What Is an EEG Used For?

What to expect when getting an electroencephalogram

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An electroencephalogram (EEG) test is used to check for seizures and diagnose epilepsy. EEG can also be used to diagnose other medical conditions like sleep disorders or brain injuries, as well as to monitor brain activity during surgery or when a person is in a coma.

An EEG records electrical activity in the brain. It is a non-invasive test that works by picking up abnormal brain waves via electrodes that are attached to the scalp. An EEG can be used by primary care providers or specialists like a neurologist.

This article covers uses for EEG, how they work, and what to expect during the test.

what to expect during an electroencephalogram
 Illustration by Cindy Chung, Verywell

What Is an EEG Test?

There are several types of electroencephalograms (EEGs) and different versions of each type. The type you have done will depend on why you need the test. Generally, there are two basic types of EEG tests that are used:

Routine EEG

This basic EEG test is usually done after a patient has a seizure for the first time. Ideally, the test should be done within 24 hours. Thus, it's important to get emergency care or go to the hospital ER right away if you or someone else has a seizure.

A routine EEG can be done with or without video monitoring—where you are videotaped during the test to see if you have abnormal brain waves during specific movements or activities.

Ambulatory EEG

This EEG test uses equipment that a patient wears to record brain activity continuously as they go about their normal activities. It can be done with video as long as there is another person to do the taping.

Use of Electrodes

No matter the EEG type, the electrical activity of the brain is picked up by small metal discs (electrodes) that are positioned in standardized patterns on the scalp. Electrodes can be wireless or have wires that attach to a computer.

The electrodes detect electrical activity produced by the brain and transmit this information to a computer, where it is processed and saved electronically or printed out. Brain waves are recorded as squiggly lines called traces, and each trace represents a different area in the brain.

EEG Uses

An EEG measures the continuous electrical activity of the brain. EEGs most often are used to look for seizure activity or evaluate a person’s risk for seizures. Seizures are abnormal electrical discharges in the brain that can cause confusion, agitation, uncontrolled movements, hallucinations, and even collapse.

If you're being evaluated for epilepsy, your neurologist will look for patterns on your EEG called epileptiform. They can look like spikes, sharp waves, or spike-and-wave discharges on the test. If abnormal activity shows up on your EEG, the trace can show where in your brain the seizure started.

For example:

  • If you're having generalized seizures (these involve both sides of your brain), there likely will be spike-and-wave discharges spread throughout your brain.
  • If you’re having focal seizures (which involve just one area of your brain), spikes and or sharp waves can be seen in a specific location.

Although the main reason an EEG is done is to diagnose epilepsy, the test has many other uses. These include looking for abnormal brain activity that could be caused by:

Sleep and Seizures

In people with abnormal movements or behaviors during sleep, it’s important to rule out seizures as a potential cause. 

An EEG can be done with a standard sleep study (polysomnogram). The test monitors sleep stages and cycles to identify disruptions in sleep patterns, and why they might be happening. 

An EEG can also be used to: 

  • Determine why a patient is in a coma, state of delirium, or brain dead
  • Evaluate a patient for drug intoxication
  • Monitor a patient in a medically-induced coma to make sure that they’re getting the correct level of anesthesia
  • Monitor a patient having brain or vascular surgery to make sure the procedure is not causing permanent damage to their brain
  • Check a person’s brain for early signs of dementia or Alzheimer’s disease

Risks and Contraindications

For most people, an EEG is safe and does not have any major risks. The electrodes used for an EEG are harmless because they only pick up electrical charges. They do not emit electricity.

In rare cases, an EEG can cause seizures in a person with a seizure disorder. In these cases, the seizures are brought on by deep breathing or flashing lights. They can also happen if the person took less or none of their medication before they had the test.

If you have seizures, you will be monitored very carefully and treated with a fast-acting anti-seizure medication right away if this happens during the test. There will also be oxygen and other safety equipment nearby in case you have a prolonged seizure.

If you're being tested in a hospital for a longer time and are at risk of having severe seizures, other precautions can be taken. For example, a belt can be placed around your waist to keep you from falling and you may not be allowed to walk around.

People who are in major danger of getting hurt during a seizure can be fitted with mitts so they do not scratch themselves. A restraint can be used to prevent them from climbing out of bed, and the sides of the bed can be padded.

