EEG

Your doctor might ask you to have an EEG (electroencephalogram) test if you have epilepsy, or if they think you might have epilepsy. This is a painless test, which records the electrical activity in your brain. The results of an EEG may help doctors when they are making a diagnosis, or when they are making decisions about your treatment.

There are several different types of EEG test. This information will tell you how they work.

What does an EEG test do?

Your brain is constantly producing tiny electrical signals. During an EEG test, electrodes (flat metal discs) are placed on your head. The electrodes pick up the electrical signals from your brain and record them on an EEG machine. 

The electrodes only pick up the electrical signals. They do not affect your brain and they do not cause you any pain.

The EEG machine records the electrical signals from your brain on a computer. They look like wavy lines and these represent 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.

Most people have brainwave patterns that look similar to everyone else’s. Sometimes, the EEG test shows that a person has different brainwave patterns to other people. These are caused by unusual electrical activity in their brain. They can sometimes indicate that the person has epilepsy.

Standard EEG tests

Usually, you have a standard EEG test at an outpatient’s appointment at the hospital. The appointment normally lasts between one and one and a half hours. You can go home as soon as the test has been done.

During the test, you sit or lie down. The person who does the test will attach the electrodes to your head with a sticky gel. They may ask you 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.

Sleep EEG tests

Your doctor may ask you to have an EEG test while you are asleep. This could be because your seizures happen when you are asleep or when you are tired. Or, you may have had a standard EEG test when you were awake, but it did not show any unusual electrical activity. When you are asleep, your brainwave patterns change and may show more unusual electrical activity.

A sleep EEG test is usually done in hospital, using a standard EEG machine. Before the test, you may be given some medicine to make you go to sleep. The test lasts for one to two hours and you usually go home once you have woken up.

Sleep-deprived EEG tests

A sleep-deprived EEG test is done when you have had less sleep than usual. When you are tired, there is more chance that there will be unusual electrical activity in your brain. Your doctor may ask you to have this test if you have had a standard EEG test, but it didn’t show any unusual electrical activity.

Before a sleep-deprived EEG test, your doctor may ask you not to go to sleep at all the night before, or to wake up much earlier than you usually do.

The beginning of the sleep-deprived EEG test is the same as a standard EEG test. You then may 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 while you are moving around. 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 epileptic activity in your brain, than during a standard 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 wear the machine on a belt, so you are able to go about your daily business. You do not 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 tests

During a video-telemetry test, you need to stay in hospital. A video-telemetry test involves wearing an ambulatory EEG (see above). At the same time, all your movements are recorded by a video camera. The test is usually carried out over a few days. Sometimes your epilepsy medicine may be 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 seizure(s). 

You usually only have a video-telemetry test if you have already been diagnosed with epilepsy. Here are some examples of why your doctor might ask you to have a video-telemetry test:

  • It is not clear what type of seizures you have
  • Your epilepsy medicine is not working well
  • There is a possibility that your seizures are not caused by epilepsy, but something else
  • You are considering having epilepsy surgery
  • What information does an EEG test give?

EEG tests give information about the electrical activity that is happening in your brain at the time the test is carried out.

With many types of epilepsy, you only have unusual electrical activity in your brain when you are having a seizure. The rest of the time your brain activity is normal. So, if your EEG test doesn’t show any unusual activity, it means that there is no epileptic activity in your brain at the time the test is being done. This doesn’t rule out the possibility that you have epileptic activity in your brain at other times. A clear EEG test does not definitely mean that you don’t have epilepsy.

People with some types of epilepsy have unusual electrical activity in their brain all the time, 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. An example of this is children who have typical absence seizures.

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 such as encephalitis (swelling of the brain) or vertigo (dizziness). Some people inherit unusual brainwave patterns from their parents, even though they don’t have epilepsy. Therefore, an EEG that shows unusual brainwave patterns doesn’t necessarily mean that you have epilepsy.