Home > Health Library > Electroencephalogram (EEG)
An electroencephalogram (EEG) is a test that
measures and records the electrical activity of your brain. Special sensors
(electrodes) are attached to your head and hooked by
wires to a computer. The computer records your brain's electrical activity on
the screen or on paper as wavy lines. Certain conditions, such as
seizures, can be seen by the changes in the normal
pattern of the brain's electrical activity.
electroencephalogram (EEG) may be done to:
Before the day of the
electroencephalogram (EEG) test, tell your doctor if you are taking any
medicines. Your doctor may ask you to stop taking certain medicines (such as
sedatives and tranquilizers, muscle relaxants,
sleeping aids, or medicines used to treat seizures) before the test. These
medicines can affect your brain's usual electrical activity and cause abnormal
Do not eat or drink foods that have caffeine (such
as coffee, tea, cola, and chocolate) for 12 hours before the test.
Since the electrodes are attached to your scalp, make sure
your hair is clean and free of sprays, oils, creams, and lotions. Shampoo your
hair and rinse with clear water the evening before or the morning of the test.
Do not put any hair conditioner or oil on after shampooing.
find certain types of abnormal electrical activity in the brain, you may have
to be asleep during the recording. You may be asked not to sleep at all the
night before the test or to sleep less (about 4 or 5 hours) by going to bed
later and getting up earlier than usual. 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 a sleep-deprived EEG, plan to have someone drive you to
and from the test.
An electroencephalogram (EEG) may be
done in a hospital or in a doctor's office by an EEG technologist. The EEG
record is read by a doctor who is specially trained to diagnose and treat
disorders affecting the nervous system (neurologist).
You will be asked to lie on
your back on a bed or table or relax in a chair with your eyes closed. The EEG
technologist will attach several flat metal discs (electrodes) to different
places on your head, using a sticky paste to hold the electrodes in place. A
cap with fixed electrodes may be placed on your head instead of individual
electrodes. In rare cases, these electrodes may be attached to the scalp with
The electrodes are hooked by wires to a computer
that records the electrical activity inside the brain. A machine can show the
activity as a series of wavy lines drawn by a row of pens on a moving piece of
paper or as an image on the computer screen.
Lie still with your
eyes closed during the recording, and do not talk to the technologist unless
you need to. The technologist will watch you directly or through a window
during the test. The recording may be stopped from time to time to allow you to
stretch and reposition yourself.
The technologist may ask you to
do different things during the test to record what activity your brain does at
An EEG takes 1 to 2 hours. After the test, you may do your
normal activities. But if you were sleep-deprived or given a sleep medicine,
have someone drive you home after the test.
There is no pain with an
If paste is used to hold the
electrodes, some paste may stay in your hair after the test, so you will have
to wash your hair to remove it. If needle electrodes are used (which is rare),
you will feel a brief, sharp prick (about like having a hair pulled out) when
each electrode is inserted. If electrodes are placed in your nose, they may
cause a tickling feeling and, rarely, some soreness or a small amount of
bleeding for 1 to 2 days after the test.
If you are asked to
breathe rapidly, you may feel lightheaded or have some numbness in your
fingers. This reaction is normal. It will go away a few minutes after you start
breathing normally again.
An electroencephalogram (EEG) is a very safe
test. The electrical activity of your brain is recorded, but at no time is any
electrical current put into your body. An EEG should not be confused with
electroshock (electroconvulsive) therapy.
If you have a seizure
disorder such as epilepsy, a seizure may be triggered by the flashing lights or
by hyperventilation. If this occurs, the technologist is trained to take care
of you during the seizure.
electroencephalogram (EEG) is a test that measures and
records the electrical activity of your brain. Special sensors (electrodes) are attached to your head and hooked by
wires to a computer. EEG test results are ready on the same or the next
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 (photic
stimulation) 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 a problem in one
area or side of the brain is present.
The EEG shows sudden bursts of
electrical activity (spikes) or sudden slowing of brain waves in the brain.
These changes may be caused by a brain tumor, infection, injury,
epilepsy. When a person has epilepsy, the location and
exact pattern of the abnormal brain waves may help show what type of epilepsy
or seizures the person has. Keep in mind that in many people with epilepsy, the
EEG may appear completely 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 affecting
the entire brain—such as drug intoxication, infections (encephalitis), or metabolic disorders (such as
diabetic ketoacidosis) that change the chemical
balance in the body, including the brain—may cause these kinds of
The EEG shows delta waves or
too many theta waves in adults who are awake. These results may mean brain
injury or a brain illness is present. Some medicines can also cause
The EEG shows no electrical
activity in the brain (a "flat" or "straight-line" EEG). This means that brain
function has stopped, which is usually caused by lack of oxygen or blood flow
inside the brain. This may happen when a person has been in a coma. In some
cases, severe drug-induced sedation can cause a flat EEG.
Reasons you may not be able to
have the test or why the results may not be helpful include:
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.
Current as of:
October 10, 2013
Anne C. Poinier, MD - Internal Medicine & Colin Chalk, MD, CM, FRCPC - Neurology
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