Electroencephalogram (EEG)


An electroencephalogram (EEG) is a recording of the brain’s electrical activity. Metal electrodes attached to the head transform electrical activity into patterns, commonly called brain waves. A polygraph machine records the brain waves and, in some cases, the waves are transmitted to a computer screen. A basic EEG takes about 45 minutes, with a range of 30 minutes to 90 minutes.

Lightweight EEG devices are now available so people can walk around and perform their normal daily activities while the devices detect and record brain waves over longer periods of time.

What It’s Used For

An EEG can be used to evaluate patients with seizure disorders, confusion, head injuries, or other conditions that may be caused by an abnormality in the brain. It also can help to diagnose certain types of dementia (brain illnesses that cause worsening mental impairment) and encephalopathy (brain dysfunction), such as the encephalopathy caused by severe liver or kidney disease. Occasionally, an EEG may be used to confirm brain death, for example, in patients on life support who are in deep comas.


In most cases, no special preparation is necessary. However, in some people, an EEG gives better results if the recording is obtained after a period of no sleep (sleep deprivation), to make it more likely that seizures or other abnormalities may occur. For these people, the doctor will give specific instructions about how and when to limit sleep before the EEG.

How It’s Done

You will sit or lie comfortably as a number of small electrodes are attached to your head. These electrodes are not painful. Your scalp will not need to be shaved, but the area under the electrodes will be cleaned to remove any excess oils from your scalp. A special EEG gel or paste helps the electrodes pick up your brain waves more efficiently.

As the electrodes detect your brain waves and feed them into the EEG machine, several wavy lines (your brain-wave patterns) appear on the polygraph’s graph paper. As your EEG continues, you will need to open and close your eyes, breathe quickly and deeply and look at a flashing light. In some people, EEG recording continues as you fall asleep.

Once the EEG is finished, the electrodes are removed and any excess EEG gel or paste is cleaned away. Then you can go home.


Contact your doctor within a few days to find out the results of your EEG. Otherwise, no specific follow-up is necessary.


An EEG is a safe procedure. Harmful side effects are not expected. In rare cases, certain people with epilepsy may experience seizures from the flashing lights or the hyperventilating they have to do during the procedure.

When To Call A Professional

Because harmful side effects are not expected, patients typically need to call their doctors only for their results.

Johns Hopkins patient information

Last revised:

Diseases and Conditions Center

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All ArmMed Media material is provided for information only and is neither advice nor a substitute for proper medical care. Consult a qualified healthcare professional who understands your particular history for individual concerns.