An electroencephalogram is a recording of brain activity.
Brain cells continually send messages to each other that can be picked up as small electrical impulses on the scalp. The process of picking up and recording the impulses is known as an EEG.
A normal EEG means that you have a normal pattern of brainwave activity. An abnormal reading means that abnormal patterns of brain activity are being produced and picked up.
EEGs can be used to help diagnose and manage a number of different medical conditions including:
People of all ages can have EEGs, including tiny babies and the very elderly.
If you have epilepsy, your brain sometimes doesn't work normally. This causes seizures (previously known as epileptic fits).
People with seizures can have normal brain activity (EEG results) or quite minor changes in between attacks. This is why specialists are needed to look at and assess your EEG results.
An EEG will help your doctor identify the type of epilepsy you have and what may be triggering your seizures. This will enable the most effective type of medication to be prescribed for you.
In rare cases, treatment may include neurosurgery (brain surgery).
Read more about how epilepsy is diagnosed and how epilepsy is treated.
Other conditions that affect brain function where EEGs can be used include:
- dementia - a group of symptoms that are responsible for the decline of brain function
- head injury
- brain tumour - an abnormal and uncontrollable growth of cells in the brain
- brain abscess - a pus-filled swelling in the brain that's caused by an infection
- encephalitis - brain inflammation that's sometimes caused by an infection
- brain haemorrhage - bleeding in the brain
- cerebral infarct - brain tissue that has died due to a blockage in blood flow
- sleep disorders
How EEG is performed
Routine EEG recordings usually take 20-40 minutes (see below), although a typical appointment will last for about an hour including some preparation time at the beginning plus some time at the end.
The test is usually carried out as an outpatient procedure by a highly trained clinical physiologist who has specialised in neurophysiology (the study of the workings of the nervous system).
The clinical physiologist will explain the procedure to you, and you’ll be able to ask any questions that you have. You'll also be asked to agree or decline to have the various parts of the test (consent). The EEG procedure is painless and you should feel comfortable throughout.
The skin on your scalp will be cleansed and about 20 electrodes (small discs) will be attached to specific areas, measured out in the correct locations.
The electrodes will be connected to an EEG machine by thin leads. The machine records your brain wave activity for later analysis. Afterwards, the electrodes will be removed and your scalp cleaned. You may also want to wash your hair afterwards
After you’ve left the EEG department, the recordings will be analysed and used for the purposes of diagnosis and treatment.
Types of EEG
The main types of EEG are explained below.
A routine EEG recording lasts for about 20-40 minutes.
During the test, you’ll be asked to rest quietly and from time to time to open or close your eyes. In most recordings you’ll be asked to breathe deeply in and out for about three minutes.
At the end of the procedure, provided you've agreed, a strobe light will be placed nearby and you'll see bright flashes of light which are repeated at different speeds.
The reasons for doing this and your consent will always be clearly established beforehand.
A sleep EEG is carried out while you're asleep. It may be used if a routine EEG doesn't show any conclusive features, or to test for sleep disorders.
While you’re asleep your brainwave patterns change significantly, and useful information related to your condition can be obtained. If necessary, to promote sleep, you may be asked to stay awake during the preceding night.
An ambulatory EEG is where brain activity is recorded throughout the day and night, over a period of one or more days.
You’ll be given a small portable EEG recorder that can be clipped onto your clothing. It will record your EEG activity during the whole day and night.
Video telemetry, also known as video EEG, is a special type of EEG that simultaneously videos you and records your brain wave activity.
It’s used when an EEG and continuous intensive monitoring are needed. For example, it can be used to see what a child is doing while they’re having a seizure. This can help diagnose the type of epilepsy that they have, where the seizure starts and how the electrical activity spreads through their brain.
Video telemetry is usually carried out on an in-patient basis in a purpose built hospital suite. It usually takes place day and night for up to five days, unless enough information about the seizure is recorded over a shorter period.
The EEG signals are transmitted wirelessly to a base-station (a computer processing unit). The video is both recorded at the base-station and kept under regular surveillance by trained staff so they can respond immediately if there are any problems.