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:
- Alpha waves have a frequency of 8 to 12 cycles per second. Alpha
waves are present only in the waking state when your eyes are closed but you
are mentally alert. Alpha waves go away when your eyes are open or you are
- Beta waves have a frequency of 13 to 30 cycles per second. These
waves are normally found when you are alert or have taken high doses of certain
medicines, such as
- Delta waves have a frequency of less than 3 cycles per second.
These waves are normally found only when you are asleep or in young
- Theta waves have a frequency of 4 to 7 cycles per second. These
waves are normally found only when you are asleep or in young children.
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.