EEG
An electroencephalogram (EEG) is a test to detect problems in the electrical activity of the brain.
How the Test is Performed
Brain cells communicate with each other by producing tiny electrical impulses. In an EEG, this faint electrical activity is measured by putting electrodes on the scalp.
The test is performed by an EEG technician in your health care provider's office, at a hospital, or at a laboratory. You will be asked to lie on your back on a bed or in a reclining chair.
The technician will place between 16 and 25 flat metal disks (electrodes) in different places on your scalp. The disks are held in place with a sticky paste. The electrodes are connected by wires to an amplifier and a recording machine.
The recording machine converts the electrical impulses into patterns that can be seen on a computer screen and stored on a computer disk. Before computers, the activity was printed on paper. In either case, the electrical activity looks like a series of wavy lines. You will need to lie still with your eyes closed because any movement can change the results.
You may be asked to do certain things during the recording, such as breathe fast and deeply for several minutes or look at a bright flashing light.
How to Prepare for the Test
You will need to wash your hair the night before the test. Do not use any oils, sprays, or conditioner on your hair before this test. If you have a hair weave, you may want to ask the laboratory for special instructions.
Your health care provider may want you to stop taking certain medications before the test. Do not change or stop taking medications without first talking to your health care provider. Bring a list of your medications with you.
Avoid all foods containing caffeine for 8 hours before the test.
Sometimes you may need to sleep during the test, so you may be asked to reduce your sleep time the night before. If you are asked to sleep as little as possible before the test, do not eat or drink any caffeine, energy drinks, or other products that help you stay awake.
How the Test Will Feel
This test causes no discomfort. Although having electrodes pasted onto your skin may feel strange, they only record activity and do not produce any sensation. Very little electricity passes from the electrode into your skin.
Why the Test is Performed
EEG is used to help diagnose seizures and their type. It is also used to evaluate:
- Abnormal changes in body chemistry that affect the brain
- Brain diseases such as Alzheimer's disease
- Confusion
- Head injuries
- Infections
- Tumors
EEG is also used to:
- Evaluate problems with sleep ( sleep disorders)
- Investigate periods of unconsciousness
- Monitor the brain during brain surgery
An EEG may be done to show that the brain has no activity, in the case of someone who is in a deep coma. It can be helpful when trying to decide if someone is brain dead.
EEG cannot be used to measure intelligence.
Normal Results
Brain electrical activity has a certain number of waves per second (frequencies) that are normal for different levels of consciousness. For example, brain waves are faster when you are awake, and slower when you are sleeping.
There are also normal patterns to these waves. The EEG looks for these frequencies and patterns.
What Abnormal Results Mean
Abnormal results on an EEG test may be due to:
- Abnormal bleeding (hemorrhage)
- An abnormal structure in the brain (such as a brain tumor)
- Attention problems
- Tissue death due to a blockage in blood flow (cerebral infarction)
- Drug or alcohol abuse
- Head injury
- Migraines (in some cases)
- Seizure disorder (such as epilepsy or convulsions)
- Sleep disorder (such as narcolepsy)
- Swelling of the brain (encephalitis)
Note: A normal EEG does not mean that a seizure did not occur.
Risks
The procedure is very safe. However, the flashing lights or fast breathing (hyperventilation) required during the test may trigger seizures in those with seizure disorders. The health care provider performing the EEG is trained to take care of you if this happens.
It may be difficult to get the paste out of your hair, but it should come out after a few washings with regular shampoo.
Alternative Names
Electroencephalogram; Brain wave test
References
Trescher WH, Lesser RP. The Epilepsies. In: Bradley WG, Daroff RB, Fenichel GM, Jankovic J, eds. Neurology in Clinical Practice. 5th ed. Philadelphia, Pa: Butterworth-Heinemann; 2008:chap 71.
Krumholz A, Wiebe S, Gronseth G, et al. Practice parameter: evaluating an apparent unprovoked first seizure in adults (an evidence-based review): report of the Quality Standards Subcommittee of the American Academy of Neurology and the American Epilepsy Society. Neurology. 2007;69(21):1991-2007.
Woo Lee J, Khoshbin S. Clinical neurophysiology and electroencephalography. In: Stern TA, Rosenbaum JF, Fava M, et al, eds. Massachusetts General Hospital Comprehensive Clinical Psychiatry. 1st ed. Philadelphia, Pa: Mosby Elsevier; 2008:chap 75.
Update Date: 1/4/2011
Updated by: Linda J. Vorvick, MD, Medical Director, MEDEX Northwest Division of Physician Assistant Studies, University of Washington, School of Medicine. Also reviewed by Joseph V. Campellone, MD, Division of Neurology, Cooper University Hospital, Camden, NJ. Review provided by VeriMed Healthcare Network. Also reviewed by David Zieve, MD, MHA, Medical Director, A.D.A.M., Inc.
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