An electroencephalograph is a useful test to help diagnose epilepsy. It records the electrical activity of the brain. However, a normal result does not rule out epilepsy.
Note: the information below is a general guide only. The arrangements, and the way tests are performed, may vary between different hospitals. Always follow the instructions given by your doctor or local hospital.
What is an electroencephalograph (EEG)?
The brain normally produces tiny electrical signals that come from the brain cells and nerves which send messages to each other. These electrical signals can be detected and recorded by the EEG machine. The EEG test is painless and harmless. (The EEG machine records electrical signals coming from your brain - it does not put any electricity into your brain or body.)
You may be advised to have an EEG if you have had symptoms which may be due to a seizure. (Old words for seizure are convulsion or 'fit'.) An EEG test is useful in diagnosing conditions such as epilepsy.
How is the test done?
When you have the test, the operator will attach several small patches (electrodes) to your scalp. Wires from the electrodes are connected to the electroencephalograph (EEG) machine. The machine detects and amplifies the electrical signals and records them on to a paper or computer. The test takes about 20-30 minutes. The electrodes are removed at the end of the test.
By Baitaal (Own work), via Wikimedia Commons
For the duration of the test you may be asked to sit in a chair or lie on a couch. At some point you may be asked to blink lots of times, or to breathe deeply. These can sometimes trigger patterns of electrical activity in the brain which are associated with certain types of epilepsy.
What can the test show?
A normal ('negative') result
This shows a typical pattern of electrical activity from the brain. Most people without epilepsy, and many people with epilepsy, have a normal result. This is because an electroencephalograph (EEG) only shows the electrical activity of the brain when the test is done. With many types of epilepsy, you only have abnormal electrical activity during a seizure. For the rest of the time, the pattern is normal.
An abnormal ('positive') result
This shows abnormal patterns of electrical activity. Some people with certain types of epilepsy have abnormal patterns all the time, not just when they have seizures. (Although, during a seizure the activity is even more abnormal.) For example, children with typical 'absence seizures' often have a characteristic EEG pattern which helps to confirm this type of epilepsy.
However, a small number of people who never have seizures and who don't have epilepsy, have some abnormal patterns of electrical activity in the brain.
Therefore, if you have symptoms which are thought to be seizures, an abnormal EEG means that the diagnosis is likely to be epilepsy. However, a normal result does not rule out epilepsy, and an abnormal result does not necessarily mean that you have epilepsy.
Children and electroencephalography (EEG)
The interpretation of a child's EEG recording is more difficult. This is because the EEG changes during childhood. An adult pattern is usually developed by the age of 15 years. As the EEG pattern in infants and children can vary considerably, careful interpretation of the test is necessary.
Some specialised types of electroencephalograph (EEG) test
In some cases, a strobe light may be used during an EEG test. This aims to detect if this alters the electrical pattern in the brain. (Usually it does not. However, a small number of people have seizures triggered by flickering or strobe lights and so this may help to identify these people.)
In this test, an EEG is performed while you are sleeping. This is usually carried out when you are in hospital. You may need to have one of these if your seizures happen when you are asleep or when you are tired.
There may be a better chance of detecting abnormal brain activity after a period of time when you are deprived of sleep. Therefore, sometimes the EEG test is done after you have stayed awake for all or most of the night. It is done in the same way as the normal test but with you asleep - after the period of 'sleep deprivation'.
This may be advised in cases where the diagnosis is not clear. It uses a portable EEG machine which records the brain's electrical activity when you are going about your normal activities. The electrodes can usually be hidden under your hair. The wires are connected to a small machine which you wear on a belt (a bit like wearing an mp3 player). You may be asked to keep a diary when you have meals, go to sleep, and have any symptoms which may be a seizure. The patterns on the EEG can be analysed to see if they change when symptoms occur. It may help to confirm whether certain symptoms are due to seizures.
Where there is doubt about a diagnosis of epilepsy, or where the type of seizures someone experiences is unclear, video-telemetry can be helpful. This is a test that uses a video camera linked to an EEG machine. The camera will visually record your movements and, at the same time, the EEG machine will record your brainwave pattern. Both the video and EEG are stored on to a computer so that they can be reviewed once the test is finished. The doctor will be able to see any seizures that you may have had, as well as any changes in your EEG at that time. The test is often carried out over a number of days in order to increase the chances of recording one of your seizures.
What should I do to prepare for an electroencephalograph (EEG)?
Your doctor should give you instructions about what you need to do to prepare for an EEG. Commonly there is no preparation necessary. You should not stop any medication you take for seizures unless advised to by your doctor.
Further reading and references
Epilepsies: diagnosis and management; NICE Clinical Guideline (January 2012)
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