What is an EEG?
An EEG is a test that detects abnormalities in your brain waves, or in the electrical
activity of your brain. During the procedure, electrodes consisting of small metal
discs with thin wires are pasted onto your scalp. The electrodes detect tiny electrical
charges that result from the activity of your brain cells. The charges are amplified
and appear as a graph on a computer screen, or as a recording that may be printed
out on paper. Your healthcare provider then interprets the reading.
During an EEG, your healthcare provider typically evaluates about 100 pages, or computer
screens, of activity. He or she pays special attention to the basic waveform, but
also examines brief bursts of energy and responses to stimuli, such as flashing lights.
Evoked potential studies are related procedures that also may be done. These studies
measure electrical activity in your brain in response to stimulation of sight, sound,
Why might I need an EEG?
The EEG is used to evaluate several types of brain disorders. When epilepsy is present,
seizure activity will appear as rapid spiking waves on the EEG.
People with lesions of their brain, which can result from tumors or stroke, may have
unusually slow EEG waves, depending on the size and the location of the lesion.
The test can also be used to diagnose other disorders that influence brain activity,
such as Alzheimer's disease, certain psychoses, and a sleep disorder called narcolepsy.
The EEG may also be used to determine the overall electrical activity of the brain
(for example, to evaluate trauma, drug intoxication, or extent of brain damage in
comatose patients). The EEG may also be used to monitor blood flow in the brain during
There may be other reasons for your healthcare provider to recommend an EEG.
What are the risks of an EEG?
The EEG has been used for many years and is considered a safe procedure. The test
causes no discomfort. The electrodes record activity. They do not produce any sensation.
In addition, there is no risk of getting an electric shock.
In rare instances, an EEG can cause seizures in a person with a seizure disorder.
This is due to the flashing lights or the deep breathing that may be involved during
the test. If you do get a seizure, your healthcare provider will treat it immediately.
Other risks may be present, depending on your specific medical condition. Be sure
to discuss any concerns with your healthcare provider before the procedure.
Certain factors or conditions may interfere with the reading of an EEG test. These
- Low blood sugar (hypoglycemia) caused by fasting
- Body or eye movement during the tests (but this will rarely, if ever, significantly
interfere with the interpretation of the test)
- Lights, especially bright or flashing ones
- Certain medicines, such as sedatives
- Drinks containing caffeine, such as coffee, cola, and tea (while these drinks can
occasionally alter the EEG results, this almost never interferes significantly with
the interpretation of the test)
- Oily hair or the presence of hair spray
How do I get ready for an EEG?
Ask your healthcare provider to tell you what you should do before your test. Below
is a list of common steps that you may be asked to do.
- Your healthcare provider will explain the procedure to you and you can ask questions.
- You will be asked to sign a consent form that gives your permission to do the procedure.
Read the form carefully and ask questions if something is not clear.
- 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.
- Tell your healthcare provider of all medicines (prescription and over-the-counter)
and herbal supplements that you are taking.
- Discontinue using medicines that may interfere with the test if your healthcare provider
has directed you to do so. Do not stop using medicines without first consulting your
- Avoid consuming any food or drinks containing caffeine for 8 to 12 hours before the
- Follow any directions your healthcare provider gives you about reducing your sleep
the night before the test. Some EEG tests require that you sleep through the procedure,
and some do not. If the EEG is to be done during sleep, adults may not be allowed
to sleep more than 4 or 5 hours the night before the test. Children may not be allowed
to sleep for more than 5 to 7 hours the night before.
- Avoid fasting the night before or the day of the procedure. Low blood sugar may influence
- Based on your medical condition, your healthcare provider may request other specific
What happens during an EEG?
An EEG may be done on an outpatient basis, or as part of your stay in a hospital.
Procedures may vary depending on your condition and your healthcare provider's practices.
Talk with your healthcare provider about what you will experience during your test.
Generally, an EEG procedure follows this process:
- You will be asked to relax in a reclining chair or lie on a bed.
- Between 16 and 25 electrodes will be attached to your scalp with a special paste,
or a cap containing the electrodes will be used.
- You will be asked to close your eyes, relax, and be still.
- Once the recording begins, you will need to remain still throughout the test. Your
healthcare provider may monitor you through a window in an adjoining room to observe
any movements that can cause an inaccurate reading, such as swallowing or blinking.
The recording may be stopped periodically to let you rest or reposition yourself.
- After your healthcare provider does the initial recording while you are at rest, he
or she may test you with various stimuli to produce brain wave activity that does
not show up while you are resting. For example, you may be asked to breathe deeply
and rapidly for 3 minutes, or you may be exposed to a bright flashing light.
- This study is generally done by an EEG technician and may take approximately 45 minutes
to 2 hours.
- If you are being evaluated for a sleep disorder, the EEG may be done while you are
- If you need to be monitored for a longer period of time, you may also be admitted
to the hospital for prolonged EEG (24-hour EEG) monitoring.
- In cases where prolonged inpatient monitoring is not possible, your doctor may consider
doing an ambulatory EEG.
What happens after an EEG?
Once the test is completed, the electrodes will be removed and the electrode paste
will be washed off with warm water, acetone, or witch hazel. In some cases, you may
need to wash your hair again at home.
If you took any sedatives for the test, you may be required to rest until the sedatives
have worn off. You will need to have someone drive you home.
Skin irritation or redness may be present at the locations where the electrodes were
placed, but this will wear off in a few hours.
Your healthcare provider will inform you when you may resume any medicines you stopped
taking before the test.
Your healthcare provider may give you additional or alternate instructions after the
procedure, depending on your particular situation.
Before you agree to the test or the procedure make sure you know:
- The name of the test or procedure
- The reason you are having the test or procedure
- What results to expect and what they mean
- The risks and benefits of the test or procedure
- What the possible side effects or complications are
- When and where you are to have the test or procedure
- Who will do the test or procedure and what that person’s qualifications are
- What would happen if you did not have the test or procedure
- Any alternative tests or procedures to think about
- When and how will you get the results
- Who to call after the test or procedure if you have questions or problems
- How much will you have to pay for the test or procedure