What Is an Electroencephalogram (EEG) Test and What Is It Used For?

Evaluation of Brain's Electrical Activity Important in Sleep, Seizures

The electrical activity of the brain is evaluated with an electroencephalogram (EEG) test used in sleep, seizures, coma, and brain death testing
The electrical activity of the brain is evaluated with an electroencephalogram (EEG) test used in sleep, seizures, coma, and brain death testing. Michel Tcherevkoff/The Image Bank/Getty Images

It is sometimes necessary to arrange further testing to evaluate conditions that affect the brain. One of the most common tests is the electroencephalogram (EEG). What is an EEG test and what is it used for? Learn about some of the conditions where it may be indicated, including the evaluation of seizures, confusion, coma, brain death, and even sleep.

What Is an Electroencephalogram (EEG) Test?

An EEG, also known by its formal name electroencephalogram, is a measurement of the continuous electrical activity of the brain.

These brain-wave patterns are recorded with the placement of small metal discs called electrodes positioned in a standardized pattern on the scalp. The resulting digital tracings reflect the summation of the activity of millions of individual neurons within the surface of the brain called the cerebral cortex. The pattern's voltage (or amplitude) and frequency is most often interpreted by a neurologist and aids in the diagnosis of various conditions. 

Why Is EEG Testing Done?

Once the electrodes are placed, information is quickly gathered about the state of the brain's function. There are certain conditions where this is especially important. 

EEGs are most often performed to evaluate for the presence or risk of seizures. A routine EEG is usually arranged after a first-time seizure occurs. Seizures are characterized by abnormal electrical discharges within the brain that may result in confusion or agitation, uncontrolled movements, hallucinations, and even collapse.

As there is a significant risk with uncontrolled seizures, treatment with medication is initiated if the EEG suggests a risk of recurrence.

Beyond assessment in epilepsy, the EEG has other utility, especially in the hospital setting. It may be useful in determining the cause of altered consciousness, such as occurs with delirium or coma.

If someone has suffered a brain injury due to trauma or lack of oxygen (hypoxia), the EEG may have prognostic value to determine the likelihood of recovery. In rare circumstances, it may provide evidence that brain death has occurred.

The electrical activity of the brain can also help to localize areas with abnormal function. For example, it might suggest a developmental abnormality that leads to recurrent seizures. In the past it was used to help narrow down the area of the brain that may be dysfunctional. Fortunately, improvements in imaging such as MRI have largely supplanted this use. It can still be useful to ensure proper function of the connections of the nervous system with testing of evoked potentials. In addition, intraoperative monitoring with EEG may ensure that orthopedic or brain surgery is not causing permanent damage.

How Does an EEG Help Diagnose Sleep Disorders?

A limited application of EEG is used with a standard sleep study called a polysomnogram. Electrodes are placed on the frontal, central, temporal, and occipital locations.

These measurements can be used to identify sleep stages. This is helpful to determine when sleep onset occurs, when awakenings from breathing disruption are present in sleep apnea, and the overall structure of sleep. These findings may be summarized as part of a hypnogram in a sleep study report.

In addition, seizures can be identified during the overnight sleep study. Often there will be other signs of seizures prior to their discovery on this testing, but it may provide additional evidence of the condition and the effectiveness of treatment. In people with abnormal movements or behaviors in sleep, it can be important to rule out seizures as a potential cause.

Are There Risks or Alternatives to EEG Tests?

There are no significant risks associated with EEG testing. The tests are inexpensive, accessible, and often quite useful. The placement of the electrodes may cause some irritation of the skin of the scalp. In addition, the glue or paste used may cause a mild allergic reaction. These are usually minor and go away quickly. 

Your doctor will determine if an EEG is necessary. At times, the results may be somewhat non-specific and further evaluation may be indicated. As mentioned previously, other adjunctive testing such as imaging may provide additional information.

If you have been recommended to have an EEG as part of your medical evaluation, you can rest assured that the testing is safe and can often provide useful information. The results must be interpreted in the clinical context, and your physicians will use them to understand the broader medical picture.


Emerson RG and Pedley TA. "Clinical Neurophysiology: Electroencephalography and Evoked Potentials." In Neurology in Clinical Practice, Elsevier, 5th edition, 2008, pp. 455-481.

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