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Home > Patient & Family Resources > Health Library > Electroencephalogram (EEG)
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.
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.
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
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
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
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 lightheaded 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.
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
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 tumor, infection, injury,
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
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 MedicineSpecialist Medical ReviewerKarin M. Lindholm, DO - Neurology
Current as ofOctober 9, 2017
Current as of:
October 9, 2017
Anne C. Poinier, MD - Internal Medicine & E. Gregory Thompson, MD - Internal Medicine & Karin M. Lindholm, DO - Neurology
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