of what is going on.
SLEEP DEPRIVED EEG
For this test it may be requested that the person stay awake all or most of the night before the test. The test is performed as for a routine EEG but it is hoped the person will sleep during the test.
DRUG-INDUCED SLEEP EEG
In some cases sleep is induced by a mild sedative medication, which is given before the EEG, to cause the person to sleep during the recording itself. The sedative medication will always be checked to ensure it is safe to be taken by the individual, and that there is no interaction with other medication that they are taking.
HOW SHOULD I PREPARE FOR AN EEG?
Your hair should be clean and free from hair gel or other substances.
DO I NEED TO STOP TAKING MY MEDICATION FOR THE TEST?
Medication should never be stopped unless specifically instructed by your consultant.
IS THE TEST PAINFUL?
No, the test is harmless and non-invasive, which means that the skin is not broken.
IS THIS SHOCK TREATMENT?
No. The EEG is a diagnostic test and does not pass electricity through the brain; it only records electrical impulses from the brain.