British Society for Clinical Neurophysiology
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My EEG Investigation
What does EEG stand for?
EEG stands for electro-encephalo-gram: a recording of the electrical activity of the brain.
What does the investigation involve?
An EEG involves recording electrical signals from the brain or “brain waves” a bit like a cardiograph (ECG) records electrical activity from the heart. Usually a number of electrodes need to be stuck to the scalp either with a paste or with special glue and compressed air to quickly dry it. Gentle scratching of the skin underneath each electrode is required to get a good recording. Your heartbeat will also be recorded using some pads usually attached to your arms. During the recording you will be asked to open and close your eyes. For most of the recording you will be asked to remain relaxed with your eyes closed. If you are sufficiently relaxed , you may be allowed to fall asleep for a short while. At some stage you may be asked to breathe deeply (controlled hyperventilation) for a few minutes. Finally a bright light may be flashed at different speeds in front of you. The person doing the test will explain what he or she wishes you to do at all times. It is important that you mention any concerns you have or if there is something that particularly worries you such as flashing lights. In some departments you may also be recorded on video during your test which can be useful if you have an attack during your test. No part of the test will be carried out without your agreement.
How long does the investigation take?
A routine EEG takes about 1 hour. It takes about 30 minutes to set up the recording and another 30 minutes for the recording itself. Some EEGs such as sleep deprived recordings may take longer (see below)
Why am I having an EEG investigation?
There are many reasons why a doctor may have arranged for you to have an EEG test. If you are not sure then you should ask the doctor who organised the test for you. One of the common reasons for having the test is when epilepsy is suspected or to help decide what type of epilepsy a patient has. In some patients who have had a seizure an EEG will help predict whether they are likely to have more. It is important to realise that not everyone who has a seizure or “a blackout” needs to have an EEG. The EEG will also be normal in some people with epilepsy. The EEG is not a test for epilepsy. The only way of diagnosing epilepsy is by getting a good description of the attacks from the patient and someone who has witnessed it. In difficult cases we may need record EEG for longer periods of time (see videotelemetry sections).
Are there any side effects?
There are no common side effects. Very rarely either the flashing lights or hyperventilation may cause a patient to have a seizure. Usually this will not be worse than the patient’s usual seizures and may help with getting the right diagnosis. However having a seizure may affect your driving license entitlement so if you are worried then please raise this with person performing your EEG or the doctor who has sent you for the test.
I have been sent for a sleep deprived EEG. Why is this?
As the name suggests you are asked to stay awake for either part or all of the night before your test. During the EEG most people fall asleep but do not worry if you stay awake. A sleep deprived EEG can sometimes give more information than a routine EEG. In people with certain types of epilepsy, a sleep deprived EEG may show up more definite abnormalities than a standard EEG. There is always a small risk that you may have a seizure because of sleep deprivation and your doctor will discuss this with you.
Do I need to do anything before my investigation?
Wash your hair the night before but avoid using any hair products such as gels. Having an EEG does tend to make a bit of a mess of hair with the special gel and glue used. If you are having a sleep deprived EEG as an outpatient then you should be accompanied by a responsible adult as you will be too tired to drive safely and there is also the small possibility you might have a seizure. If possible it is best not to come on public transport.
How will I find out the result?
The EEG recording first needs to be looked at by a doctor who has special training in interpreting it, usually a clinical neurophysiologist. In most departments the result will be sent out a few days later to the doctor who referred you for the EEG. Therefore you will NOT normally be given the result on the day of your EEG.