An electroencephalogram (EEG) is a test that finds problems in your brain waves — the electrical activity of your brain. During an EEG, electrodes are pasted onto your scalp. These are small metal disks with thin wires. They detect tiny electrical charges that result from the activity of your brain cells. The charges appear as a graph on a computer screen. Or the recording may be printed out on paper. Your doctor then interprets the reading.

During an EEG, a doctor typically looks at about 100 pages, or computer screens, of activity. They pay special attention to the basic waveform. But your doctor also looks at brief bursts of energy and responses to stimuli, such as flashing lights.

You may also have tests called evoked potential studies. These studies measure electrical activity in your brain in response to things that stimulate sight, sound, or touch.

An EEG is used to look for several types of brain disorders. When epilepsy is present, seizure activity will appear as rapid spiking waves on the EEG.

People with lesions on their brain, which can result from tumors or stroke, may have very slow EEG waves. It depends on the size and the location of the lesion.

The test can also be used to diagnose other disorders that influence brain activity. These may include Alzheimer disease, certain psychoses, and a sleep disorder called narcolepsy.

An EEG may also be used to look at the overall electrical activity of the brain. For example, it may be used to evaluate trauma, drug intoxication, or the extent of brain damage in a person who is in a coma. Depending on where the injury is, an EEG is one test of many to help decide brain death in critically ill patients. An EEG may also be used to monitor blood flow in the brain or neck's blood vessels during surgery.

There may be other reasons for your doctor to advise an EEG.

An EEG has been used for many years. It's considered a safe procedure. The test causes no discomfort. The electrodes record activity. They don't produce any sensation. There is also no risk of getting an electric shock.

In rare cases, 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 doctor will treat it right away.

There may be other risks, depending on your specific health condition. Talk with your doctor before the procedure about any concerns.

Certain things may interfere with the reading of an EEG test. These include:

• Low blood sugar (hypoglycemia) caused by fasting.
• Body or eye movement during the test, which rarely, if ever, majorly interferes with the results of the test.
• Lights, especially bright or flashing ones.
• Certain medicines, such as sedatives.
• Drinks that have caffeine, such as coffee, cola, and tea, which can sometimes change the EEG results. (They almost never interfere in a major way with the outcome of the test.)
• Oily hair or the presence of hair spray.

Your doctor will explain the procedure to you, and you can ask questions. Ask your doctor what you should do before your test. Common steps that you may be asked to do include the following:

• You'll be asked to sign a consent form. It gives your permission to do the procedure. Read the form carefully and ask questions if something isn't clear.
• Wash your hair with shampoo. Don't use a conditioner the night before the test. Don't use any hair styling products, such as hairspray or gel.
• Tell your doctor about all prescription and over-the-counter medicines and any herbal supplements that you're taking.
• Stop taking medicines that may interfere with the test if your doctor has told you to do so. Talk with your doctor first before you stop taking any medicines.
• Don't consume any food or drinks that have caffeine for 8 to 12 hours before the test.
• Follow any directions your doctor gives you about reducing your sleep the night before the test. Some EEG tests require that you sleep through the procedure, and some don't. 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.
• Don't fast the night before or the day of the procedure. Low blood sugar may influence the results.
• Based on your health condition, your doctor may request other preparations.

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 doctor's practices. Talk with your doctor about what you should expect.

In general:

1. You'll be asked to relax in a reclining chair or lie on a bed.
2. Between 16 and 25 electrodes will be attached to your scalp with a special paste. Or a cap with the electrodes will be used.
3. You'll be asked to close your eyes, relax, and be still.
4. Once the recording begins, you'll need to remain still during the test. Your doctor may watch you through a window in another room to look for any movements that can cause an inaccurate reading, such as swallowing or blinking. The recording may be stopped now and then to let you rest or reposition yourself.
5. After your doctor does the first recording while you're at rest, they may test you with various stimuli to make brain wave activity that doesn't show up while you're 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.
6. This study is generally done by an EEG technician. It may take 45 minutes to 2 hours.
7. If you're being tested for a sleep disorder, the EEG may be done while you are asleep.
8. If you need to be watched for a longer period of time, you may also be admitted to the hospital for prolonged EEG (24-hour EEG) monitoring.
9. In cases where prolonged inpatient monitoring isn't possible, your doctor may consider doing an ambulatory EEG.

When the test is finished, 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 need to rest until the sedatives have worn off. You'll need to have someone drive you home.

You may have skin irritation or redness where the electrodes were placed. If so, it will wear off in a few hours.

Your doctor will inform you when you may restart any medicines you stopped taking before the test.

Your doctor may give you other instructions after the procedure, depending on your 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 you will get the results.
• Who to call after the test or procedure if you have questions or problems.
• How much you will have to pay for the test or procedure.