There are also factors that can affect the results of an EEG test and may make the results less accurate:

  • Body functions (e.g., heart rate, breathing, sweating, blood sugar)
  • Movement (e.g., talking, twitching)
  • Medications and substances (e.g., sedation, caffeine)
  • Personal factors (e.g., hair that is very oily, products in the hair like spray or gel) 
  • Environmental factors (e.g., flashing lights)

Before the Test

If you will be having an EEG, the following information will help you to prepare for the test.

Timing

The length of the encephalogram will depend on the type of test you're having.

In general, a routine EEG can take as little as 20 to 30 minutes as an outpatient test, or up to 24 hours to several days if you’re having it at a hospital. This time ensures that brain waves can be measured during sleep (a prolonged or 24-hour EEG).

In both cases, make sure you allow extra time for checking in and test preparation (30 to 60 minutes should be enough). An ambulatory EEG can be as short as a day or up to three days long. 

Often EEGs are named based on the length of recording time required (e.g., two-hour EEG, 24-hour EEG). If your healthcare provider does not tell you, ask how long your test will last from start to finish so you can plan accordingly.

Location

In most cases, an EEG test is done as an outpatient procedure in a provider’s office, hospital, lab, or clinic. If extended monitoring is needed, you might be admitted to the hospital for a few days.

If you have an ambulatory EEG, the test will take place at your home.

What to Wear

You'll be sitting or lying down for some time during the test, so wear something that will let you do so comfortably. Choose a top that buttons or zips up, so you do not have to pull anything over your head.

You can wear jewelry, but keep in mind that large or dangling earrings could get in the way of the test, depending on where the electrodes are placed. If you're being admitted to the hospital, you’ll change into a gown to wear while you’re there. 

Food and Drink

On the day of an EEG, or for at least eight to 12 hours before, you should not eat or drink anything that has caffeine in it, such as coffee, tea, or cola. Caffeine can affect the results of the test. You do not need to fast the night before or the day of your test. In fact, not eating before the test could result in low blood sugar, which could affect your results.

If you take prescription medications or regularly take over-the-counter (OTC) drugs or supplements, including herbal remedies, tell your healthcare provider.

Most medications are fine to take before an EEG, but anything that acts as a sedative may affect the test. In some cases, a sedative is given for an EEG to help you relax, but this is done by providers and is a very specific dose. 

If you have a seizure disorder that you take medication for, you might be asked to lower your dose or not take your prescription at all before the test to "bring on" abnormal brain activity. Make sure to follow your healthcare provider's instructions about medications on the day of your test.

Cost and Health Insurance

If you have health insurance, your EEG will be covered as long as it's considered medically necessary according to the terms of your policy. You could still be responsible for a copay or coinsurance (typically 10 to 50% of the total cost of the test) if you have not met your deductible.

The cost of an EEG depends on the type of test you have, where the test is done, where you live, and other factors. 

In general, the cost of a routine in-office EEG ranges from around $200 to $800 or more. If video monitoring is included or if the test is longer or takes place overnight in a hospital, the total could be $3,000 or more. An ambulatory EEG typically runs from $500 to more than $3,000. The average cost is around $780.

Some hospitals offer discounts of up to 30% for patients who do not have health insurance or who pay out-of-pocket for an EEG.

What to Bring

If you know that you will be given a sedative for your EEG, you will need to bring someone with you on the day of the test to drive you home after. You can also arrange for someone to pick you up when the test is over.

If you're going to a hospital or testing site for many hours and not required to sleep, you may want to bring something to do, such as a book to read. You might be able to use your cell phone, tablet, or laptop, but ask first.

Other Considerations

If you're supposed to sleep during your routine EEG, you might be told to only sleep for four or five hours—or not at all—the night before. 

Instead of keeping you up the night before, your provider may have you go in for your EEG very early in the morning while you're still drowsy.

You should wash your hair the night before or the morning of the test so that your head and hair are clean and free of natural oils that could make it hard for the electrodes to stick to your scalp. Do not use conditioner, hairspray, or other styling products, as these could affect the electrodes.

During the Test

The way in which each EEG test is done depends on the type you’re having.

Routine EEG

When you check in for your routine EEG test, you will probably be asked to sign a consent form. You will then be taken to a testing room where a technician will do the EEG. If you're at a hospital, you might be admitted to an epilepsy monitoring unit.

The room where the test takes place will be quiet and dimly lit to help you relax. Sometimes, a sedative is given to help you feel more comfortable. 

The technician will have you either sit back in a reclining chair or lie down on a bed. They will measure your head so they can put the electrodes in the correct spots, which they will mark using a special wax crayon.

