A Guide to Electroencephalogram (EEG) Procedure and Results

Medically Reviewed By Seunggu Han, M.D.
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An electroencephalogram (EEG) is an important test that doctors use to diagnose brain-related conditions. It records your brain’s electrical activity so that experts can study the patterns and check for issues. With this painless test, a person wears an electrode cap or has electrodes gently attached to their scalp and is connected to an EEG machine. The machine records brainwaves and illustrates them in a graph. Typically, a neurologist will analyze the graph and provide results to the person’s doctor within 48 hours.

Here is a look at the EEG process, including what to expect and how to understand the results.

What is an EEG?

Child getting an EEG test
Bisual Studio/Stocksy United

An electroencephalogram (EEG) is a noninvasive, painless test that detects unusual electrical activity in the brain. A technician attaches electrodes to your scalp to record the electrical impulses that form brain wave activity. An EEG helps doctors diagnose or evaluate brain disorders and conditions. 

An EEG is only one method that your doctor can use to diagnose or evaluate brain disorders and conditions. They may request other tests in addition to an EEG.

Why is an EEG performed?

Doctors use an electroencephalogram (EEG) to diagnose or evaluate conditions affecting the brain and nervous system and neurological disorders.

These include:

  • seizure disorders, including epilepsy
  • sleep disorders
  • brain damage from a stroke, a tumor, an injury, or another form of trauma
  • dementia
  • behavioral and learning disorders
  • some psychoses

Read about 10 conditions that an EEG can diagnose.

Understanding what EEG results mean

EEG Results
These are EEG results. Iryna Imago/Getty Images

During an electroencephalogram (EEG) test, electrodes are placed on the scalp to record brain activity in different parts of your brain called lobes. Different lobes are responsible for different functions, such as memory, speech, and vision. If your doctor detects unusual activity, they will know what part of the brain is not working as expected.

Your doctor will also study the patterns of your brainwave activity. Brainwaves are categorized by frequency, which is the number of waves generated per second. There are different categories of brainwaves.

Brainwave categories, ranging from most to least active, include:

  • Gamma waves: These are the fastest waves. They indicate intense concentration.
  • Beta waves: These are fast waves. They are present when you are awake and engaged.
  • Alpha waves: These are slower waves related to relaxation.
  • Theta waves: These slow waves are a typical part of light sleep or daydreaming.
  • Delta waves: These are the slowest waves. They are present in adults who are in deep sleep.

A sudden change in brainwave pattern or an interruption in brainwave activity may indicate a seizure. Slow brain waves might indicate autism or attention deficit hyperactivity disorder, while rapid activity might indicate epilepsy, post-traumatic stress disorder, psychosis, or drug use. 

Learn more about understanding your EEG results.

How long do EEG results take?

After an electroencephalogram (EEG) test, a neurologist analyzes the EEG recording. They will share the results with your doctor. Your doctor will then discuss the results at a follow-up appointment with you. Typically, the test results are available to your doctor within 48 hours of your test. 

Questions to ask your doctor about your results

To prepare for your follow-up appointment, you may want to make a list of questions to ask regarding your results. These may include:

  • What in the EEG results supports this diagnosis?
  • Are there other tests that can confirm this diagnosis?
  • What factors, if any, may have influenced the results of this EEG test?
  • Should I repeat the EEG test?
  • What other tests or treatments might I need at this time?
  • What are my next steps?
  • When should I follow up with you again?

Who performs an EEG?

A specially trained medical technician performs the electroencephalogram (EEG). The medical technician consults with the neurologist or neurosurgeon, who typically orders the test. These two doctors specialize in conditions affecting the brain and spinal cord. 

Specifically, a neurologist specializes in the medical treatment of conditions affecting the brain and spinal cord. A neurosurgeon, on the other hand, specializes in the surgical treatment of conditions affecting the brain and spinal cord. 

How is an EEG performed?

Doctor performing an EEG on a woman
This is a doctor performing an EEG on a woman.. diego_cervo/Getty Images

An electroencephalogram (EEG) is performed in a hospital or outpatient setting. The procedure is noninvasive and takes 1–2 hours. 

