NSE : Information on epilepsy: Electroencephalography (EEG), epilepsy tests

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Information on epilepsy: Electroencephalography (EEG)

If a person is believed to be having seizures, they may be referred to hospital for an EEG.

An EEG is used to record the electrical activity of the brain by picking up the electrical signals from the brain cells. These signals are picked up by electrodes on the head and are recorded on paper or on a computer. This shows how the brain is working. The electrodes only record electrical activity, they do not give out electrical signals, so both tests are painless.

What happens during the test?

At the start of the test, the technician sticks the electrodes to the person's head with paste. For young children, the electrodes may be held in place with a rubber cap. The electrodes are connected to the recording machine. The test lasts about 30 minutes and during it the person usually sits or lies down.

What does the EEG show?

The EEG gives information about the electrical activity of the brain during the time the test is happening. When someone has an epileptic seizure, their brain activity changes and is irregular. If they have a seizure during the test, the change in brain activity can be seen on the EEG recording. This is sometimes called 'epileptiform activity'. Some people have epileptiform activity even when they are not having a seizure so the EEG can also be useful for them.

Epileptiform activity can sometimes be seen more clearly during deep breathing. So part of the test might include deep breathing or repeated blinking to see if there are any changes.

'Positive' and 'negative' results

While the EEG can show if someone is having a seizure at that moment, it can't show what happens in the person's brain before or after the test. So unless the person has a seizure, the test might not give much information.

The test may come back 'negative' or 'normal', which just means there was no change seen in brain activity at the time. This does not mean that the person does not have epilepsy.

Some people who do not have epilepsy may have some irregular activity on their EEG. So a 'positive' result, where there is irregular activity, does not always mean that the person has epilepsy.

Photosensitive epilepsy

Some people with epilepsy have their seizures brought on or 'triggered' by flashing lights. This is called photosensitivity. Part of the EEG test is to see if the person is photosensitive. This is done by flashing a light in front of them. The technician can see any changes in brain activity while the test happens so they can stop the lights before a seizure happens.

More information on photosensitive epilepsy.

Sleep deprived EEG

For some people, there is more chance of irregular brain activity happening when they are tired or when they are going to sleep. Having a sleep-deprived EEG might help.

This test is done in the same way as a normal EEG but the person goes to sleep during the test. To help the person sleep, they may be asked to stay awake some, or all, of the night before. In some cases a mild sedative might be given to help the person sleep.

What does the EEG show?

Positive and negative results

Photosensitive epilepsy

Sleep-deprived EEG

Ambulatory EEG

Video telemetry

Photo: attaching the electrodes

Photo: EEG technician

Ambulatory EEG

This is similar to a normal EEG. However the electrodes are stuck in place with glue and attached to a small recording machine worn on a belt around the waist, like a personal stereo.

This type of EEG allows the brain activity to be recorded for several hours, days or weeks. The person can move around and carry on with their normal daily activities while the recording happens. As the brain activity is recorded for longer, there is more chance of a seizure being recorded on the EEG than during the normal 30 minute test.

Video telemetry

This test is done in hospital and may last a couple of days. Video telemetry means that as well as having an EEG, the person is videoed at the same time. The EEG electrodes are attached in the same way as in ambulatory EEG, so the person is able to move around the room. In the room, often mounted on the wall, there is a video camera that records what the person is doing.

This test means that the electrical activity of the brain can be compared with what the person does during a seizure. This can help to work out what type of seizures the person has.

More information on diagnosing epilepsy

UK Epilepsy Helpline: 01494 601400
Monday - Friday 10am - 4pm

NSE is the largest medical charity for epilepsy in the UK. We provide information and support to professionals and people affected by epilepsy. We also provide care for people with epilepsy through medical, residential and rehabilitation services.