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 electroencephalograph (EEG) machine. The EEG test is painless and harmless. 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'.)

How is the test done?

The operator will attach several small patches (electrodes) to your scalp. Wires from the electrodes are connected to the 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. 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.

What can the test show?

  • A normal ('negative') result
  • 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.

Children and electroencephalography

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 test

Strobe lighting

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.

Sleep deprived EEG

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'.

Ambulatory EEG

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.


Where there is doubt about a diagnosis of epilepsy. 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 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?

Commonly there is no preparation necessary. You should not stop any medication you take for seizures unless advised to by your doctor.

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