An EEG records the electrical activity of the brain through small sensors on the scalp. It is painless and safe, and it plays an important part in understanding seizures and some other conditions.
Imagine your teenage son collapsing and shaking at school. Everyone is frightened. A few weeks later he is booked for an EEG. You are not sure what it involves, or whether a normal result means he does not have epilepsy.
What an EEG is
An electroencephalogram (EEG) uses small metal discs (electrodes) attached to the scalp to record the tiny electrical signals produced by brain cells. These appear as wavy lines on a screen.,EEG does not put any electricity into the brain, cannot read thoughts and is completely safe for adults, children and babies.
Why an EEG might be done
- To help diagnose epilepsy and identify the type of seizures
- To investigate blackouts, collapses or unusual episodes
- To monitor brain activity in intensive care, for example after a cardiac arrest
- To investigate confusion or reduced consciousness
- Sometimes in sleep disorders or before epilepsy surgery
How to prepare
- Wash your hair the night before or on the morning of the test, and do not use conditioner, gel, spray or oils
- Remove hair extensions, braids or weaves if possible, or ask the department for advice
- Take your usual medicines unless told otherwise
- Eat normally, as low blood sugar can affect the recording
- For a sleep-deprived EEG, follow instructions about staying awake the night before
What happens during a routine EEG
A physiologist measures your head and marks where the electrodes go. About 20 electrodes are attached using a sticky paste or a cap. You may be asked to sit or lie down, relax and close your eyes.,During the recording, which lasts about 20 to 40 minutes, you may be asked to open and close your eyes, breathe deeply and quickly for a few minutes (hyperventilation), and look at a flashing light (photic stimulation). These can bring out abnormal activity in some types of epilepsy. You can say if you do not want to do any part of the test. The whole appointment usually takes about an hour, and a video may be recorded at the same time.
Other types of EEG
- Sleep-deprived EEG: done after reduced sleep, which makes abnormal activity more likely to show
- Sleep EEG: recorded while you nap, sometimes with a medicine to help sleep
- Ambulatory EEG: a portable recorder worn at home for one to several days
- Video telemetry: a stay in hospital with continuous EEG and video, to capture episodes and work out exactly what they are
What results can show
An EEG may show patterns typical of certain types of epilepsy, such as spikes or sharp waves. It can also show slowing of brain activity in some conditions.,Importantly, many people with epilepsy have a normal EEG between seizures, and some people without epilepsy have minor EEG changes. Diagnosis is based mainly on a detailed description of the episodes, ideally from someone who saw them, and the EEG is one piece of evidence.
How witnesses can help
- Note how the episode started and what the person was doing
- Record how long it lasted
- Describe movements, colour changes, breathing and responsiveness
- Note how the person was afterwards: confused, sleepy, or quickly back to normal
- A short video on a phone, taken safely and with consent where possible, can be extremely helpful
After the test
The paste is removed, though some may remain in your hair until you wash it. You can usually go home and return to normal activities straight away, unless you had sedation or a sleep-deprived EEG, in which case you should not drive. Results are reviewed by a specialist and usually sent to the healthcare provider who requested the test.
Seizure first aid
- Stay calm, note the time and protect the person from injury
- Do not put anything in their mouth or restrain them
- Once the shaking stops, place them on their side
- Call emergency services if a seizure lasts more than five minutes, if it is their first seizure, if they are injured, or if another seizure follows
Frequently asked questions
Does an EEG hurt?
No. Some people find the paste slightly cold or sticky.
Can I drive to my appointment?
Usually yes for a routine EEG, unless you have been told not to drive because of seizures.
Can the flashing light cause a seizure?
Very rarely. Staff watch carefully and stop immediately if needed.
But my EEG was normal. Does that mean it was not a seizure?
Not necessarily. A normal EEG does not rule out epilepsy. Your specialist will consider the full history.
Your next step: If someone you know has had a blackout or seizure, write down exactly what you saw as soon as possible, to share with their healthcare team.
Related: Fainting: causes and first aid
Sources: Epilepsy Action, International League Against Epilepsy, National Institute for Health and Care Excellence (NG217), US National Institute of Neurological Disorders and Stroke.
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