An electroencephalogram (EEG) is a test used to evaluate the electrical activity in the brain. Brain cells communicate with each other through electrical impulses, and an EEG can be used to help detect problems associated with this activity.
An EEG is used to detect problems in the electrical activity of the brain that may be associated with certain brain disorders. The measurements given by an EEG are used to confirm, rule out, or provide information about disorders such as:
- seizure disorders, including epilepsy
- head injury
- encephalitis, or inflammation of the brain
- brain tumor
- encephalopathy, or brain dysfunction resulting from various causes
- memory problems
- sleep disorders
An EEG can be used to monitor activity during brain surgery. It is also performed to determine the level of brain activity in someone who is in a coma.
An EEG cannot provide a measurement of intelligence and is not used to diagnose mental illness.
Electrical impulses in the brain are evaluated using an EEG. The test measures this electrical activity through several electrodes placed on your scalp. An electrode is a conductor through which an electric current can pass safely. The electrodes transfer information from your brain through wires to an amplifier and a machine that measures and records the data.
The test is administered at a hospital, at your healthcare provider’s office, or at a laboratory by a specialized technician. The test usually involves the following steps:
- You will be asked to lie down on your back in a reclining chair or on a bed.
- The technician will measure your head and use a pencil to mark where electrodes will be attached to your scalp. These spots are then scrubbed with a special cream that helps the electrodes get a high-quality reading.
- The technician will put a sticky gel adhesive on 16 to 25 electrodes and will place these electrodes at various spots on your scalp. The electrodes look like flat metal disks.
- Once the test begins, the electrodes send electrical impulse data from your brain to the recording machine. This machine converts the electrical impulses into visual patterns that can be seen on a screen and are saved to a computer. On the screen, the electrical impulses look like wavy lines with peaks and valleys.
- You may be directed by the technician to do certain things while the test is in progress, such as lie still, close your eyes, breathe deeply or quickly, or look at stimuli like a flashing light or a picture.
- The EEG usually takes 30 to 60 minutes.
- After the test is complete, the technician will remove the electrodes.
During the test, very little electricity is passed between the electrodes and your skin. The electrodes do not send any sensations, and you will feel little to no discomfort.
There are no risks associated with an EEG. The test is painless and safe.
In the case of testing people with epilepsy and other seizure disorders, the stimuli presented during the test, such as a flashing light or fast breathing, may cause a seizure. The technician performing the EEG is trained to safely manage patients, if this is to occur.
The following should be done prior to having an EEG:
- Wash your hair the night before the EEG and do not put any products, such as conditioners, sprays, or gels in your hair before the test.
- Check with your doctor to see if you should stop taking any medications before the test and bring a list of your medications to give to the technician performing the EEG.
- Avoid any food or drinks with caffeine for eight hours prior to the test.
- If you are required to sleep during the EEG, your doctor may ask you to sleep as little as possible before the test. Also, you may be given a sedative to help you to relax and sleep before the test begins.
After the EEG is finished and the technician removes the electrodes from your scalp, you should not feel any side effects from the test, and you can continue with your regular routine for the day.
However, if you were given a sedative, the medication will be in your system for a short while. You will need someone to take you home after the test, and you will be instructed by the technician to rest and not drive for the rest of the day.
A specialized doctor interprets the recordings taken from the EEG and then sends the results to the doctor that ordered the test for you. Your doctor may schedule an appointment to go over the test results. It may be helpful to write down any questions you might want to ask your doctor about the results. Be sure to speak up if there is anything about the results that you do not understand.
Electrical activity in the brain is seen in an EEG as a pattern of waves. Different levels of consciousness, such as sleeping and waking, have a specific range of frequencies of waves per second that are considered to be normal. For example, the wave patterns move faster when you are awake than when you are asleep. The EEG will show if the frequency of waves or patterns are normal.
Abnormal EEG results may be due to:
- epilepsy or other seizure disorder
- abnormal bleeding or hemorrhage
- sleep disorder
- swelling of the brain or encephalitis
- problems with attention
- death of tissue because of a blockage of blood flow or cerebral infarction
- alcohol or drug abuse
- head injury