Electroencephalogram (EEG)

What is an EEG?

An EEG (electroencephalogram) is a test that measures your brain activity. Healthcare providers order EEGs to diagnose conditions that affect your brain. An EEG test measures the naturally occurring electrical activity arising from your brain.

During an EEG, a technician places small metal disks (electrodes) on your scalp. The electrodes attach to a machine that monitors the electrical signals that your brain cells (neurons) make to communicate with each other. Monitoring that communication shows how well the different areas of your brain are working.

Why is an EEG done?

Typically, healthcare providers order EEGs if you have seizures or if they think that you may have epilepsy. Your provider may order an EEG if you have symptoms that could be a result of an undiagnosed epileptic condition, including:

• Confusion.
• Fainting (syncope).
• Memory loss.
• Changes in behavior.

They may use EEGs to watch for signs of epilepsy if you have conditions like:

• Alzheimer’s disease or dementia.
• Coma.
• Infections, including encephalitis.
• Traumatic brain injury.
• Tumors.

Types of EEG tests

Healthcare providers order different EEG tests depending on the medical issue. EEG test types include:

• Routine EEG: Routine EEG scans may involve looking at flashing lights or taking breaths at different times during the test. This type of EEG test may take 20 to 30 minutes.
• Prolonged EEG: This test gives your provider more information than a routine EEG. Your provider may order a prolonged EEG test to determine if certain symptoms, like a memory lapse, happen because you have seizures. Prolonged EEGs may take about an hour or longer to complete.
• Ambulatory EEG: In this test, you’re able to go about your day while an EEG machine tracks your brain activity. Ambulatory EEGs involve wearing a small EEG recorder that you wear for one or more days.
• Video EEG: If you’re having an EEG because your provider wants to record seizures, they may ask you to have a video EEG so they can see and hear what you’re doing. They may call this test EEG monitoring, EEG telemetry or video EEG monitoring.
• Sleep EEG: Your provider may order a sleep EEG along with a sleep study (polysomnogram) to obtain more information than a sleep study alone provides. They may order this test if you have a sleep disorder that could be a result of an epileptic condition.

How does an EEG work?

An EEG works by measuring the electrical signals or impulses that travel between your brain cells. EEGs track those signals by using electrodes that attach to wires that sense electrical impulses. The electrodes:

1. Send information about the signals to an EEG machine.
2. The EEG machine records brain waves and generates a visual output in the form of waveforms (traces) on a computer screen that recreate the pattern of brain activity recorded.
3. Your brain wave patterns change when you’re awake, asleep or if something affects your brain cells.
4. The image on a computer screen is your EEG.
5. Your healthcare provider will review your EEG for abnormal patterns.

How do I prepare for an EEG?

There are different types of EEGs. Your healthcare provider will explain the kind of test you’ll have, including why they’re doing a specific test, what will happen during the test, how long it will take and if you need to have someone to take you home after the test. Regardless of the type of EEG test, you should:

• Tell your provider about medications you take.
• Wash your hair the night before your test, but don’t use conditioner or styling products that could affect how electrodes transmit brain activity information.
• Not sleep the night before your test if they ask you not to.

What happens during a routine EEG?

During a routine EEG:

1. You sit in a chair or lie on a bed.
2. A technician places about 23 electrodes on your scalp with glue or paste. The electrodes don’t cause any sensation or pain. They’re simply there to record your brain activity.
3. You relax with your eyes either open or closed.
4. Your technician may ask you to look at a bright light to see activity in the part of your brain that manages vision.
5. They may ask you to take a series of deep breaths.

A routine EEG may take 20 to 30 minutes to complete.

What happens after a routine EEG?

Your technician will remove the electrodes and clean the part of your scalp where they placed the electrodes. Unless your provider tells you otherwise, you can go home and go about your day. Your hair and skin may feel sticky from the glue or paste that kept the electrodes in place during your EEG, so you may want to wash your hair when you get home.

What are the risks or side effects of having an EEG?

EEGs rarely cause side effects. Some people may feel dizzy while taking deep breaths during the test. If you have certain forms of epilepsy, deep breathing (hyperventilation) or light (photic) stimulation during the EEG may trigger an epileptic seizure. This doesn’t happen very often, and if it does, your technician will know what to do to help you during a seizure. You may lose some hair or have skin irritation where the electrodes were on your head.

When do I get my test results?

You’ll have a follow-up appointment with your healthcare provider. They’ll review the brain wave patterns that the test shows.

What do EEG test results mean?

Your healthcare provider is your best resource for insight into your test results. They’ll look for abnormal brain waves or rhythms, which may indicate brain dysfunction or epileptic activity.

What are next steps if I have an abnormal EEG test result?

That depends on your situation. Your healthcare provider may refer you to a specialist, like a neurologist, who can diagnose, treat or manage your condition.