Understanding diagnostic tests
Electroencephalogram (EEG)
An EEG records the brain’s electrical activity. It can provide important information about seizures and epilepsy when considered alongside your symptoms and medical history.
About this information
This webpage is subject to continuous review by Epilepsy Ontario to help keep the information current. This material is for informational purposes only and should not be used for self-diagnosis or as a substitute for professional medical advice, diagnosis, or treatment. Consult your healthcare provider about your own health and care.
What does an EEG do?
Small sensors called electrodes are attached to the scalp to record electrical signals produced by the brain. A standard scalp EEG records these signals; it does not send electricity into the brain.
A specialist examines the recording for patterns that may help identify the type of epilepsy or guide further investigation.
Routine EEG
A recording made during an appointment, usually while you are awake. The team may also try to record sleep.
Sleep or sleep-deprived EEG
A recording that includes sleep. You may receive specific instructions about changing your sleep beforehand.
Ambulatory EEG
A portable recorder collects information over a longer period while you follow an agreed routine outside the testing department.
Video-EEG monitoring
Video and EEG are recorded together so the team can compare an episode’s appearance with the electrical recording.
Before your EEG
- Follow the instructions from your testing centre.
- Arrive with clean, dry hair and avoid styling products as directed.
- Bring an up-to-date medication list.
- Ask about meals, caffeine, sleep, and transportation.
- Explain any accessibility, communication, or sensory needs in advance.
Do not stop medication or deprive yourself of sleep unless specifically instructed. Sleep deprivation can increase seizure risk. If a sleep-deprived EEG is ordered, ask about safe travel arrangements afterward.
During the test
The technologist measures your head and attaches electrodes using paste or another suitable method. You will be asked to sit or lie comfortably while the recording is made.
Depending on the test, you may be asked to open and close your eyes, breathe deeply, or look at flashing lights. These procedures can bring out changes in the recording and occasionally trigger a seizure. The team will explain them and monitor you.
Tell the technologist if you feel unwell or uncomfortable. Do not try to reproduce these procedures at home.
After the test
The electrodes are removed. Some paste may remain in your hair until it is washed. Follow any instructions related to sleep deprivation, medication, or sedation.
What do the results mean?
A normal EEG does not rule out epilepsy. A short recording may not capture changes that happen at other times.
An abnormal finding also needs clinical interpretation. Your healthcare provider considers the EEG alongside your history, witnessed events, examination, and other tests.
Further EEG testing may be recommended if questions remain.
Questions to ask
- Which type of EEG am I having, and why?
- Should I change anything about sleep, meals, or medication?
- How long should I allow for the appointment?
- Who will explain the results, and when?
