An electroencephalogram (EEG) is a test that measures and records the electrical activity of your brain. Special sensors called electrodes are attached to your head. They're hooked by wires to a computer. The computer records your brain's electrical activity on the screen. Or it may record the activity on paper as wavy lines. Changes from the normal pattern of electrical activity can show certain conditions, such as seizures.
An EEG may be done to:
Before the day of the EEG test, tell your doctor if you are taking any medicines. Your doctor may ask you to stop taking certain medicines before the test. They include sedatives and tranquilizers, muscle relaxants, sleeping aids, and medicines used to treat seizures. These medicines can affect your brain's usual electrical activity. Taking them may affect your test results.
Do not eat or drink things that have caffeine for 12 hours before the test. This includes coffee, tea, cola, and chocolate.
The electrodes will be attached to your scalp. Make sure that your hair is clean before the test. Don't put sprays, oils, creams, or lotions in your hair. Shampoo your hair and rinse with clear water the night before or the morning of the test. Do not put any hair conditioner or oil in your hair after you wash it.
To find certain types of abnormal electrical activity in the brain, you may have to be asleep during the test. You may be asked not to sleep at all the night before the test. Or you may need to sleep less (about 4 or 5 hours) by going to bed late and getting up early. If your child is going to be tested, try to keep him or her from taking naps just before the test. If you know that you are going to have an EEG with little or no sleep, plan to have someone drive you to and from the test.
An EEG may be done in a hospital or in a doctor's office. An EEG technologist does the test. The EEG record is read by a doctor who is trained to diagnose and treat problems that affect the nervous system (neurologist).
You will be asked to lie on your back on a bed or table. Or you may sit in a chair with your eyes closed. The EEG technologist will attach several flat metal discs (electrodes) to different places on your head. A sticky paste is used to hold them in place. Instead of separate electrodes, you may wear a cap with several fixed electrodes. In rare cases, the electrodes may be attached to the scalp with tiny needles.
The electrodes are hooked by wires to a computer that records the electrical activity in the brain. A machine can show the activity as a series of wavy lines on a piece of paper. Or the activity may be shown as an image on the computer screen.
You will need to lie still with your eyes closed during the recording. The technologist will watch you directly or through a window during the test. The recording may be stopped from time to time. This allows you to stretch and change your position.
The technologist may ask you to do different things during the test to see what activity your brain does at that time.
An EEG takes 1 to 2 hours. After the test, you may do your normal activities. But if you had little or no sleep or were given a sleep medicine, have someone drive you home after the test.
There is no pain during an EEG.
Paste may be used to hold the electrodes in place. Some of the paste may stick in your hair after the test. You will have to wash your hair to get it out. If needle electrodes are used (which is rare), you will feel a brief, sharp prick when each electrode is put in. It will feel kind of like having a hair pulled out. If electrodes are placed in your nose, they may tickle. Rarely, this may cause some soreness or a small amount of bleeding for 1 to 2 days after the test.
If you are asked to breathe fast, you may feel light-headed or have some numbness in your fingers. This is normal. It will go away a few minutes after you start breathing normally again.
An EEG is a very safe test. The electrical activity of your brain is recorded. But no electrical current is put into your body. An EEG is not the same as electroshock (electroconvulsive) therapy.
If you have a seizure disorder such as epilepsy, the flashing lights may trigger a seizure. Or a seizure may happen if you hyperventilate. If it happens, the technologist is trained to take care of you during the seizure.
An electroencephalogram (EEG) is a test that measures and records the electrical activity of your brain. Special sensors called electrodes are attached to your head. They're hooked by wires to a computer. EEG test results are ready on the same day or the next day.
There are several types of brain waves.
In adults who are awake, the EEG shows mostly alpha waves and beta waves.
The two sides of the brain show similar patterns of electrical activity.
There are no abnormal bursts of electrical activity and no slow brain waves on the EEG tracing.
If flashing lights are used during the test, one area of the brain (the occipital region) may have a brief response after each flash of light. But the brain waves are normal.
The two sides of the brain show different patterns of electrical activity. This may mean that there's a problem in one area or side of the brain.
The EEG shows sudden bursts of electrical activity called spikes. Or the test shows sudden slowing of brain waves in the brain. These changes may be caused by a brain tumour, infection, injury, stroke, or epilepsy. When a person has epilepsy, the location and exact pattern of the abnormal brain waves may help show the type of epilepsy or seizures. In many people with epilepsy, the EEG may appear normal between seizures. An EEG by itself does not diagnose or rule out epilepsy or a seizure problem.
The EEG records changes in the brain waves that may not be in just one area of the brain. A problem that affects the whole brain may cause these kinds of changes. This includes drug intoxication, infections (encephalitis), and metabolic disorders (such as diabetic ketoacidosis). These problems change the chemical balance in the body, including the brain.
The EEG shows delta waves or too many theta waves in adults who are awake. This may mean that there is a brain injury or brain illness. Some medicines can also cause this.
The EEG shows no electrical activity in the brain. This is a "flat" or "straight-line" EEG. This means that brain function has stopped. It's usually caused by lack of oxygen or blood flow inside the brain. It may happen when a person has been in a coma. In some cases, severe sedation from drugs can cause a flat EEG.
You may not be able to have the test, or the results may not be helpful, if:
Other Works Consulted
Chernecky CC, Berger BJ (2013). Laboratory Tests and Diagnostic Procedures, 6th ed. St. Louis: Saunders.
Fischbach FT, Dunning MB III, eds. (2009). Manual of Laboratory and Diagnostic Tests, 8th ed. Philadelphia: Lippincott Williams and Wilkins.
Pagana KD, Pagana TJ (2010). Mosby's Manual of Diagnostic and Laboratory Tests, 4th ed. St. Louis: Mosby Elsevier.
ByHealthwise StaffPrimary Medical ReviewerAnne C. Poinier, MD - Internal MedicineE. Gregory Thompson, MD - Internal MedicineAdam Husney, MD - Family MedicineMartin J. Gabica, MD - Family MedicineSpecialist Medical ReviewerColin Chalk, MD, CM, FRCPC - NeurologyKarin M. Lindholm, DO - Neurology
Current as ofOctober 14, 2016
Current as of: October 14, 2016
Anne C. Poinier, MD - Internal Medicine
& E. Gregory Thompson, MD - Internal Medicine & Adam Husney, MD - Family Medicine & Martin J. Gabica, MD - Family Medicine & Colin Chalk, MD, CM, FRCPC - Neurology & Karin M. Lindholm, DO - Neurology
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