An electroencephalogram (EEG) is a painless test that records brain activity.
How it works
When the brain cells send messages to each other, they produce tiny electrical signals. In an EEG test, electrodes (flat metal discs) are placed onto your scalp using a sticky substance. These electrodes pick up the electrical signals from your brain and send them to an EEG machine, which will record the signals as wavy lines onto paper or on a computer.
The EEG rarely causes any side effects and takes 30 to 45 minutes (see How an EEG is performed for more information).
Why it is used
The pattern of electrical activity is useful for diagnosing a number of conditions that affect the brain, particularly epilepsy. EEGs are also used to investigate other conditions that may affect brain function, such as dementia, infection of the brain, head injuries, brain tumours or bleeding on the brain (haemorrhage).
The EEG may help to show your doctor the section of your brain that is not working as it should. It will also help them to decide which treatment will be most suitable for you.
EEGs are also sometimes used for patients in a coma to help determine their level of brain function. For more information, see Why an EEG is carried out.
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