Understanding Your EEG Results

Medically Reviewed By William C. Lloyd III, MD, FACS
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Your doctor may recommend an EEG (electroencephalogram) to diagnose the cause of symptoms, such as seizures or memory loss. An EEG evaluates brain function by measuring the electrical activity within the brain. It records patterns of activity during rest and in response to certain stimuli. This helps your doctor diagnose and evaluate brain disorders and conditions.

Keep in mind that electroencephalography is only one part of a comprehensive diagnostic workup. A doctor who specializes in neurology or neurosurgery will interpret your EEG results in the context of other tests and clinical information. The test can only reveal what is happening in the brain; it can’t explain why it’s happening. That requires the expertise of your doctor. When discussing your test results with your doctor, it's helpful to have background information on the test itself and normal and abnormal EEG results.

What Does a Normal EEG Look Like?

Electrical activity in the brain is recorded as a ‘wave.’ There are many types of brain waves that an EEG records. The basic brain waves are alpha, beta, theta, and delta waves. There are other more complex waves. Each type of brain wave has a normal frequency, height, shape, and location. A person’s age can also determine whether a certain brain wave is normal or not. For example, delta waves are normal in young children. They are not normal for adults who are awake. Your doctor examines each facet of a wave to determine if it is normal or not.

What Does an Abnormal EEG Look Like?

Abnormal EEG results can show up in two ways. First, normal brain activity may be suddenly interrupted and changed. This happens in epileptic seizures. In partial seizures, only part of the brain shows the sudden interruption. The whole brain shows it in generalized seizures.

The other way an EEG can show abnormal results is called non-epileptiform changes. This can be a general change in the way a normal brain wave looks. It may have an abnormal frequency, height or shape. It can also be a brain wave showing up that should not. For example, a delta wave occurring in an adult who is awake is not normal. This wave typically occurs in adults when they are in deep sleep.

How Do Doctors Use the EEG Results? 

Doctors use information from an EEG to gain insight into brain activity.

1. Alpha waves are related to relaxation and attention. They are present when you are awake with your eyes closed. They usually disappear when you open your eyes and pay attention to something.

2. Beta waves are normal in people who are awake. It doesn’t matter whether your eyes are open or closed. Certain drugs, such as sedatives, can influence these waves.

3. Theta waves are related to sleep. These slow waves are normal for all ages during sleep. They generally aren’t obvious when adults are awake.

4. Delta waves are also related to sleep. These waves are normal in adults who are in deep sleep and in young children.

Other brain waves and EEG patterns give doctors information about sleep stages, how easily you can be roused from sleep, and problems with the structure of the brain.

What Do Abnormal EEG Results Mean?

An abnormal EEG means that there is a problem in an area of brain activity. This can offer a clue in diagnosing various neurological conditions. Read 10 Conditions Diagnosed With an EEG to learn more.

EEG testing is one part of making a diagnosis. The results may not pinpoint a specific diagnosis, but it can narrow the possibilities. 

Second opinions can be a valuable tool when trying to establish a neurological diagnosis. Remember that a second opinion on your test results is not a negative reflection on your doctor. It offers additional input and reassurance about your situation.

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Medical Reviewer: William C. Lloyd III, MD, FACS
Last Review Date: 2020 Aug 3
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THIS TOOL DOES NOT PROVIDE MEDICAL ADVICE. It is intended for informational purposes only. It is not a substitute for professional medical advice, diagnosis or treatment. Never ignore professional medical advice in seeking treatment because of something you have read on the site. If you think you may have a medical emergency, immediately call your doctor or dial 911.

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  2. Smith SJM. EEG in neurological conditions other than epilepsy: when does it help, what does it add? J Neurol Neurosurg Psychiatry 2005;76:ii8-ii12.

  3. Electroencephalogram (EEG). Johns Hopkins Medicine. http://www.hopkinsmedicine.org/healthlibrary/test_procedures/neurological/electroencephalogram_eeg_9