what is an EEG?

An EEG which is short for ‘electroencephalogram’, is a test which records the patterns of electrical charges from the millions of nerve cells in the brain. This test is often carried out on people who are suspected of having epilepsy as it shows abnormal activity of the sort commonly associated with the condition.

The EEG gives information about the workings of the brain, but not its structure {the CT and MRI scans give this information}.

In patients with generalised seizures {absences, tonic clonic seizures myoclonic jerks} any EEG abnormalities are usually seen on both sides of the brain. In partial epilepsy abnormalities may be seen localised in one particular area.

This test is not to be confused with ECT which is form of treatment in which an electrical current is applied to the head under anaesthetic. During an EEG there is no current administered, merely the recording of tiny electrical signals coming out of the head.

what is an EEG for?

It is used as an aid but only as an aid in the diagnosis of epilepsy and other conditions. The diagnosis is made on the basis of the patients history and any available witnesses as well as from the EEG. More specialised EEG recordings, however, are sometimes helpful if there is diagnostic doubt.

what happens in an EEG?

You are asked to lie down. The technician explains what is going to happen and tries to ensure that you feel relaxed. The technician then places about 20 small discs called ’electrodes’ onto your head, either by attaching them with a glue like substance to the scalp or by inserting them under a rubber cap placed on your head. The electrodes are then connected by wires to the EEG machine which is then switched don.

You are asked to keep very still as movements of the muscles or eyes can cause large signals which may obscure signals from the brain.

You are then asked to do simple tasks such as:Opening and closing your eyes, taking deep breaths. Looking at flashing light called ‘stroboscope’. In most people it is unusual for the flashing lights to produce excess activity at the back of the brain. If the flashing light does produce an abnormal discharge in the EEG, the light is immediately switched off so there is no risk of seizure occurring.

Some people, especially whose seizures occur during sleep or just after waking, may have longer recording taken during natural sleep or during sleep induced by a sleeping tablet.

is the test painful?

NO the EEG is a painless investigation. The cap on the head may be a little uncomfortable but you will not experience any pain during, or as a consequence of the test. The EEG machine cannot ‘read minds’ nor can it give an electrical shock.

where would an EEG be carried out?

In the EEG department of a hospital or special centre.

how long does the EEG last?

Approximately one hour is required for a routine EEG. This allows time for the technician to explain the procedure, for you to feel reassured, and for the technician to complete the recording.

what does the EEG machine do?

The EEG machine is an amplifier. The electrical charges from the brain are magnified by the EEG machine sufficiently to be recorded on paper or on a computer. This provides a permanent record for the EEG Consultant to interpret.

what is an ambulatory EEG

Ambulatory means walking and an ambulatory EEG allows you to walk around and carry out normal duties, daytime activities and sleep while recordings of electrical charges from your head are being made. The EEG is recorded on a small EEG recorder which you wear on a waistbelt. When a seizure occurs, the EEG recording of the event can be viewed subsequently on a special machine in the EEG laboratory. You must keep an account of activities during the recording so that it is possible to time all events accurately and relate them to the EEG recording.

how often will an EEG need to be done?

The doctor will decide if and when such a test is needed. It may be that only one EEG will be required. Sometimes, as 40% of people with epilepsy may have a normal EEG between attacks, several EEG’s may need to be taken at times when the patient is more liable to seizures. It may also be necessary to carry out further EEG’s if seizures become more frequent, or to monitor responses to treatment.

should medication be stopped before an EEG?

There is no need to stop medication before having a routine EEG

However DECISIONS ABOUT ALTERING MEDICATION SHOULD ONLY BE TAKEN BY A DOCTOR.

who gives the results of the EEG?

The hospital doctor or the GP will provide you or your parents with the results.

can the results be wrong?

No test is absolutely foolproof. If no unusual activity shows up on the EEG, it does not necessarily mean that you do not have epilepsy. A single EEG only records brain activity for a short period of time and during this time there may be no abnormality occurring. A normal EEG does not exclude epilepsy and an abnormal EEG does not necessarily indicate epilepsy. However certain patterns are commonly associated with epilepsy especially spikes and waves.