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 an independent laboratory. You will be asked to lie on your back on a bed or in a reclining chair.
The technician will apply between 16 and 25 flat metal disks (electrodes) in different positions 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, as well as stored on a computer disk. Before computers, the activity was printed on paper. In either case, the electical activity looks like a series of wavy lines. You will need to lie still with your eyes closed because any movement can alter the results.
You may be asked to do certain things during the recording, such as breathe deeply and rapidly for several minutes or look at a bright flashing light.
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.
Your health care provider may want you to stop taking certain medications before the test. Do not change or stop medications without first talking to your health care provider.
You should avoid all foods containing caffeine for 8 hours before the test.
Sometimes it is necessary to sleep during the test, so you may be asked to reduce your sleep time the night before. If you're asked to sleep as little as possible before the test, don't consume any caffeine, energy drinks, or other products that help you stay awake.
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. No significant electricity passes from the electrode into your skin.
EEG is used to help diagnose if you're having seizures and if so, what type. An EEG is also used to find the causes of confusion, and to evaluate head injuries, tumors, infections, degenerative diseases such as Alzheimer's disease, and abnormal changes in body chemistry that affect the brain.
It is also used to:
- Evaluate problems with sleep ( sleep disorders)
- To investigate periods of unconsciousness
- To monitor the brain during brain surgery
The EEG may be done to show that the brain has no activity, in the case of someone in a deep coma. It can be helpful when trying to decide if someone is brain dead.
EEG cannot be used to measure intelligence.
Brain electrical activity has certain frequencies (the number of waves per second) that are normal for different levels of consciousness. For example, brain waves are faster when you are awake, and slower when you're sleeping. There are also normal patterns to these waves. These frequencies and patterns are what the EEG reader looks for.
Abnormal results on an EEG test may be due to:
- 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
- Inflammation of the brain (encephalitis)
- Hemorrhage (abnormal bleeding caused by a ruptured blood vessel)
- Migraines (in some cases)
- Seizure disorder (such as epilepsy or convulsions)
- Sleep disorder (such as narcolepsy)
Note: A normal EEG does not mean that a seizure did not occur.
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.
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.