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Electroencephalography Procedure and Purpose

written by: R. Elizabeth C. Kitchen•edited by: Leigh A. Zaykoski•updated: 5/23/2011

If you are getting ready to have an electroencephalography procedure read on to learn more about how it is done, how to prepare, and what the results mean.

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    An electroencephalography procedure is done to look at the brain's electrical activity to detect problems. Electrodes (special sensors) are placed on the patient's head and they have wires attached to them that are hooked into a computer. Certain medical conditions will cause noticeable wavy lines to occur helping doctors diagnose these conditions.

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    Why is Electroencephalography Done?

    This diagnostic test may be done to:

    • Look for problems with dementia or loss of consciousness
    • To determine whether a coma patient is brain-dead
    • Keep an eye on a patient's brain activity while they are under general anesthesia during brain surgery
    • To determine which type of seizure a patient is experiencing and to diagnose epilepsy
    • To determine how much of a chance a patient has to recover after experiencing a change in consciousness
    • Study narcolepsy and other sleep disorders
    • Help determine is the patient has a mental health problem or a physical problem (such as problem with their spinal cord, brain, or nervous system)
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    Preparing for Electroencephalography

    Prior to having this test done, the patient's doctor will talk to them about which medications can cause abnormal test results by affecting the normal electrical activity of the brain. Some of these medications include:

    • Sedatives
    • Muscle relaxers
    • Seizure medications
    • Tranquilizers
    • Sleeping aids

    Caffeine should be avoided at least eight hours prior to having this test done because it can also affect the results. Hair must be clean and product free. Hair conditioner should also be avoided at least 24 hours prior to this test. Patients may also be asked to avoid sleep before this test so that they can sleep through it. Those who are asked not to sleep should have a ride to and from the hospital or testing facility.

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    How Is Electroencephalography Done?

    An electroencephalography procedure can be done in a doctor's office or a hospital and is done by an EEG technologist. The EEG record, however, will be read and analyzed by a medical doctor who has special training in diagnosing and treating nervous system disorders (neurologist).

    Patients will lie down on their back on a table or bed, or they will be asked to relax in a chair and close their eyes. 16 to 25 electrodes will be strategically placed on their head. These electrodes are connected to a computer by wires. The patient should be still and quiet during the test, but are often allowed to reposition and stretch when the technologist tells them they can do so. Throughout the test, the patient may be asked to hyperventilate, look at strobe lights, or sleep.

    This test will take about one to two hours to complete. Once the test is done the patient will be able to go about their normal day, but will need a ride home if they were sleep-deprived for the test.

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    How Does Electroencephalography Feel?

    There is no pain or discomfort during this test. If needle electrodes are used the patient may feel a small, brief prick when they are placed. This is often described as feeling like having a hair pulled out. If the patient needs to hyperventilate during this test, they may have some temporary finger numbness and lightheadedness that will go away within a few minutes of breathing normally.

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    What Do the Results Mean?

    Normal results include:

    • Mostly beta waves and alpha waves when an adult patient is awake
    • No slow brain waves or abnormal bursts of electrical activity on the EEG tracing
    • Normal brain waves, but a possible brief response to the flashing lights

    Abnormal results include:

    • Two sides of the brain showing different electrical activity patterns
    • Brain wave changes that may not be in just one part of the brain could indicate drug intoxication, metabolic disorders, infections such as encephalitis, or changes in the brain or body's chemical balance
    • Showing too many theta waves or delta waves in awake adults could indicate brain illness or injury. This could also be caused by certain medications
    • No electrical activity in the brain could indicate coma, severe drug-induced sedation, lack of blood flow or oxygen inside the brain, or a stop in brain function
    • Sudden spikes of electrical activity or a sudden slowing of brain waves could indicate an infection, stroke, brain tumor, injury, or epilepsy
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    Are There Any Risks?

    This test is very safe. While the brain's electrical activity is recored, no electrical current is introduced into the body. Patients with a seizure disorder may experience a seizure when they are asked to hyperventilate or look at the strobe lights, but the technologist doing the electroencephalography procedure is trained to take care of patients who are having a seizure.

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    WebMD. (2008). Electroencephalogram (EEG). Retrieved on July 22, 2010 from WebMD:

    Medline Plus. (2010). EEG. Retrieved on July 22, 2010 from Medline Plus: