An electroencephalogram (EEG) is a test to detect problems in the electrical activity of the brain.

Brain cells communicate by producing tiny electrical impulses. In an EEG, electrodes are placed on the scalp over multiple areas of the brain to detect and record patterns of electrical activity and check for abnormalities.

The test is performed by an EEG technician in a specially designed room that may be in your health care provider's office or at a hospital. You will be asked to lie on your back on a table or in a reclining chair.

The technician will apply between 16 and 25 flat metal discs (electrodes) in different positions on your scalp. The discs are held in place with a sticky paste. The electrodes are connected by wires to an amplifier and a recording machine.

The recording machine converts the electrical signals into a series of wavy lines that are drawn onto a moving piece of graph paper. You will need to lie still with your eyes closed because any movement can alter the results.

You may be asked to do certain things during the recording, such as breathe deeply and rapidly for several minutes or look at a bright flickering light.

You will need to wash your hair the night before the test. Do not use any oils, sprays, or conditioner on your hair before this test.

Your health care provider may want you to stop taking certain medications before the test. Do not change or stop medications without first talking to your health care provider.

You should avoid all foods containing caffeine for 8 hours before the test.

Sometimes it is necessary to sleep during the test, so you may be asked to reduce your sleep time the night before.

This test causes no discomfort. Although having electrodes pasted onto your skin may feel strange, they only record activity and do not produce any sensation.

EEG is used to help diagnose the presence and type of seizure disorders, to look for causes of confusion, and to evaluate head injuries, tumors, infections, degenerative diseases, and metabolic disturbances that affect the brain.

It is also used to evaluate sleep disorders and to investigate periods of unconsciousness. The EEG may be done to confirm brain death in a comatose patient.

EEG cannot be used to measure intelligence or diagnose mental illness.

Brain waves have normal frequency and amplitude, and other characteristics are typical.

What does the EEG show?

The EEG gives information about the electrical activity of the brain during the time the test is happening. When someone has an epileptic seizure, their brain activity changes and is irregular. If they have a seizure during the test, the change in brain activity can be seen on the EEG recording. This is sometimes called 'epileptiform activity'. Some people have epileptiform activity even when they are not having a seizure so the EEG can also be useful for them.

Epileptiform activity can sometimes be seen more clearly during deep breathing. So part of the test might include deep breathing or repeated blinking to see if there are any changes.

Abnormal findings may suggest the following:

• Seizure disorders (such as epilepsy or convulsions)

• Structural brain abnormality (such as a brain tumor or brain abscess)

• Head injury

• Attention deficit problems

• Encephalitis (inflammation of the brain)

• Hemorrhage (abnormal bleeding caused by a ruptured blood vessel)

• Cerebral infarct (tissue that is dead because of a blockage of the blood supply)

• Sleep disorders (such as narcolepsy)

• Drug or alcohol abuse

• Migraines (in some cases)

• Hallucinations

• Syncope

• Vertigo

EEG may confirm brain death in someone who is in a coma. Note: a normal EEG does not mean that a seizure did not occur.

Additional conditions under which the test may be performed:

• Arteriovenous malformation (cerebral)

• Benign positional vertigo

• Cerebral aneurysm

• Complicated alcohol abstinence (delirium tremens)

• Creutzfeldt-Jacob disease

• Delirium

• Dementia

• Dementia due to metabolic causes

• Febrile seizure (children)

• Generalized tonic-clonic seizure

• Hepatic encephalopathy

• Hepatorenal syndrome

• Insomnia

• Labyrinthitis

• Meniere's disease

• Metastatic brain tumor

• Multiple sclerosis

• Optic glioma

• Partial (focal) seizure

• Partial complex seizure

• Petit mal seizure

• Pick's disease

• Senile dementia (Alzheimer's type)

• Shy-Drager syndrome

• Syphilitic aseptic meningitis

• Temporal lobe seizure

The procedure is very safe. If you have a seizure disorder, a seizure may be triggered by flashing lights or by hyperventilation. The health care provider performing the EEG is trained to take care of you if this happens.