What is an EEG test?
An electroencephalogram (EEG) records activity in the brain. Your brain constantly produces tiny electrical signals. During an EEG test, small sensors, called electrodes, are placed on your scalp. The electrodes are connected by wires to an EEG machine. Electrodes are attached using a special paste glue or paste. After the electrodes are removed, you may still have a lot of glue in your hair. Hair conditioner can help to remove the glue afterwards.
The electrodes pick up the electrical signals from your brain and record them on a computer. They only pick up the electrical signals. They don’t interfere with your brain and they don’t cause you any pain.
The electrical signals look like wavy lines that show your brainwave patterns. The EEG test can only show your brainwave patterns at the time the test is carried out. At different times, your brainwave patterns may be different.
A highly trained specialist, called a clinical neurophysiologist, can recognise if your EEG test shows a brainwave pattern that is different to most other people. Some of these patterns point to a strong possibility of epilepsy.
When are EEG test done?
Your doctor might ask you to have an EEG (electroencephalogram) test for one of the following reasons:
- They think you might have epilepsy
- You have epilepsy, and they need to know more about it
- They are unsure whether or not your seizures are due to epilepsy
- You are being considered for epilepsy surgery
- They want to withdraw your epilepsy medicines
Sometimes an EEG is done to check whether someone is in non-convulsive status epilepticus. Non-convulsive status epilepticus can change a person’s level of awareness or cause confusion, but might not be recognised as a seizure without an EEG.
The results of an EEG can help doctors to make the correct diagnosis and decide on the best treatment. They should always be interpreted by a doctor who specialises in reading EEG results. This is because reading an EEG incorrectly is one of the most common reasons for people to be given the wrong diagnosis.
Where are EEG tests done?
An EEG is usually done in hospital in an outpatient clinic. People with epilepsy have told us that they found it helpful to go with someone to the appointment. Some people find the process quite tiring and there is a very small risk you might have a seizure during the test. You may find it useful to plan how you will get home after the appointment.
You might be asked to take a simple piece of portable recording equipment home with you. You will be shown how to operate this.
An EEG test gives information about the electrical activity that is happening in your brain at the time the test is carried out.
Many people with epilepsy only have unusual electrical activity in their brain when they are having a seizure. The rest of the time the brain activity may be entirely normal. So, if your EEG test doesn’t show any unusual activity, it only means that there is no epileptic activity in your brain at the time the test is being done. This doesn’t prove that you don’t have abnormal activity in your brain at other times. And it doesn’t exclude you having epilepsy.
People with some types of epilepsy have unusual electrical activity in their brain even when they are not having a seizure. When they have an EEG test, the results can show certain brainwave patterns that doctors recognise. This information is very helpful for doctors when they are making a diagnosis.
A small number of people have unusual EEG test results, even though they never have seizures and they don’t have epilepsy. These could be caused by other medical conditions, problems with their vision, or brain damage. So, an EEG that shows unusual brainwave patterns doesn’t always mean that you have epilepsy.
Can an EEG test show what type of seizures I have?
When an EEG test picks up unusual electrical activity, the doctor will look at where it is happening in the brain. With focal (partial) seizures the activity is only happening in some areas of your brain. With generalised seizures the activity is more widespread. But unless you have a seizure during the recording the doctor cannot be absolutely sure what type of seizures you have.
Can an EEG test show if there is any damage in my brain?
An EEG test only gives information about the electrical activity in your brain. It doesn’t show if there’s any damage or physical abnormalities in your brain. An MRI can do this.
Will I have to have more than one EEG test?
There are several ways an EEG test can be done. And there are several reasons why you might be asked to have further EEGs.
- If you have an EEG test that doesn’t show any unusual electrical activity in your brain, your doctor might ask you to have another
- It can be helpful, if possible, to have an EEG test at times when you are more likely to have a seizure. For example, this might be early in the morning. For some women, it might be around the time of having a period
Will an EEG test cause me to have a seizure?
There’s a very small risk that you could have a seizure during an EEG test. This could be caused by looking at a flashing light or breathing deeply. These activities are usually part of the test. You will be closely monitored and medical professionals will be there to help if you do have a seizure.
Your doctor might ask you to have less sleep than usual before you have some types of EEG tests. This is done to increase the chances of catching abnormal activity during the test. This might also increase the risk that you would have a seizure around the time of having the test.
Will having a seizure during an EEG test affect my right to drive?
If you hold a driving licence, having a seizure could mean that you have to stop driving until you have been seizure free for 12 months. If you are concerned about the risk of having a seizure, talk to the doctor who has asked you to have the test.
Will I have to give my consent (permission) for an EEG test to be done?
You will usually have to give your consent to having an EEG test done. This is because it is important that you are aware of why the test is done and of its risks, however small they might be. A video recording is usually recorded as part of the EEG test. Your consent will also be needed for this.
If you give your consent, but then change your mind, you can withdraw your consent at any time. The NICE guidelines say you should be given information on the reasons for tests, their results and meaning, the reasons for of specific investigations, and how they will be carried out.
Standard EEG tests
You will usually have a standard EEG test at an outpatient’s appointment at the hospital. During the test, you sit or lie down. You may be asked to breathe deeply for some minutes and also to look at a flashing light. These activities can change the electrical activity in your brain, and this will show on the computer. This can help the doctor to make a diagnosis.
You will be asked to keep as still as possible during the test. Any movement can change the electrical activity in your brain, which can affect the results
Routine outpatient EEG recordings usually take 20 to 40 minutes, although a typical appointment will last about an hour, including some preparation time at the beginning and some time at the end. Other types of EEG recording may take longer. You can go home as soon as the test has been done.
If you don’t remember anything about your suspected seizures it can be very helpful to bring someone with you who has seen them. They will be able to describe what happens for you during a seizure.
