EEG (Electroencephalogram)

What is an EEG?
An electroencephalogram (EEG) is a test used to evaluate the electrical activity in the brain.

Brain cells communicate with each other through electrical impulses. An EEG can be used to help detect potential problems associated with this activity.

An EEG tracks and records brain wave patterns. Small flat metal discs called electrodes are attached to the scalp with wires. The electrodes analyze the electrical impulses in the brain and send signals to a computer that records the results.

The electrical impulses in an EEG recording look like wavy lines with peaks and valleys. These lines allow doctors to quickly assess whether there are abnormal patterns. Any irregularities may be a sign of seizures or other brain disorders.

An EEG is used to detect problems in the electrical activity of the brain that may be associated with certain brain disorders. The measurements given by an EEG are used to confirm or rule out various conditions, including:

  • Seizure disorders (such as epilepsy)
  • Head injury
  • Encephalitis (inflammation of the brain)
  • Brain tumor
  • Encephalopathy (disease that causes brain dysfunction)
  • Memory problems
  • Sleep disorders
  • Stroke
  • Dementia

When someone is in a coma, an EEG may be performed to determine the level of brain activity. The test can also be used to monitor activity during brain surgery.

How do I prepare for an EEG?
Let your doctor know about any medications - both prescription and over-the-counter -- and supplements you're taking.

Wash your hair the night before the test. Don't use any leave-in conditioning or styling products afterward.


The test usually takes 30 to 60 minutes to complete. You lie down on the exam table or bed, and a technician puts about 20 small sensors on your scalp. These sensors, called electrodes, pick up activity from cells inside your brain called neurons and send them to a machine, where they show up as a series of lines recorded on moving paper or displayed on a computer screen.

You'll relax with your eyes open first, then with them closed. The technician may ask you to breathe deeply and rapidly or to stare at a flashing light, because both of these can change your brain wave patterns.

What do the EEG test results mean?
A neurologist (someone who specializes in nervous system disorders) interprets the recordings from the EEG and then sends the results to your doctor. Your doctor may schedule an appointment to go over the test results with you.

Normal results
Electrical activity in the brain appears in an EEG as a pattern of waves. Different levels of consciousness, like sleeping and waking, have a specific range of frequencies of waves per second that are considered normal. For example, the wave patterns move faster when you’re awake than when you’re asleep. The EEG will show if the frequency of waves or patterns are normal. Normal activity typically means you don’t have a brain disorder.

Abnormal results
Abnormal EEG results may be due to:

  • Epilepsy or another seizure disorder
  • Abnormal bleeding or hemorrhage
  • Sleep disorder
  • Encephalitis (swelling of the brain)
  • Tumor
  • Dead tissue due to a blockage of blood flow
  • Migraines
  • Alcohol or drug abuse
  • Head injury

Information Sources, on 2018.06.02

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