What Is an Electroencephalogram (EEG)?

What to expect when undergoing this test

An electroencephalogram (EEG) is a non-invasive test that records electrical activity in the brain. It works by picking up abnormal brain waves via electrodes that are attached to the scalp. EEGs are usually done to detect seizures and to diagnose epilepsy, but they can be used to evaluate or diagnose other conditions, such as sleep disorders or brain injuries. EEGs are also often used to monitor brain activity in someone who is in an induced coma or undergoing certain types of surgery. An EEG may be ordered by a general practitioner or by a neurologist—a doctor who specializes in disorders that affect the brain, spinal cord, and nerves.

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

Purpose of Test

An EEG is a measurement of the continuous electrical activity of the brain. This is detected via small metal discs called electrodes that are positioned in standardized patterns on the scalp. Each electrode has wires that attach to a computer, although according to the Epilepsy Foundation of America (EFA), wireless systems are being used more and more. 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.

EEGs most often are used to evaluate the presence or risk of seizures—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 that can manifest as spikes, sharp waves, or spike-and-wave discharges. If abnormal activity shows up on your EEG, the trace can show where in your brain the seizure originated.

For instance, if you're having generalized seizures, which means they involve both sides of your brain, there likely will be spike-and-wave discharges spread throughout your brain. If you’re having focal seizures, meaning that they involve just one area of your brain, there will be spikes and or sharp waves can be seen in that specific location.

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

  • A head injury
  • A brain tumor
  • An infection such as encephalitis (inflammation of the brain that's usually caused by a virus)
  • Stroke
  • Sleep disorders caused by seizures. For this purpose, an EEG may be done in conjunction with a standard sleep study called a polysomnogram, which monitors sleep stages and cycles to identify disruptions in sleep patterns, and why they may be occurring. In people with abnormal movements or behaviors during sleep, it can be important to rule out seizures as a potential cause.

An EEG might also be used to determine why someone is in a coma or state of delirium, if a person in a persistent coma is brain-dead, or to evaluate drug intoxication.

Someone in a medically induced coma may have continual brain wave monitoring using an EEG to make sure they're getting the correct level of anesthesia. A patient undergoing brain or vascular surgery may be monitored with an EEG to make sure the surgery is not causing permanent damage.

Types of EEG

There are several types of electroencephalograms, as well as various versions of each. Your experience will be based on your specific situation. Very generally speaking, however, there are two basic types of EEG:

  • Routine EEG: This basic test is typically done after someone has a seizure for the first time. This is ideally performed within 24 hours, which is why it's important to call for emergency help or go to a hospital ER right away if you or someone else has a seizure. A routine EEG can be done with or without video monitoring, in which you will be videotaped during the test to see if you have abnormal brain waves during specific movements or activities.
  • Ambulatory EEG: This test uses equipment that a person wears so that brain activity can be recorded 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.

Risks and Contraindications

For most people, an EEG is perfectly safe and poses no significant risks. Note that the electrodes used for an EEG only pick up electrical charges; they do not emit electricity and are harmless.

In rare instances, an EEG can cause seizures in a person with a seizure disorder, which are brought on by deep breathing or flashing lights or if the person took less or none of their medication for the test. Rest assured that you will be monitored very carefully for this and treated with a fast-acting anti-seizure medication right away if it happens to you. There will also be oxygen and other safety equipment nearby in the event of a prolonged seizure.

If you're being tested in a hospital for a prolonged period of time and are at risk of having severe seizures, other precautions may be taken during the test. For instance, a belt may be placed around your waist to keep you from falling or you may not be allowed to walk around. People who are truly in danger of getting hurt during a seizure may even be fitted with mitts so they don't scratch themselves, or a restraint to prevent them from climbing out of bed. The sides of the bed may be padded.

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 your 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 procedure, or as many as 24 hours to several days in a hospital, so that brain waves can be measured during sleep. This is sometimes referred to as a prolonged or 24-hour EEG. In both cases, allow extra time for checking in and test preparation (30 to 60 minutes should be sufficient).

An ambulatory EEG may be as short as a day or as many as three days.

Often EEGs are named based on the length of recording time required (two-hour EEG, 24-hour EEG). If your doctor doesn't 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 an outpatient procedure performed in a doctor's office, hospital, lab, or clinic. In some cases of extended monitoring, you may need to be admitted to the hospital for a few days.

If you have an ambulatory EEG, it will take place at home.

What to Wear

Since you'll be sitting or lying down for some time, you should wear something that allows you to do that comfortably. Choose a top that buttons or zips up, so you don't 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 depending on where the electrodes are placed. If you're being admitted to the hospital overnight or for longer, you'll change into a hospital gown.

Food and Drink

On the day of an EEG, or for at least eight to 12 hours beforehand, you should not eat or drink anything that contains caffeine, such as coffee, tea, or cola, as it can affect the test. However, it's important that you not fast the night before or day of your test. Low blood sugar can interfere with your results.

