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    Electroencephalogram (EEG)

    What Affects the Test

    You may not be able to have the test, or the results may not be helpful, if:

    • You move too much.
    • You take certain medicines. This includes medicines used to treat seizures (antiepileptic medicines), sedatives, tranquilizers, and barbiturates.
    • You drank coffee, soda, or tea, or you ate other foods that have caffeine before the test.
    • You are unconscious from severe drug poisoning or a very low body temperature (hypothermia).
    • Your hair is dirty, oily, or covered with hair spray or other hair products. This can cause a problem with how the electrodes are placed.

    What To Think About

    • If the doctor thinks that a person has epilepsy but the EEG is normal, the EEG technologist may have the person look at a flashing light. Or the person may be asked to breathe fast and deep (hyperventilate) or sleep during the test. These techniques sometimes show epileptic EEG patterns that did not show up at first. If epilepsy is suspected after the first EEG, the doctor may repeat the EEG more than once.
    • An EEG done during a seizure will almost always show electrical patterns that aren't normal. This makes an EEG useful when a doctor thinks that a person is having psychogenic nonepileptic seizures. These are also called pseudoseizures. They have no physical cause. But they can be caused by stress, emotional trauma, or mental illness. These seizures do not cause abnormal electrical activity in the brain. They will not show abnormal EEG results.
    • There could still be a problem in the brain even when the EEG is normal.
    • Other tests that may also be done include:
      • Video EEG. Video EEG records seizures on video and on a computer. This way the doctor can see what happens just before, during, and right after a seizure. This test can help find the specific area of the brain that the seizures may be coming from. It also helps in diagnosing psychogenic seizures, which may look like real seizures but do not affect the electrical activity in the brain. Video EEG may be used short-term or long-term.
        • Short-term monitoring is done on an outpatient basis. It may last up to 6 hours.
        • Long-term monitoring is done in the hospital. It may last 3 to 7 days.
      • Brain mapping. This is a fairly new method that is very similar to EEG. Electrodes are placed on the person's scalp to transmit the brain's electrical activity. Then a computer makes a color-coded map of signals from the brain. It is sometimes done to find a specific problem area in the brain that has already shown up on a regular EEG. Doctors are still not sure how brain mapping could be best used.
      • Ambulatory EEG monitoring. During this test, the person is able to move around. The test allows for electrical activity in the brain to be recorded for a long time. Fewer electrodes are attached to the person. The person carries a small, portable recording unit. The test may last for a full day or more. The person can leave the hospital. Ambulatory EEG monitoring is not as accurate as a regular EEG.

    WebMD Medical Reference from Healthwise

    Last Updated: August 21, 2015
    This information is not intended to replace the advice of a doctor. Healthwise disclaims any liability for the decisions you make based on this information.

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