EEG

An EEG (electroencephalogram) records the electrical activity of the brain. Small metal discs called electrodes are stuck to the scalp, and wires carry the signals to a machine that draws them as wavy lines. The machine only records. An EEG cannot measure intelligence.

A doctor may ask for an EEG to look for the cause of seizures or blackouts, and to help diagnose and follow epilepsy. It can also help assess unexplained fainting or memory loss, sleep conditions, an infection of the brain such as encephalitis, and the brain after a head injury.

Your clinic will tell you what to do. In general:

A routine EEG goes like this:

1. You sit in a chair or lie on a bed. Your head is measured and marked where the discs will go.
2. The discs are stuck to your scalp with sticky paste, and wires join them to the recording machine.
3. You lie still with your eyes closed, as movement affects the recording. A staff member watches it.
4. You may be asked to open and close your eyes, breathe deeply and fast for a few minutes, look at a flashing light, or sleep.
5. The discs are taken off and your scalp is cleaned.

In a sleep-deprived EEG you stay up for part of the night before. A portable EEG is worn at home for up to 3 days. A video EEG also films you, often over a few days in hospital.

The paste may leave your hair sticky until you wash it out. Most people go home and carry on as normal. Results can take a few days or weeks, as a specialist has to read the recording, and you may be asked to come back to discuss them. If you have heard nothing after a few weeks, contact the doctor who sent you.

An EEG is very safe. The discs may feel sticky and strange, but the test should not be uncomfortable. Flashing lights or fast, deep breathing can bring on a seizure in people who have seizure disorders. Trained staff are there to care for you if this happens.

A specialist reads the recording and sends a report to your doctor, who explains it and what comes next. Brain waves are faster when you are awake and slower in some stages of sleep. An EEG that is not normal can come from epilepsy, but also from a head injury, a stroke, a brain infection, some medicines or a liver or kidney problem. It does not name the cause on its own.

What an EEG cannot show: a normal EEG does not mean a seizure did not happen, as a short recording may not catch one. Repeating the test, recording during sleep, after missed sleep or for longer can find more. Doctors also use the story of what happened, an examination and often a brain scan such as MRI or CT. Reading an EEG takes training, so it should be read by an experienced specialist.