EEG, which stands for electroencephalogram, is a diagnostic test used to detect abnormal brain activity that might indicate a serious medical condition. Through the use of electrodes carefully placed on the scalp, the EEG records the activity of thousands of neurons in the brain.


What is an EEG used for?

EEGs are used to help physicians diagnose and treat patients with the following conditions:

  • Seizure disorders such as epilepsy
  • Confusion, dementia, and other brain disorders
  • Encephalitis (swelling of the brain) and other infections
  • Head injuries
  • Brain tumors
  • Sleep disorders

EEG can also be used to evaluate:

  • Periods of unconsciousness
  • The potential for recovery from cardiac arrest or other major trauma
  • Comatose patients by evaluating or ruling out brain death
  • Brain activity during surgery when a patient is under general anesthesia

How does it work?

EEG measures brain waves and is able to detect abnormal brain activity through the use of electrodes. Using a mild adhesive, these electrodes are strategically placed on the scalp and connected to the EEG machine with thin wires. Electrical impulses that naturally occur throughout the brain are recorded and amplified by the machine and, using a row of pens, are illustrated as a series of wavy lines on a moving piece of paper or on a computer screen. At no time do electrical currents enter the brain. The electrodes only record activity; they do not produce any sensations.

During an EEG, brain activity is recorded while the patient is at rest. Normal brain activity changes depending on the stimulus. When a patient is relaxed, EEG waves are slow, and when a patient is excited, the waves move faster. If necessary, the patient may be asked to change breathing patterns, look at a bright or flashing light, or even go to sleep to see how the brain functions during these conditions.


What can I expect during an EEG?

An EEG test is a painless procedure that is usually performed in your doctor’s office or in a hospital. You will be asked to sit or lie down and close your eyes (to keep brain stimulation at a minimum). About 20-25 electrodes, which look like small silver buttons or discs, will be attached to your scalp and then to the EEG machine. You will be asked to relax and keep still for the duration of the test, which is usually 30-60 minutes in length. The test may be stopped to allow you to stretch or re-position yourself for comfort.

To prepare for an EEG test, you may be asked to:

  • Stop taking certain medications (but only do this if your doctor says it’s necessary).
  • Refrain from eating or drinking any foods that contain caffeine for at least 8 hours before the test.
  • Eat a small meal before the test.
  • Wash your hair the day of the test but do not use any oils, sprays, or other hair products.
  • If your EEG will be monitored while you are sleeping, you may be asked to reduce the amount of sleep you get the night before.

After the EEG, you will be allowed to return to your normal daily activities.