Electroencephalogram (EEG)

An EEG (electroencephalogram) measures the electrical activity in your brain. Healthcare providers often use this test to diagnose conditions like epilepsy.

Providers may use the term EEG to describe the test itself and what it shows.

During this test, a technician places small, metal disks (electrodes) on your scalp. The electrodes connect to a machine that records the electrical signals your brain cells (neurons) use to communicate. Tracking these signals shows how well different parts of your brain are working.

Why is an EEG done?

Healthcare providers often order this test if you have seizures or if they suspect epilepsy. They may also recommend one if you have symptoms like:

• Confusion
• Fainting
• Memory loss
• Changes in behavior

EEGs can also monitor brain activity in certain conditions, including:

• Alzheimer’s disease or dementia
• Brain infections like encephalitis
• Brain tumors
• Coma
• Traumatic brain injury

Types of EEG tests

Providers order different types of EEGs depending on the medical issue. These include:

• Routine EEG: This may use flashing lights or changes in breathing to record brain activity.
• Prolonged EEG: This records brain activity for about an hour or longer. Providers may use it to see if symptoms like memory lapses link to seizures.
• Ambulatory EEG: This lets you go about your normal day while a small recorder tracks brain activity for one or more days.
• Video EEG: This records brain activity along with video and audio. It helps providers see what you’re doing during a seizure. It’s also called EEG monitoring, EEG telemetry or video EEG monitoring.
• Sleep EEG: Sometimes done with a sleep study (polysomnogram), this helps gather more information during sleep. Providers may order it if they suspect a sleep disorder linked to epilepsy.

How does an EEG work?

This test measures the electrical signals (impulses) that pass between your brain cells. Electrodes on your scalp pick up these signals through small wires. The process works like this:

1. The electrodes send the signals to an EEG machine.
2. The machine records brain waves and displays them on a screen.
3. Brain wave patterns change when you’re awake, asleep or when something affects your brain.
4. The image on the screen is your EEG.
5. Your healthcare provider reviews the image to look for abnormal patterns.

How do I prepare for an EEG?

There are different types of EEGs. Your healthcare provider will explain which test you need and why. They’ll also tell you what to expect, how long it’ll take and if you’ll need a ride home afterward. No matter the type you need, you should:

• Tell your provider about any medications you take
• Wash your hair the night before, but don’t use conditioner or styling products — these can affect how the electrodes work
• Follow instructions about sleep — your provider may ask you to stay awake the night before
• Follow any changes to your eating and drinking as directed

Most EEGs need little preparation, and your provider will give you clear instructions.

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 using glue or paste. These don’t cause pain. They only record brain activity.
3. You relax with your eyes open or closed.
4. The technician may ask you to look at a bright light to measure brain activity related to vision.
5. They may also ask you to take a series of deep breaths.

How long does an EEG take?

A routine EEG may take 20 to 30 minutes to complete. The time depends on the type. But your provider will tell you in advance so you know what to expect.

EEG test side effects

This test rarely causes side effects. Some people feel dizzy while taking deep breaths. If you have certain types of epilepsy, deep breathing (hyperventilation) or flashing lights (photic stimulation) may trigger a seizure. This is uncommon. Your technician will know how to help if anything happens.

You may also notice minor and temporary hair loss or skin irritation where the electrodes were placed.

What can I expect after an EEG?

Your technician will remove the electrodes and clean your scalp. Unless your provider gives you other instructions, you can go home and return to your normal activities.

Your hair or skin may feel sticky from the glue or paste used for the electrodes. You may want to wash your hair when you get home.

When should I know the results?

You’ll have a follow-up appointment with your doctor after the test. The timing can vary, but they’ll see you as soon as possible. During the visit, they’ll review your brain wave patterns and explain the next steps.

If the results are abnormal, what are the next steps?

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