What Happens During an EEG

Portrait of a nice handsome man wearing electrodes
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You might have seen them in your doctor's office or have had them used on yourself—a weird hairnet-like cap with a bunch of electrodes. These electrodes on your head, known as an electroencephalogram (EEG), can tell you a lot about your brain. An EEG records electrical activity in the brain. Brain cells create tiny electrical impulses for communicating with each other. The EEG picks up these impulses through tiny wires (electrodes) placed on your scalp. The impulses are amplified and digitally recorded by a computer. The recordings look like wavy lines (sometimes called brain waves).

Why Have an EEG

An EEG is usually done to see if a person is having seizures, and if so, what type of seizures they are. Generally speaking, there are two types of seizures—generalized and partial. EEGs can be useful in differentiating between the two. An EEG can also look for changes in brain activity caused by head injury, tumor, infection, sleep disorders, or other problems that can affect the brain. In addition, an EEG may be used to evaluate brain activity in someone who is unconscious or in a coma. An EEG may also be used to monitor the brain during surgery.

What to Expect

The EEG test is painless and can be performed in a doctor's office, hospital, lab, or outpatient clinic. Prior to having an EEG done, you will need to avoid caffeine and any medications that affect the nervous system. Do not wash your hair the day of the test or use any hair styling products.

For the EEG, a number of small metal discs are attached to your scalp with a special conducting paste or extremely fine needles. You do not need to have your head shaved or haircut. The electrodes are attached to wires that carry the brain's electrical impulses to the recording computer. You will be seated in a reclining chair or reclining in a bed.

Once the electrodes are attached and the computer is recording, you may be asked to do things like open or close your eyes, or change how fast or slow you are breathing. It is important to lay still unless directed otherwise. The test is very sensitive and any movements can result in inaccurate readings. You might be exposed to bright or flashing lights or noise. The EEG usually takes about an hour. If the EEG is done while you are sleeping, it usually takes about 3 hours.

If your doctor needs a longer test, they may order you an ambulatory EEG. This type of EEG takes about three days. During this test, you will be asked—and able to—go about your normal routine. However, you will have to wear the electrodes as well as a special recording device.

Understanding Your Results

It is possible to get normal results even if you have epilepsy. EEGs may not be able to pick up all seizures. So it is possible for seizures to go undetected by an EEG. Abnormal results can indicate: 

  • Migraines
  • Bleeding (hemorrhage)
  • Head injury
  • Tissue damage
  • Seizures
  • Swelling (edema)
  • Substance abuse
  • Sleep disorders
  • Tumors  

Any abnormal results may require further testing and imaging to make a definitive diagnosis. 

View Article Sources
  • "Neurological Diagnostic Tests and Procedures." Disorders A - Z. 15 May 2009. National Institute of Neurological Disorders and Stroke.