If you're on any prescription medications or regularly take over-the-counter drugs or supplements, including herbal remedies, make sure your doctor knows. Most medications are fine to take before an EEG, but anything that acts as a sedative may interfere with the test. (In some cases, a sedative may be given to help a patient undergoing an EEG relax, and it's important that that dose be specific.)

If you have a seizure disorder that you take medication for, you may be asked to lower your dose or not take your prescription at all prior to the test in order to "bring on" abnormal brain activity. Follow your doctor's instructions.

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. Of course, you may be responsible for a copay or coinsurance (typically 10 percent to 50 percent of the total cost, if you haven't met your deductible).

The cost of an EEG depends on the type of test you have, where the procedure takes place, the region of the country you live in, and other factors. In general, a routine in-office EEG will range from around $200 to $800 or more; if video monitoring is included or the test is prolonged or takes place overnight in a hospital, the total could reach $3000 or more. Some hospitals may offer discounts of up to 30 percent for patients who don't have health insurance or who pay out-of-pocket for an EEG; be sure to ask.

An ambulatory EEG typically runs from $500 to more than $3,000. The average is around $780.

What to Bring

If you know that you will be given a sedative for your EEG, you will need to bring someone along to drive you home afterward or arrange for a pick-up.

If you know you'll be at the 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 may 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 may be instructed to only sleep for four or five hours, or not at all, the night before. Alternatively, your doctor may have you go in for your EEG very early in the morning, when 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 adhere to your scalp. For the same reason, don't use conditioner, hairspray, or other styling products.

During the Test

How each test is carried out depends on the type being performed.

Routine EEG

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

The room where the test takes place will be quiet and dimly lit, to help you stay as relaxed as possible. Sometimes a sedative is given for this purpose. The technician will have you either sit back in a reclining chair or lie down on a bed. She will measure your head in order to put the electrodes in the correct spots, which she will mark using a special wax crayon.

Next, the technician will attach the electrodes, around 16 to 25 in total. She may gently scrub each area where an electrode will be placed with a cream that's mildly abrasive, which will help the disc stick better and also improve the quality of the recording. Each electrode will be attached using a special paste that, like the wax and cream, will wash out of your hair with no problem.

Sometimes a cap with the electrodes already attached will be used.

During the Test: With the electrodes in place, the technician will have you close your eyes and relax. She 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 to do, but the technician will watch you (probably through a window in an adjoining room) so that she can stop the recording periodically to allow you to change your position or just take a break from being motionless.

You will stay still for an initial reading at rest. The technician then may ask you to do specific things, such as breathe deeply and rapidly, or open and close your eyes; or you may 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. She may wash the electrode paste off using warm water, acetone (which is like nail polish remover), or witch hazel.

If you took 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 resume your regular activities.

Ambulatory EEG

To be set up for an ambulatory EEG, you will go to a doctor's office, clinic, or hospital where a technician will attach electrodes to your scalp as in 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 may be used. It can be removed easily with acetone or a similar solution after the test is complete. Your head will be covered with gauze or a cap.

The wires from the electrodes will be attached to a recording device that's a little bigger than a portable cassette player and can be worn on your waist with the wires running inside or outside of your shirt. Once everything is in place and you have received specific instructions, you can leave to go home for the next 24 hours to 72 hours.

At home, you'll be encouraged to go about your normal activities as much as possible, with a few notable exceptions: You may be instructed not to chew gum or suck on candy or breath mints, as the action of your jaw could affect the test. It's also important to keep the electrodes and the recorder dry, so you may not be able to bathe or shower.

You may be instructed to keep a record of what you do during the day and to note seizures or other symptoms. Even something as simple as scratching your head because the electrodes make your scalp itchy may show up as abnormal brain activity, so your notes should be as detailed as possible. If you're to be videotaped, a friend or family member will do this according to instructions from your doctor or technician.

After the Test

You can resume 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. This should not last long.

Your healthcare provider will inform you as to when you may resume medicines you stopped taking before the test, if any.

Interpreting Results

The results of your EEG will be sent to a neurologist to be interpreted, who will convey them to the doctor who ordered your test. This will affect how long you will have to wait: You may hear back from your doctor within a day or so, or it could be as long as week or two.

An EEG will come back as either normal or abnormal. In other words, it will show that you did not have abnormal brain wave patterns or seizures during the test or you did. Note that 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

Your doctor will probably have you come in to discuss next steps if you have an abnormal EEG. This may involve further testing, such as a high-density EEG (in which more electrodes may be used and spaced very closely together to hone in exactly where in the brain seizures are originating), or imaging such as computerized tomography (CT), magnetic resonance imaging (MRI), or positron emission tomography (PET) to look for lesions or other abnormalities on your brain that could be causing you to have seizures.

If you need treatment, it will depend on your final diagnosis. Epilepsy, for example, usually can be managed with medication or surgery.

A Word From Verywell

Symptoms such as seizures and injuries or illnesses that affect the brain can be scary. The same can be said of having an encephalogram, especially one that requires hours or even days of continuous monitoring. If you're feeling anxious about your upcoming EEG, try to focus on the value of the test. Ultimately, the results will help to put your mind at ease, either by determining that nothing is wrong or pinpointing a potential problem so that it can be addressed.

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