There are videos on the internet about having an EEG. If you want to know what it is like for a young patient to have an EEG you might want to look at this video from the Evelina Hospital. The experience for an adult is not much different.
Sleep EEG tests
Your doctor might ask you to have an EEG test while you are asleep. This could be because your seizures happen when you are asleep. Or you may have had a standard EEG test when you were awake, but it didn’t show any unusual electrical activity. When you are asleep, your brainwave patterns change and may show more unusual electrical activity. There are also some types of seizure that mainly happen during sleep.
A sleep EEG test is usually done in hospital, using a standard EEG machine. Before the test, you may be given some medicine to help you go to sleep. The test lasts for one to two hours and you usually go home once you have woken up.
Sleep EEGs can be particularly useful in young children. This is because there are some epilepsy syndromes which are more common in young children, where seizures mainly happen in sleep. In some cases, the brain activity can become constantly abnormal during sleep and this might affect behaviour. This is true in electrical status epilepticus during slow wave sleep (ESES).
For some older people, a sleep EEG can also be useful as brainwave patterns related to focal seizures are more likely to be seen in sleep. Focal seizures are the most common type of seizure in older people.
Sleep-deprived EEG tests
A sleep-deprived EEG test is done when you have had less sleep than usual. Before you have a sleep-deprived EEG test, your doctor may ask you not to go to sleep at all the night before. Or they may ask you to wake up much earlier than you usually do.
When you are tired, there is more chance that there will be unusual electrical activity in your brain. Your doctor might ask you to have this test if you have had a standard EEG test, but it didn’t show any unusual electrical activity. A sleep-deprived EEG can be helpful if you are thought to be having absence, myoclonic or focal (partial) seizures.
The beginning of the sleep-deprived EEG test is the same as a standard EEG test. You may then fall asleep or doze while the EEG is still recording the activity in your brain. The test lasts for a few hours and you usually go home once you have woken up.
Ambulatory EEG tests
‘Ambulatory’ means designed for walking. So you can have an ambulatory EEG test during your normal daily activities. An ambulatory EEG test is designed to record the activity in your brain over a few hours, days or weeks. This means there is more chance that it will pick up unusual electrical activity in your brain or a seizure, than during a standard or sleep EEG test.
An ambulatory EEG uses electrodes similar to those used on a standard EEG test. However, the electrodes that are attached to your head are plugged in to a small machine that records the results. You can carry the machine in a small bag, so you are able to go about your daily business. You don’t usually stay in hospital while the test is being done.
Your doctor will ask you to keep a diary of your activities, such as sleeping and eating, while you are wearing the ambulatory EEG. They will also ask you, or somebody who is with you, to keep a detailed record of any seizures you have. They will then be able to match up what has been happening with the results of your brainwave activity on the EEG test results.
Video-telemetry is the term used to describe a procedure involving an EEG and a video recording. If more information about your seizures is needed and the doctors want to see what happens during your seizures, then there are two options for longer recordings which also include video.
During a video-telemetry test (VT), you need to stay in hospital. A VT involves using a piece of equipment similar to the one used for ambulatory EEG. At the same time, all your activity is recorded by a video camera. The test is usually carried out over a few days. Sometimes your epilepsy medicine is reduced or withdrawn. This is to increase the chances that you will have a seizure that can be recorded.
After the test, doctors can watch the video to see any seizures that you had. They can also look at the EEG results for the time you were having the seizure. This will tell them about any changes to your brainwave patterns at the time of the seizures.
Home video-telemetry / Home long term monitoring (LTM)
Home video-telemetry can be done at home in a similar way to an ambulatory EEG (see above). You will be shown how to set up the equipment, which will include a video recorder.
You will usually have had other types of EEG tests before you are considered for a home VT. Your doctor might ask you to have a home VT for one of the following reasons:
- It’s not clear what type of seizures you have
- Your epilepsy medicine isn’t working well
- There’s a possibility that your seizures are not caused by epilepsy, but something else
- You’re being assessed for epilepsy surgery
Evelina Children’s Hospital has made a film about home video-telemetry. The film shows how the home video-telemetry service works at Evelina. Services at other hospitals may be different.
Some people who are being considered for epilepsy surgery will have a procedure called invasive EEG-telemetry (iEEG). This is a less common type of EEG for people with more complex epilepsy. A neurosurgeon will do an operation to place the EEG electrodes directly onto the surface of the brain or into the brain. The electrodes are called ‘strip’ or ‘grid’ or ‘stereo-EEG’ electrodes.
The reason for iEEG is to find out where exactly your seizures are coming from. Another part of this test is called ‘cortical mapping’. Cortical mapping is done to see exactly which part of your brain is responsible for things like your memory or speech. It is done to reduce the risk of complications after surgery.
How long will it take to get my EEG results?
You will not normally get your EEG test results on the same day. This is because the recordings will need to by studied by the specialist first and then sent to the doctor who requested the tests. It will usually be a few days or weeks before they can discuss your results with you.
If you have any other questions about an EEG test, you could ask your family doctor, epilepsy specialist, epilepsy specialist nurse or the person who will carry out the test.
If you would like to see this information with references, visit the Advice and Information references section of our website. If you are unable to access the internet, please contact our Epilepsy Action Helpline freephone on 0808 800 5050.
Epilepsy Action would like to thank Professor Stefano Seri, Consultant Clinical Neurophysiologist, and Ms Caroline Scott, Senior Clinical Scientist Birmingham Women’s and Children’s NHS Foundation Trust, UK, for their contribution.
Professor Seri and Ms Scott have declared no conflict of interest.
This information has been produced under the terms of Epilepsy Action's information quality standards.
- Updated May 2020To be reviewed May 2023