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TESTS IN EPILEPSY: ELECTROENCEPHALOGRAPHY (EEG)

The EEG is the principal investigation used in epilepsy. Many patients with epilepsy will have an EEG performed, usually after a clinical diagnosis has been made, and before treatment is started. The EEG detects the brain’s electrical activity by sensitive sensors called ‘electrodes’ which are placed on the scalp; these electrodes detect the normal and abnormal electrical activity of the nerve cells within the brain. Most routine EEGs are recorded with the child or adult awake, but EEGs may be arranged after deprivation of sleep or during sleep (spontaneous or induced by drugs).

All hospitals with neurological or neurosurgical departments and some larger, non-specialized hospitals will have facilities for recording a routine EEG. The procedure is simple and painless and, in the case of a routine EEG takes only about 20-30 minutes to complete. The EEG detects and records the brain’s activity; at no time is there any electrical discharge passing from the equipment to the patient. The EEG should not be confused with electroconvulsive therapy or ECT, which is used to treat depressive illnesses, and has nothing to do with epilepsy.

The recording technician first measures the patient’s head for correct placement of the electrodes, which are then placed according to an international system based on the patient’s head size and on measurements taken from the bridge of the nose, and the bony protuberance at the back of the head. Silver electrodes are fastened to the head with a sticky substance called collodion. Alternative electrodes are gauze pads moistened with a salt solution and secured with a rubber cap. Sometimes the patient’s scalp is gently rubbed beneath the electrodes to reduce the electrical resistance of the skin which improves the recording. Twelve electrodes are used in small infants, 20 in older children and adults. Wires from each electrode are then connected to a junction box (head-box), connected in turn to the amplifiers of the EEG machine by a cable. After amplification, the EEG machine records the signals on tape or disc, or displays them directly by ink-jets, pens, or laser on to paper which moves at constant speed, usually 3 cm/second. It is this paper with the written waves that is known as ‘the EEG’ and which is examined and analysed by doctors. The advantage of recording the electrical signals from the different electrodes on to magnetic tape or disc is that they can be recombined in other ways for subsequent more detailed analysis. They can then of course be displayed on paper again at any subsequent time.

During an EEG the child or adult is asked to lie quite still. This is because movement of any part of the body may obscure, or make it difficult to detect the electrical activity of the brain. The technician also in the course of the recording will ask the patient to open and close the eyes (to look for normal patterns of activity which vary according to whether the eyes are opened or not), to breathe deeply for 3 minutes, and to look at a flashing light. Overbreathing (also called hyperventilation) and the flashing-light test (called photic stimulation) are useful ways of activating or provoking abnormal electrical activity from the brain, and are often important in helping to decide what type of seizure or what epilepsy syndrome a person has.

The appearance of the EEG is dependent upon the age of a patient because the brain is developing and maturing rapidly, particularly from birth to 7 or 8 years of age. Generally speaking, a normal adult EEG pattern is reached by the age of 10-12 years and there is then little change until the age of 60 or 70 years. Doctors who analyse EEGs must have some knowledge and understanding about EEG patterns (normal and abnormal) in infants and children, as well as in adults.

The hallmark or typical EEG finding in a patient with epilepsy between seizures is a ‘spike’ or ‘spike and slow wave’ or ‘sharp wave,. A ‘spike’ is a sudden change in voltage that shows up against the background activities. An example of a very abnormal EEG seen in infants with West syndrome. However, even in patients who have epilepsy these abnormalities are not always seen, and this is why the EEG must not be relied upon to make or exclude a diagnosis of epilepsy. The first 20 minute recording of an adult who has had an undoubted tonic-clonic seizure is normal in 40-50 per cent of cases.

For most people with epilepsy, a routine (20-30 minute) EEG is the only necessary test. However, this is only a short period to record the brain’s electrical activity, and it is unlikely that a clinical attack or seizure will occur in this time. If more information is required, then other types or systems of EEG recording may be performed.

(a) EEG after deprivation of sleep: In this situation a patient is asked to make sure they get only 4-5 hours sleep for two consecutive nights. This encourages the occurrence of seizure discharges. Deprivation of sleep may also lead the patient to drowse or to sleep during the recording, and again this encourages the appearance of abnormal EEG discharge.

(b) Drug-induced sleep EEG: A small dose of a sedative drug may encourage the patient to fall asleep during the recording, and again drowsiness and sleep may show abnormalities which may not be present whilst awake.

(c) Ambulatory EEG monitoring: This is a technique of recording an EEG for not just 20 or 30 minutes but for up to 24 or even 48 hours. The electrodes (six, eight or 12, rather than the twenty electrodes in a routine EEG) are wired up to a small tape recorder (like a Walkman cassette player) which is strapped to the waist. After this the child or adult can leave the EEG department, go home and carry on their normal activities, and then return to the EEG department after 24 hours to have the tape analysed or the tape replaced. This procedure is more likely, by the length of the recording alone, to pick up abnormal electrical activity, and is particularly valuable if the person has a fit or seizure during the 24 hours when the electrical activity is being recorded. The tape can be analysed in a special fast-pace display unit, so the doctor does not have to sit watching the EEG for 24-48 hours!

(d) Depth electrodes: On rare occasions, special depth electrodes are used. These are fine wires inserted under sterile conditions into areas of the brain thought possibly to be the site of origin of seizure discharge. This is an important test in those patients who are being considered for surgical treatment of their epilepsy.

(e) Video-telemetry: This is another way of obtaining an EEG over a longer period of time. In this technique the patient has to stay in a room in the hospital for 24 hours or longer. At the same time as the electrical activity is recorded on the EEG, a video camera records the activities of the patient. In this way it is possible to replay repeatedly both simultaneous video and EEG recordings and observe the pattern of the EEG during an attack or seizure. This provides valuable information on the type of epileptic seizure and from where within the brain the seizure may be starting. If no abnormalities are seen on the EEG during an ‘attack’, then almost certainly the attacks are not epileptic. Video-telemetry is really only of practical benefit if the patient is having frequent attacks, as it is otherwise impractical to keep the patient in hospital attached to expensive equipment on the remote chance that a seizure may occur.

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