A routine EEG generally includes these steps:

  1. You will be asked to lie down or sit in a chair.
  2. A technician will either attach about 20 electrodes to your head or help you put on an electrode cap. Electrode caps are particularly helpful for the precise placements of electrodes while ensuring that they maintain proper contact with your scalp. The electrodes or electrode cap will connect to the EEG machine.
  3. You will be asked to rest quietly during the test, which may last 20–40 minutes or possibly longer.
  4. You may be asked to look at flashing lights, open and close your eyes, or take deep breaths at times during the test to see if any of these activities affect your brain or trigger a seizure.
  5. The technician will verify the resulting graphs to see that the test is complete.
  6. The technician will remove the electrodes or electrode cap.

What are the types of EEGs?

Besides the routine electroencephalogram (EEG) explained above, other types of EEGs have different processes. These include:

  • Ambulatory EEG: This uses a portable EEG machine to record brain activity throughout the day and night for 1 day or longer.
  • Sleep EEG: This is done while you sleep to check for sleep disorders.
  • Video EEG: This involves filming you while the EEG is recording brainwaves to gather more information.
  • Invasive EEG: This involves surgery to place electrodes on or into the brain to identify the location causing seizures. It is rarely used.

What are the risks and potential complications of an EEG?

Complications of an electroencephalogram (EEG) are not common, but any medical procedure involves risk.

If you have a seizure disorder, there is a risk of an EEG triggering a seizure. Your care team will treat any seizure activity immediately.

How do I prepare for an EEG?

The steps you take before an electroencephalogram (EEG) can improve your comfort and help your doctor obtain the most accurate test results. 

You can prepare for an EEG by doing the following:

  • Wash and dry your hair, but do not apply hair products.
  • Eat as you typically do unless your doctor tells you otherwise.
  • Avoid foods and beverages that contain caffeine.
  • Only stop medications as directed by your doctor.

You may also be asked to restrict your sleep the night before your EEG.

What can I expect after my EEG?

An electroencephalogram (EEG) should not have any aftereffects. Most people go right home and go about their typical activities after an EEG.

How will I feel after an EEG?

An EEG is a painless, noninvasive test. However, tell your doctor or care team right away if you have any pain or discomfort during or after the procedure.

When can I go home?

You will likely go home right after the testing is complete.

When should I call my doctor?

It is important to keep your follow-up appointments after an EEG. Your doctor will discuss your EEG results at the first follow-up appointment. Call your doctor if you have new or worsening symptoms or questions between appointments.

Frequently asked questions

Here are some other questions that people commonly ask about electroencephalogram (EEG) tests.

Are you awake during an EEG?

You are usually awake during an EEG unless it is being performed to test for sleep disorders. You may be asked to try not to sleep the night before a sleep EEG.

Will I feel pain during an EEG?

An EEG is a painless, noninvasive procedure. However, tell your care team if you have any pain or discomfort.

Can an EEG detect past seizures?

An EEG cannot detect past seizures. However, they may show current or predict future seizures. The medical technician may show you flashing lights or ask you to take deep breaths to stimulate a seizure. The EEG can also record activity between seizures, called interictal brain waves, which are seen as sharp waves or spikes. These can help show how likely it is that a seizure may occur.

What can an EEG show that an MRI scan cannot?

An EEG depicts the electrical activity in your brain rather than its physical condition. An MRI scan can show damage or anomalies in the tissue, whereas an EEG can show anomalies in how your brain is functioning.

What do you wear during an EEG?

You should wear comfortable clothing because you will be asked to lie or sit still during the test. During the test, you will wear an electrode cap or have about 20 electrodes attached gently to your scalp.

Make sure that your hair is clean and dry with no styling products in it. Bring a brush or comb, as your hair may become mussed and have some residue in it from the electrode glue.

Summary

An electroencephalogram (EEG) is a recording of the electrical activity in your brain. It is a diagnostic tool that doctors use to help identify some conditions related to unusual brain activity, such as seizures, strokes, tumors, behavioral problems, and sleep disorders. It is a painless, noninvasive test that takes place in an outpatient setting.

A specially trained technician will place electrodes on your scalp or have you wear an electrode cap. The electrodes pick up the electrical activity in your brain and send it to an electroencephalograph, which will create a graph of your brainwaves. Your doctor will examine the EEG report and give you the results.

An EEG is typically used in conjunction with other tests, but it is a valuable tool for doctors. You should have no aftereffects from an EEG, and you should be able to go about your typical daily activities soon after.

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Medical Reviewer: Seunggu Han, M.D.
Last Review Date: 2022 Jul 25
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