Why do I need one?

The brain’s electrical activity fluctuates from second to second, but routine EEGs provide only a 20- to 40-minute sample of this activity. If epilepsy waves occur in your brain only once every 3 or 4 hours, or if they happen only after an hour of sleep, for instance, a routine EEG will usually be normal. Then the doctor may want to see a longer recording that includes prolonged periods when you are both awake and asleep. This kind of recording is called an ambulatory EEG. ("Ambulatory" [AM-byew-lah-TOR-ee] means able to walk around.)

An ambulatory EEG may be performed to confirm the diagnosis of epilepsy if you've continued to have seizures despite trying various seizure medicines. It would generally be performed at a specialized epilepsy center.

What's the procedure like?

This kind of recording is made by using a special recorder that is slightly larger than a portable cassette player. You can wear it on your waist, with the wires running either under your shirt or outside of it. Then you can go about your normal routine for up to 24 hours. The electrodes are pretty well covered if you have a full head of hair, but most people prefer not to go to school or work while wearing them.

Because the electrodes must stay on your head longer than for a regular EEG, the technologist will probably use a special glue called "collodion" to keep them in place. Acetone or a similar solution is used to remove them easily at the end of the test.

You will usually be asked to keep a diary of your actions during the day to help the doctor identify the cause of abnormal activity on the recording. For instance, the electrodes may make your head itchy, and if you scratch it, that may appear as abnormal activity on the EEG.

What are the special features of ambulatory EEG recorders?

Most recorders have an "event" button for you to press if you experience any of the symptoms for which you are being tested, such as feeling "spacey" or confused. If you are unable to press the button during a seizure, someone else can do it for you.

Newer recorders have built-in programs to identify epilepsy waves and seizures. Some even have video recording capability.

Topic Editor: Steven C. Schachter, M.D.
Last Reviewed: 12/15/06