Electroencephalography (EEG), a neurophysiological study technique that has been in use for over 75 years, is a recording of the electrical potentials generated by firing neurons in the cerebral cortex. The EEG is used primarily for seizure diagnosis, although it can provide helpful information for other disorders. It is an important component of the evaluation for epilepsy surgery, which may require invasive monitoring to define the seizure onset zone and crucial functional areas.
Preparation for the EEG: What to Expect
The routine EEG is the most common test for epilepsy. The EEG technologist first measures the patient’s head so that the electrodes, which are small, metal, cup-shaped disks attached to wires, can be placed in the correct position. A wax crayon, which can be easily washed off later, is used to mark the points on the scalp. Next, the technologist applies the electrodes, usually using a paste that holds them in place for up to several hours. The technologist often scrubs each position on the scalp with a mildly abrasive cream before applying the electrodes. This will help improve the quality of the recording. The electrodes only record the brain waves. They do not stimulate the head with electricity, and pose no danger to the patient. The EEG machine then records the brain waves as a series of squiggly lines called traces. Recordings on paper are now commonly being replaced by computerized, paperless EEGs.
During the EEG: What to Expect
The patient may fall asleep briefly during a routine EEG, because the room is quiet and often dimly lit. That is fine and is often helpful, because an EEG obtained during both wakefulness and sleep may provide extra information. During the EEG, the technologist may ask patients to open and close their eyes several times, may shine flashing lights into their eyes (photic stimulation), or may ask them to breath rapidly or deeply (hyperventilation). Patients who have a medical problem, such as asthma or heart disease, which makes it unsafe to hyperventilate should tell the EEG technologist or the doctor. Similarly, pregnant women usually should not undergo hyperventilation or have photic stimulation. In some cases, the doctor may ask the patient to stay up the entire night before the EEG is performed. This sleep deprivation can increase the likelihood that epilepsy waves will be recorded. If the patient experiences any possible seizure symptoms during the test, he or she should tell the technologist.
Routine EEG in Children: What to Expect
Obtaining an EEG in children is usually easy, but it can pose a significant challenge. For babies, it is helpful to perform the EEG around naptime. Electrodes can be applied while the mother holds the child; a bottle may help to calm the baby. Then the baby is allowed to sleep naturally. Sedation is required for some babies and young children, to allow the technologist to apply the electrodes and to record sleep activity. Children have difficulty lying still during EEG recordings, and the doctors who interpret these studies must separate the waves caused by movement and muscle activity from the brain waves.
After the EEG: What to Expect
After the EEG recording is done, the technologist will remove the electrodes from the patient’s scalp, and the patient is free to go home and wash the paste out of his or her hair. The paste is lanolin- or water-based, so it can be easily washed off. The doctor usually reads the EEG after the test is completed and the patient has left.
Are there any risks to the test?
The EEG has been used for many years and is considered a safe procedure. The test causes no discomfort. The electrodes only record activity and do not produce any electrical current.
Skin irritation or redness may be present at the locations where the electrodes were placed, but this will generally wear off in a few hours.
What is the purpose of an ambulatory EEG?
The brain’s electrical activity fluctuates from second to second. The routine EEG provides a 20- to 40-minute sample of brain electrical activity, which is often sufficient. In some patients with epilepsy, however, this recording is normal or shows only minor, nonspecific findings. In such cases, an extended recording that includes long periods of wakefulness and sleep is desired. For example, in some people, epilepsy waves occur only once every 3 or 4 hours or only after an hour of sleep, and a routine EEG will almost always be normal.
An ambulatory EEG, on the other hand, can record up to 72 hours of EEG activity with a special recorder that is slightly larger than a portable cassette player. This recorder allows you to go about your normal routine while the EEG is being recorded.
Why do I need the test?
Your doctor may order an ambulatory EEG for you for a number of reasons:
- To diagnose epilepsy and determine what type of seizures are occurring. EEG is the most useful and important test in confirming a diagnosis of epilepsy.
- Distinguish fainting spells from seizures.
- To quantify the number of abnormal discharges or seizures in patients with epilepsy.
- Evaluate periods of unconsciousness or memory loss.
- Monitor how effective medications are.
- Study events that occur sporadically.
How is the test done?
The first day of an Ambulatory EEG set up procedure lasts one hour. The EEG technologist will attach 16 to 25 flat metal discs called electrodes to different places on your head, using a special glue called collodion to keep them in place. The electrodes are connected by wires to a small recorder that amplifies and records the electrical activity inside the brain. You can wear the recorder on your waist, with the wire running either under or outside of their shirt. For people with a full head of hair, the electrodes can be fairly well camouflaged. Even so, most of you may prefer not to go to work or school with the electrodes on their scalp.
You will be asked to keep a diary of activities during the day. Most recorders have an “event” button for patients to press if they experience any of the symptoms for which they are being tested, such as episodes of feeling “spacey” or confused. A family member should press the button if the patient is unable to do it.
An ambulatory EEG test may last anywhere from 24 to 72 hours. Please do not take a shower during this time as water will damage the machine.
An appointment will be made for the technologist to remove the electrodes from your scalp with acetone or similar solutions at the end of the test. You still may wish to wash your hair when returning home to remove any remaining paste.
Are there any risks to the test?
The EEG has been used for many years and is considered a safe procedure. The test causes no discomfort. The electrodes only record activity and do not produce any electrical current.
Skin irritation or redness may be present at the locations where the electrodes were placed, but this will generally wear off in a few hours.
How do I prepare for the test?
Wash your hair with shampoo but do not use a conditioner the night before the test. Do not use any hair care products, such as hairspray or gels.
Please have clean hair without any hair styling products, braids or weaves.
Eat your usual meals and take any needed medication prior to testing, unless otherwise instructed by your physician. Do NOT use any stimulants (coffee, tea, cola, etc.) starting the night before the test.
