EEG (Electroencephalogram)

What Is an EEG (Electroencephalogram)?

An EEG, or electroencephalogram, is a test that records the electrical signals of the brain by using small metal discs (called electrodes) that are attached to your scalp. Your brain cells communicate with each other using electrical impulses. They’re always working, even if you’re asleep. That brain activity will show up on an EEG recording as wavy lines. It’s a snapshot in time of the electrical activity in your brain. 

EEG Uses

EEGs are used to diagnose conditions like:

  • Brain tumors
  • Brain damage from a head injury
  • Brain dysfunction from various causes (encephalopathy)
  • Inflammation of the brain (encephalitis)
  • Seizure disorders including epilepsy
  • Sleep disorders
  • Stroke

An EEG may also be used to determine if someone in a coma has died or to find the right level of anesthesia for someone in a coma.

EEG Risks

EEGs are safe. If you have a medical condition, talk with the doctor about it before your test.

If you have a seizure disorder, there’s a slight risk that the flashing lights and deep breathing of the EEG could bring on a seizure. This is rare. A medical team will be on hand to treat you immediately if this happens.

In other cases, a doctor may trigger a seizure during the test to get a reading. Medical staff will be on hand so the situation is closely monitored.

Preparing for an EEG

There are some things you should do to prepare for EEG:

  • Don’t eat or drink anything with caffeine for 8 hours before the test.
  • Your doctor may give you instructions on how much to sleep if you’re expected to sleep during the EEG.
  • Eat normally the night before and day of the procedure. Low blood sugar could mean abnormal results.
  • 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. If you are wearing extensions that use glue, they should be removed.

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EEG Procedure

  1. 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 electrical 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 paper or displayed on a computer screen.
  2. Once the recording begins, you’ll be asked to remain still.
  3. 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 brainwave patterns. The machine is only recording the activity of the brain and doesn’t stimulate it.
  4. It's rare to have a seizure during the test.
  5. You can have an EEG at night while you're asleep. If other body functions, such as your breathing and pulse, are also being recorded, the test is called polysomnography. 
  6. In some cases, you may be sent home with an EEG device, which will either send the data directly back to your doctor's office or record it for later analysis.

After an EEG

Once the EEG is over, the following things will happen:

  • The technician will take the electrodes off and wash off the glue that held them in place. You can use a little fingernail polish remover at home to get rid of any leftover stickiness.
  • Unless you're actively having seizures or your doctor says you shouldn't, you can drive home. But if the EEG was done overnight, it's better to have someone else drive you.
  • You can usually start taking medications you'd stopped specifically for the test.
  • neurologist, a doctor who specializes in the brain, will look at the recording of your brain wave pattern.

EEG Results

Once the EEG results have been analyzed, they will be sent to your doctor, who will go over them with you. The EEG will look like a series of wavy lines. The lines will look different depending on whether you were awake or asleep during the test, but there is a normal pattern of brain activity for each state. If the normal pattern of brain waves has been interrupted, that could be a sign of epilepsy or another brain disorder. Having an abnormal EEG alone doesn’t mean you have epilepsy. The test just records what is happening in your brain at that moment.  Your doctor will do other tests to confirm a diagnosis.

WebMD Medical Reference Reviewed by Arefa Cassoobhoy, MD, MPH on July 01, 2020

Sources

SOURCES:

Epilepsy Foundation.

Mayo Clinic: “EEG (Electroencephalogram).”

Johns Hopkins Medicine: “Electroencephalogram (EEG).” 

Epilepsy Society: “A Closer Look at EEG.”

USFC Benioff Children’s Hospital: “Understanding EEG Video Telemetry.”

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