Electroencephalography (EEG)

What is an electroencephalograph (EEG)?

The EEG machine records (graph) the tiny electrical signals (electro) that come from brain neurons (encephalo). The EEG test is painless and harmless. (The EEG machine only records electrical signals coming from your brain - it does not put any electricity into your brain or body.)

You may be advised to have an EEG if your physician suspects that you have had a seizure ( also referred to as spells, fits, convusions). Though seizure is usually thought to be the same as “epilepsy” but patients sometimes pass out (syncope due to transient reductions in blood flow to the brain) and have other events that look like epileptic seizures but are not. Epileptic seizures result from abnormal, excessive or hypersynchronous neuronal activity in the brain causing surges of electrical impulses that change behavior or even cause convulsive jerking of the body.

How is the test done?

During the test our technician will attach several small patches (electrodes) to your scalp with paste. Wires from the electrodes are connected to the EEG machine, which detects and amplifies the electrical signals and records them on a computer. The test takes about 20-30 minutes. The electrodes are removed at the end of the test.

For the duration of the test you will lie on an examination table. At some point you may be asked to blink lots of times, or to breathe deeply. These can sometimes trigger patterns of electrical activity in the brain which are associated with certain types of epilepsy.

What can the test show?

A normal ('negative') result

A “normal” EEG shows typical waking patterns defined by the frequency coming from the back part of the brain (occipital lobes primarily) and may show slowing consistent with drowsiness and sleep. With many types of epilepsy, the EEG is only abnormal during a seizure and appears normal at all other times. Thus, a normal EEG does not mean you do not have epilepsy.

An abnormal ('positive') result

Abnormal brain waves (spikes and sharp waves) may be seen coming from specific areas of the brain (partial onset) or the entire brain at once (generalized). These are occasionally observed on EEG even when the patient is not experiencing any symptoms. These are called interictal (between the ictus, which usually means the spell, seizure, symptom for which you are being studied) discharges. The EEG activity during a seizure is even more abnormal. For example, children with typical 'absence seizures' often have a characteristic EEG pattern which helps to confirm this type of epilepsy.

However, a small number of people who never have seizures, and who don't have epilepsy, have some abnormal patterns of electrical activity in the brain.

If your physician suspects epilepsy and you have an abnormal brain wave pattern consistent with epilepsy then the diagnosis is usually felt to be confirmed. However, a normal result definitely does not rule out epilepsy, and there are other abnormalities, such as slowing of the brain wave pattern, that does not necessarily mean you have epilepsy.

Children and electroencephalography

An adult pattern is usually not fully developed until the age of 15 years. Therefore, the EEG changes during development making interpretation more difficult. Normal background patterns are usually established between the ages of 6-8 and for this reason the Oxford Neurology Clinic only records from children younger than age 8 on a case by case basis.

Some specialised types of electroencephalograph test

Strobe lighting

This aspect of EEG testing may help to identify these people who have seizures triggered by flickering or strobe lights.

Sleep deprived EEG

Sleep deprivation increases the likelihood that an abnormality will be recorded. Therefore, you may be given specific directions on limiting sleep to 5-6 hours and avoiding caffeine consumption on the day of your test. It is done in the same way as the normal test, but this protocol makes it easier for you to become drowsy and even fall asleep during the EEG.

Ambulatory EEG

These are ordered much less frequently than routine EEG. You will be hooked up by our technicians with electrodes in a fashion similar to the routine test but the wires will be connected to a pack that straps like a purse or backpack over your shoulder and it is worn at home for 2-3 days. Specific directions about keeping a diary of symptoms/events and sleep times will be provided by the technician. It is very important to keep accurate times and descriptions of symptoms so our interpreting physician can look very closely at the EEG during these events. Typically ambulatory EEG is used when the routine EEG is not helpful, the patient does not respond to treatment or the type of spell may not be epileptic in origin.


This is an extension of the above tests ordered to correlate video recording of your activity during a seizure with the patterns observed on the EEG. You may be sent to a special epilepsy center to have this testing done if you have not responded to treatment or the other testing done does not fully answer the question as to what type of seizures you are having. Video EEG is also an integral part of assessing whether brain surgery is appropriate to consider as a treatment of epilepsy that does not respond to medications.

What should I do to prepare for an electroencephalograph?

Your doctor should give you instructions about what you need to do to prepare for an EEG. Commonly there is no preparation necessary. You should not stop any medication you take for seizures unless advised to by your doctor.