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Neurophysiology
A guide for patients and carers
What is an EEG?
The many nerve cells that make up the brain produce continuous electrical activity when a person is awake, asleep or even in a coma. This can be recorded using small metal discs called electrodes, which are placed on the scalp. The electrical signals are then amplified by specialised equipment to produce what is seen in the EEG tracing (see figure 2 in printed bound version or pdf).
The signals show up on the EEG tracing as wavy lines, representing the fluctuations in electrical activity from moment to moment. Doctors can gain a great deal of information about the workings of the brain by examining these tracings.
What can I expect during an EEG?
After the EEG technician has asked you a few questions about your medical history, he or she will explain what will happen during the test. The whole procedure normally takes one to one-and-a-half hours, with the recording itself usually running for about 30-45 minutes. The EEG is carried out either with you sitting up in a comfortable chair or lying on a couch.
The first step is to apply the electrodes - usually about 20 in all - to the scalp. These are placed in standard positions, according to an internationally agreed convention. The technician will begin by measuring around the head with a tape measure to determine the position for each electrode, which he or she will mark with a skin pencil.
The skin where each electrode is to be placed will be cleansed to ensure that the electrical contact is good enough to allow the weak signals from the brain to be recorded properly. You will usually find that your appointment letter asks you to come for your EEG with ‘clean, dry hair’. This is not meant to be insulting. The request is made because hair products and even the hair’s natural oils can make it difficult for the technician to get a good electrode contact with the scalp.
The electrodes, which are like little cup-shaped discs, are usually stuck in place with a special paste which also helps to conduct the electrical signals. Some of them may have a special conducting jelly put in them before they are stuck onto the scalp. The technicians are very skilled at doing this and it is not uncomfortable. When everything is ready, the technician will ask you to sit or lie in as relaxed a state as you can. This can be quite difficult if you’re feeling nervous in unfamiliar surroundings, but once the test is underway, people usually find that they are gradually able to settle down. It is quite important to relax, since a tense person will have tense muscles around the face and forehead and these will produce electrical signals of their own. The electrodes on the scalp will then pick these signals up too, and they will obscure the EEG and make the recording less useful.
During the recording, you will be encouraged to close your eyes, relax and drowse, since this may give more information than if the recording takes place when you are fully awake. Despite being in strange surroundings, people often drop off to sleep during an EEG.
The technician will ask you to open or close your eyes from time to time. This is so that they can observe certain changes in the brainwaves. The brainwaves of someone sitting quietly with their eyes closed doing nothing in particular have a characteristic appearance. In this state, the alpha rhythm will commonly be recorded from the back of the head. This is one of the brain’s ‘resting rhythms’. It may become slowed or disappear altogether in many conditions affecting the brain, such as infections, coma or dementia.
When the eyes are opened, the alpha rhythm may either disappear altogether or become less prominent. The technician will be testing for this as he or she gets you to open and close your eyes.
As well as asking you to open and close your eyes, the technician is also likely to carry out so-called activation procedures. These are methods that are known to
reveal abnormalities that might not otherwise be seen. The first of these procedures is hyperventilation or overbreathing. The technician will ask you to breathe more deeply than usual, taking regular deep breaths in and out for about three minutes. Most people will tolerate this quite well, although some may feel slightly light-headed or giddy.
Overbreathing in this way will commonly produce a change in the brain’s electrical activity and may bring out abnormalities not otherwise seen in the EEG. In children with absence seizures, overbreathing may well provoke a fit. (The words ‘fit’ and ‘seizure’ mean the same thing, and both terms are used in this information). Observing the EEG during the fit will allow the diagnosis to be confirmed. Occasionally, overbreathing may provoke a more severe fit, such as a generalised convulsion, although this is uncommon. However, technicians are well aware of this possibility and will watch the EEG - and the person undergoing the test - closely during overbreathing.
In people with some medical conditions - such as bad asthma or other breathing difficulties, or those who have suffered a recent heart attack or stroke - overbreathing may be unwise and will not be attempted.
The second activation procedure used routinely is photic stimulation. In this, a quite bright strobe light is flashed in front of the person at different speeds - producing an effect very similar to the strobe lighting in a disco. This is usually done with the eyes open and then closed. Some people find that this makes them feel slightly peculiar, but most find it doesn’t trouble them. Flashing lights are known to produce a standard response in the brain, but in some people they provoke abnormal responses, including epileptic fits.
People with photosensitive epilepsy may find that fits are brought on by things such as disco strobe lighting or sitting too close to a flickering TV screen. Photic stimulation during the EEG will help identify people who are photosensitive, and these people will then be advised to avoid the kinds of situations that could provoke a seizure. As with overbreathing, the technician will keep a very close eye on you and your EEG during photic stimulation.
Does an EEG have any after effects?
A routine EEG is a straightforward test and there are no after effects. After the test you can go home or back to work and carry on as usual.
Is any special preparation needed beforehand?
No, and you should continue with any anti-epilepsy treatment or other drugs which you have been prescribed unless your doctor tells you otherwise.
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Contents
- Introduction
- What is clinical neurophysiology?
- Who works in a neurophysiology department?
- What will happen in the clinical neurophysiology department?
- What is an EEG?
- Why is my doctor sending me for a sleep EEG?
- Can an EEG be carried on yound children and babies?
- What will the EEG show in someone with epilepsy?
- What is an ambulatory EEG?
- When is video-telemetry used?
- Can the EEG help with the decision about possible surgery for epilepsy?
- What are evoked potentials?
- What is evoked potential testing used for?
- What can I expect during the test?
- What is EMG and nerve conduction studies?
- EMG in measuring electrical activity of the muscles
- Summary
- Other organisations that may be able to help