Next, the technician will attach the electrodes to your head, around 16 to 25 in total. They may gently scrub each area where an electrode will be placed with a cream that's mildly abrasive. This helps the disc stick better and also improves the quality of the recording.

Each electrode will be attached using a special paste that, like wax and cream, will wash out of your hair easily when the test is over. Sometimes, a cap with the electrodes already attached to it will be used instead. 

During the Test

Once the electrodes are in place, the technician will have you close your eyes and relax. They may suggest you take deep breaths. It's important to stay very still while your brain waves are being recorded—if you even blink or swallow, it can throw off the reading.

This may sound hard, but the technician will watch you (probably through a window in an adjoining room) and can stop the recording from time to time to give you a chance to change your position or just take a break from having to stay still. 

You will need to be still for an initial reading at rest. Then, the technician may ask you to do specific things, such as breathing deeply and quickly, or opening and closing your eyes. You might be exposed to bright or flashing lights or noise. 

The entire process should take between 45 minutes and two hours.

Post-Test

When the recording is complete, the technician will gently remove the electrodes from your scalp. They may wash the electrode paste off using warm water, acetone (like nail polish remover), or witch hazel.

If you had a sedative for the test, you may need to rest until it wears off before your ride can take you home. Otherwise, you should be able to go back to your regular activities after the test is over. 

Ambulatory EEG

To be set up for an ambulatory EEG, you will go to a provider’s office, clinic, or hospital where a technician will attach electrodes to your scalp as they do for a routine EEG, but with a few differences.

Since the electrodes will have to stay in place longer than for a regular EEG, a stronger glue called collodion is often used. It can be removed easily with acetone or a similar solution after the test is done. 

Your head will be covered with gauze or a cap during the test. The wires from the electrodes will be attached to a recording device that's a little bigger than a portable tape player. It can be worn on your waist with the wires running inside or outside of your shirt.

Once everything is in place and you have been given specific instructions on what to do next, you can leave to go home for the next 24 hours to 72 hours.

When you’re at home, you’ll be encouraged to go about your normal activities as much as possible, with a few exceptions: You might be instructed not to chew gum or suck on candy or breath mints, as the movement of your jaw could affect the test.

It's also important to make sure the electrodes and the recorder stay dry, so you may not be able to bathe or shower.

You might be instructed to keep a record of what you do during the day and take note of any seizures or other symptoms you have. Even something simple like scratching your head because the electrodes are making your scalp itchy could show up as abnormal brain activity, so your notes need to be as detailed as possible. 

If you have to be videotaped, your healthcare provider or technician will give a friend or family member instructions on how to do so.

After the Test

You can go back to your normal activities after your test is done. You'll probably want to wash your hair to get rid of any remaining glue. You may find that your scalp is red and irritated in the spots where the electrodes were placed, but it shouldn’t last long. 

Your provider will tell you when you can go back to taking any medicines you stopped before the test.

What Do the EEG Results Mean?

The results of your EEG will be sent to a neurologist to be read. They will tell the provider who ordered your EEG what the test showed.

You may hear back from your healthcare provider about your results within a day, but it could be as long as a week or two. 

An EEG will come back as either normal or abnormal. If your results are normal, it means you did not have abnormal brain wave patterns or seizures during the test. However, it is possible to get normal results even if you have a history of seizures or epilepsy.

Abnormal results from an electroencephalogram can indicate:

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

Follow-Up

If you have an abnormal EEG, your healthcare provider will probably have you come in to talk about the next steps, which could include more tests like:

The treatment you need, if any, will depend on your final diagnosis. For example, epilepsy can usually be managed with medication or surgery.

Summary

An EEG is a test that records the electrical activity in your brain. It is most often used to diagnose epilepsy, but it can also be used for head injuries, brain tumors, stroke, and infections such as encephalitis. 

There are two types of EEGs. A routine EEG is typically performed within 24 hours of your first seizure. An ambulatory EEG is a longer test that monitors brain wave activity over time.

If an EEG detects abnormal brain wave patterns, it can be a sign of conditions like epilepsy, migraines, sleep disorders, and head injury. Your provider will talk to you about the next steps after an EEG based on your diagnosis. 

11 Sources
Verywell Health uses only high-quality sources, including peer-reviewed studies, to support the facts within our articles. Read our editorial process to learn more about how we fact-check and keep our content accurate, reliable, and trustworthy.
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Brandon Peters, M.D.

By Brandon Peters, MD
Brandon Peters, MD, is a board-certified neurologist and sleep medicine specialist.