An electroencephalogram, or EEG, is a test that measures the electrical activity of the brain. It can be used to diagnose or monitor conditions that affect the brain, such as epilepsy and sleep disorders. Your doctor will let you know if you need to have an EEG.
What is an EEG test?
An EEG records the electrical signals sent between brain cells.
This electrical activity is shown on a computer as ‘brain waves’, which are interpreted by a specialist doctor.
If something is unusual about the brain’s electrical activity, it will show up in the EEG recording.
The test is safe and doesn’t hurt, as it is merely recording the brain’s electrical activity.
What conditions can an EEG test help diagnose?
Your doctor may recommend an EEG to diagnose or monitor conditions including:
- epilepsy and other types of seizures
- sleep disorders
- brain tumours
- head injuries
- encephalopathy (brain dysfunction that may have a variety of causes)
- encephalitis (brain inflammation)
- dementia, as in Alzheimer’s disease
An EEG cannot diagnose mental illnesses. Although an EEG is one of the main tools for diagnosing epilepsy, a negative or normal EEG test does not necessarily rule out epilepsy.
What does an EEG test involve?
During the EEG, flat metal discs (electrodes) will be placed all over your scalp. They are usually kept in place with a sticky paste. You won’t need to cut your hair.
These discs are also attached to wires that send the electrical signals to a computer to record the brain waves. You won’t feel any sensations from the discs.
You will need to keep still during the test. You may be asked to do some deep breathing, look at a flashing light or sleep during the test.
An EEG takes about an hour, but may be longer for a sleep recording. Sometimes EEGs are conducted when you have purposely been sleep-deprived.
How to prepare for an EEG test
Wash your hair the night before or the day of the test, so that the discs stay attached to your scalp. Avoid using conditioners, hairsprays, styling gels or other hair products.
It’s also best to avoid food or drinks that contain caffeine (including coffee, tea, cola and chocolate) for at least 8 hours before the test.
Your doctor will let you know if you need to stop taking any of your medicines beforehand.
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Last reviewed: April 2020