In electroencephalography, small metal sensors (electrodes) are applied to your scalp to record the electrical signals produced by nerve cells in the brain. The electrodes transmit this information to a machine that magnifies the electrical activity and provides a graphic representation of brain waves.
This test is usually done while you are awake, but in one variation (called a sleep EEG), you must stay awake the night beforehand, and brain activity is then recorded after you are given a sedative to help you sleep. Another test, called polysomnography, measures EEG activity and other body functions, such as breathing and heart rate, during one full night of sleep.
Purpose of the Electroencephalography
- To identify and evaluate the cause of seizures
- To aid in the diagnosis of intracranial (within the skull) lesions, such as an abscess or tumor
- To evaluate brain wave activity in people with brain or spinal cord infections (such as meningitis or encephalitis); head injury; and psychiatric conditions
- To diagnose a stroke and determine the extent of damage that has occurred
- To diagnose Alzheimer's disease, certain psychoses, a sleep disorder called narcolepsy, and other disorders that influence brain activity
- To monitor brain wave activity during surgery
- To help evaluate sleep disorders
- To diagnose a coma
- To confirm brain death
Who Performs Electroencephalography
- A doctor, a nurse, or a technician who is trained in EEG
Special Concerns about Electroencephalography
- This test is usually conducted in a special room designed to eliminate electrical interference and minimize distractions.
- A variety of medications (including sedatives, anticonvulsants, tranquilizers, and barbiturates), caffeinated beverages, failure to eat before the test, or the presence of bright or flashing lights may alter the results of the test.
- You must remain still during the test. Any excess movement can interfere with the accuracy of results.
Before the Electroencephalography
- Inform your doctor of any medications you regularly take. Certain of these agents may need to be discontinued for 1 or 2 days before the test.
- Wash your hair the night before the test, and do not use hair spray, gel, or other hair care products after shampooing.
- Be sure to eat a normal meal before the test, since low blood sugar (hypoglycemia) may interfere with test results. However, you should avoid caffeine-containing beverages, such as coffee, tea, or cola for 8 hours before the test.
- You are asked to stay up as late as possible on the night before a sleep EEG.
- If you are at risk for seizures, arrange for transportation to and from the test.
What You Experience during Electroencephalography
- You will either sit in a reclining chair or lie down on a bed.
- For most studies, 20 electrodes are attached to your scalp with a special adhesive paste or gel in a specified pattern, plus 1 on each earlobe and 1 on the forehead. (Less commonly, electrodes with tiny needles are placed in the skin of the scalp. These may cause a pricking sensation during insertion.)
- You are asked to relax, close your eyes, and lie still as the EEG recording is being taken. The examiner will note any movements, such as blinking, swallowing, or talking that may affect the test results.
- Periodically, the examiner will stop the procedure to allow you to reposition yourself and get comfortable. Fatigue and restlessness can alter brain wave patterns.
- You may be asked to breathe deeply and rapidly (hyperventilate) for 3 minutes to stimulate hidden brain wave abnormalities not evident during the resting EEG.
- “Photostimulation” may also be conducted by flashing a light at variable speeds over your face when your eyes are open or closed. This is intended to produce abnormal activity present in seizures that are stimulated by light.
- If you are undergoing a sleep EEG, you are given a sedative medication and brain wave activity is measured as you fall asleep, while you are asleep, and as you wake.
- The procedure typically takes 1 hour.
Risks and Complications of EEG
- EEG is a safe procedure. However, people who are susceptible to seizures may have one during the test.
After the Electroencephalography
- The electrodes are removed and the adhesive paste is washed away with acetone or warm water. You may need to use acetone and wash your hair at home to remove any residue.
- Unless you are actively having seizures or you underwent a sleep EEG, you may drive home immediately. If you’ve had a sleep EEG, you will be observed until the sedative wears off, and someone should drive you home.
- Resume taking any medications withheld before the test, according to your doctor’s instructions.
Results of Electroencephalography
- A neurologist will evaluate your brain wave patterns for evidence of any abnormality. For example, seizure activity is indicated by rapid, spiking waves, while cerebral lesions such as tumors or blood clots will result in abnormally slow EEG waves. The presence of an inflammatory brain disease, such as meningitis or encephalitis, may cause diffuse and slow brain waves.
- If a definitive diagnosis can be made, appropriate therapy will be initiated.
- In most cases, additional imaging tests of the brain—such as a CT scan, an MRI, or a nuclear scan—are necessary to pinpoint the location of any abnormality.
The Johns Hopkins Consumer Guide to Medical Tests
Simeon Margolis, M.D., Ph.D., Medical Editor
Updated by Remedy Health Media