Electroencephalogram (EEG)

What’s an EEG?

An EEG (electroencephalogram) measures and records your brain’s electrical signals. During an EEG, a technician places small metal disks (electrodes) on your scalp. The electrodes attach to a machine that gives your healthcare provider information about your brain’s activity. Brain activity can help your provider diagnose and monitor conditions that affect your brain.

Why is an EEG performed?

Most commonly, healthcare providers use an EEG to check for seizure activity related to epilepsy. EEGs can also help monitor health conditions or find out what’s causing certain symptoms.

Healthcare providers may use an EEG during brain surgery or to test the brain activity of someone in a coma.

EEGs can also check the status of brain-related conditions such as:

• Alzheimer’s disease or dementia.
• Brain injury.
• Infections, including encephalitis.
• Tumors.

EEGs help diagnose the causes of symptoms such as:

• Confusion.
• Fainting (syncope).
• Memory loss.
• Seizures.

Who performs an EEG?

A specially trained EEG technician performs the procedure. You may have an EEG in an outpatient laboratory or inpatient per your healthcare provider’s order. Some EEG tests record your brain’s activity while you go about your usual activities with an ambulatory device.

What are the different types of EEG tests?

There are several types of EEG tests:

• Routine EEG: Routine EEG scans take 23 minutes. Your EEG technologist may ask you to breathe differently or look at flashing lights during the procedure.
• Prolonged EEG: A prolonged EEG test usually takes one hour and 15 minutes, but some types can last several days. A prolonged EEG gives your healthcare provider more information than a routine EEG. Your provider may use a prolonged EEG test to diagnose or manage seizures disorders. Prolonged EEGs use video.
• Ambulatory EEG: Ambulatory EEGs last one to three days. Ambulatory EEGs take place at home or at an EEG monitoring unit. During an ambulatory EEG, electrodes connect to a small EEG recorder. You can do most of your daily activities while the machine tracks your brain activity. You or family member can press a button if you have a seizure or event that your healthcare provider is trying to capture.
• Video EEG: The technician makes a video recording of you during your EEG. Video recording helps your healthcare provider see and hear what you’re doing when you have a seizure or other brain event. Your provider may also call this test EEG monitoring, EEG telemetry or video EEG monitoring.
• Sleep EEG: A technician performs an EEG test while you sleep. Healthcare providers may order sleep EEGs if a routine EEG doesn’t offer enough information. You might have a sleep study to test for sleep disorders with a sleep disorders center.

How does an EEG work?

Electrodes on your scalp measure electrical signals (impulses) as they travel between brain cells. Electrodes are small metal disks that a technician secures to your scalp with removable glue.

Electrodes attach to wires that sense nerve signals, which are electrical impulses. The electrodes send information about the signals to an EEG machine.

The EEG machine records the impulses with lines (traces) that show brain wave patterns. The brain has specific wave patterns when you’re awake and asleep. If you have a seizure, the wave patterns change.

How do I prepare for an EEG?

To prepare for an EEG, you:

• Wash your hair the night before your EEG. (Don’t use anything on it — like conditioner or styling product – afterward.)
• Avoid food or drink with caffeine, like coffee, black tea or energy drinks, for eight hours before the test.
• Follow your healthcare provider’s instructions, including any changes to medicine.

What should I expect during an EEG?

Here’s what happens during an EEG:

• You lie in a comfortable bed.
• A technician places about 23 electrodes on your scalp with glue or paste.
• You relax with your eyes either open or closed.
• You may look at a bright light or breathe differently to see if your brain has changes during these activation procedures
• If you have a seizure, the technologist will note the activity in the record.
• During routine EEGs, the technician will record for 23 minutes, and strive to obtain at least a portion of drowsiness or sleep. During an EEG that spans multiple hours, they strive to obtain a longer sleep recording and record the study for one hour and 15 minutes.
• During an ambulatory EEG, you usually go home and go about your usual activities. You carry or wear a portable EEG recorder for one to three days.

What happens after an EEG?

After your EEG, the technician will remove the electrodes and clean your scalp. Your hair and skin may feel sticky, so you’ll want to wash your hair at home. You can drive and return to your usual activities unless your healthcare provider says you shouldn’t.

Are there side effects from an EEG?

Some people may feel dizzy when they deep breathe during the EEG.

There is a small risk of seizure during an EEG. If this happens, your healthcare provider is there to help.

When will I get my EEG results?

You will find out the results of your EEG at a follow-up appointment. Your healthcare provider will explain your EEG results to you.

What do the EEG results mean?

Your healthcare provider will review the brain wave patterns that your EEG identified. The test results describe patterns as normal or abnormal.

Abnormal patterns have different causes, such as:

• Alcoholism or substance use disorders (drug abuse).
• Bleeding in the brain.
• Brain swelling (edema).
• Brain tumor.
• Head injury.
• Migraines.
• Seizure disorder like epilepsy.
• Sleep disorder, such as sleep apnea or narcolepsy.
• Stroke.

What happens if I have an abnormal EEG reading?

Your healthcare provider may refer you to a specialist, like a neurologist. A specialist can diagnose, treat or manage your condition.

An EEG (electroencephalogram) is a safe, painless test that measures brain activity. An EEG can help your healthcare provider learn the cause of symptoms like seizures, confusion or memory loss. With a diagnosis, your provider can treat and manage a brain-related condition appropriately.