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Alternate Names

  • electroencephalogram


An electroencephalogram (EEG) is a recording of the electrical waves in the brain. It measures electrical impulses that are sent between nerve cells.

Who is a candidate for the test?

The healthcare professional may order an EEG to investigate the cause of these symptoms:
  • amnesia
  • confusion
  • dizziness
  • fainting
  • headaches
  • seizures
  • staring or other attention problems
  • unusual behavior
An EEG may be used to monitor blood flow to the brain during surgery on the carotid arteries. It is also used to determine brain death in a person who is in a coma.

How is the test performed?

An EEG uses small electrodes to measure the electrical activity within the brain. It does not deliver any electricity of its own. The technician will use a paste to attach about 20 small electrodes to the person's scalp.
The electrodes carry information about the brain's electrical activity to an amplifier. A special machine records the amplified brain waves and produces a pattern of tracings on paper.
During the recording, the patient may be asked to breathe deeply or to look at a flashing light. Occasionally, an EEG is performed both during sleep and while the patient is awake.

What is involved in preparation for the test?

Instructions for an EEG usually include the following preparation:
  • Avoid caffeine for 24 hours before the EEG.
  • eat normally and take all prescribed medications.
  • get no more than 6 hours of sleep.
  • shampoo the night before the EEG. Do not apply hair spray or gel.

What do the test results mean?

A normal EEG does not necessarily rule out abnormalities in the brain. However, the following conditions can be detected on an EEG:
  • brain death
  • brain infection
  • brain tumors
  • chemical disturbances that affect the brain
  • degenerative neurological diseases
  • epilepsy
  • head injury
  • intracerebral hemorrhage, or bleeding into the brain
  • sleep disorders
  • stroke

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