Electroencephalography

Definition
Purpose
Demographics
Description
Diagnosis/Preparation
Aftercare
Risks
Normal results
Morbidity and mortality rates
Alternatives

Definition

Electroencephalography, or EEG, is a neurological test that involves attaching electrodes to the head of a person to measure and record electrical activity in the brain over time.

Purpose

The EEG, also known as a brain wave test, is a key tool in the diagnosis and management of epilepsy and other seizure disorders. It is also used to assist in the diagnosis of brain damage and diseases such as strokes, tumors, encephalitis, mental retardation, and sleep disorders. An EEG may also be used to monitor brain activity during surgery to assess the effects of anesthesia. It is also used to determine brain status and brain death.

Demographics

The number of EEG tests performed each year can only be estimated. It is not a reportable event and is used in the diagnostic workup for a number of disorders. The number of EEG tests per year is estimated to be in the range of 10-25 million.

Description

Before an EEG begins, a nurse or technologist attaches approximately 16-21 electrodes to a person’s scalp using an electrically conductive, washable paste. The electrodes are placed on the head in a standard pattern based on head circumference measurements. Depending on the purpose for the EEG, implantable, or invasive, electrodes are occasionally used. Implantable electrodes include sphenoidal electrodes, which are fine wires inserted under the zygomatic arch, or cheekbone. Depth electrodes, or subdural strip electrodes, are surgically implanted into the brain and are used to localize a seizure focus in preparation for epilepsy surgery. Once in place, even implantable electrodes do not cause pain. The electrodes are used to measure the electrical activity in various regions of the brain over the course of the test period.

For the test, a person lies on a bed, padded table, or comfortable chair and is asked to relax and remain still while measurements are being taken. An EEG usually takes no more than one hour, although long-term monitoring is often used for diagnosis of seizure disorders. During the test procedure, a person may be asked to breathe slowly or quickly. Visual stimuli such as flashing lights or a patterned board may be used to stimulate certain types of brain activity. Throughout the procedure, the electroencephalography unit makes a continuous graphic record of the person’s brain activity, or brain waves, on a long strip of recording paper or computer screen. This graphic record is called an electroencephalogram. If the display is computerized, the test may be called a digital EEG, or dEEG.

KEY TERMS

Encephalitis— Inflammation of the brain.

Fast Fourier transfer— A digital processing of the recorded signal resulting in a decomposition of its frequency components.

Ictal EEG— An EEG done to determine the type of seizure characteristic of a person’s disorder. During this EEG, seizure medicine may be discontinued in an attempt to induce as seizure during the testing period.

Sphenoidal electrodes— Fine wire electrodes that are implanted under the cheek bones, used to measure temporal seizures.

Subdural electrodes— Strip electrodes that are placed under dura mater (the outermost, toughest, and most fibrous of the three membranes [meninges] covering the brain and spinal cord). They are used to locate foci of epileptic seizures prior to epilepsy surgery.

Zygomatic arch— Cheekbone. A quadrilateral bone forming the prominence of the cheek. It articulates (touches or moves) with the frontal, sphenoid, and maxillary, and temporal bones.

The sleep EEG uses the same equipment and procedures as a regular EEG. Persons undergoing a sleep EEG are encouraged to fall asleep completely rather than just relax. They are typically provided a bed and a quiet room conducive to sleep. A sleep EEG lasts up to three hours, or up to eight or nine hours if it is a night’s sleep.

In an ambulatory EEG, individuals are hooked up to a portable cassette recorder. They then go about normal activities and take normal rest and sleep for a period of up to 24 hours. During this period, individuals and their family members record any symptoms or abnormal behaviors, which can later be correlated with the EEG to see if they represent seizures.

An extension of the EEG technique, called quantitative EEG (qEEG), involves manipulating the EEG signals with a computer using the fast Fourier transform algorithm. The result is then best displayed using a colored gray scale transposed onto a schematic map of the head to form a topographic image. The brain map produced in this technique is a vivid illustration of electrical activity in the brain. This technique also has the ability to compare the similarity of the signals between different electrodes, a measurement known as spectral coherence. Studies have shown the value of this measurement in diagnosis of Alzheimer’s disease and mild closed-head injuries. The technique can also identify areas of the brain having abnormally slow activity when the data are both mapped and compared to known normal values. The result is then known as a statistical or significance probability map (SPM). This allows differentiation between early dementia (increased slowing) or otherwise uncomplicated depression (no slowing).

Diagnosis/Preparation

An EEG is generally performed as one test in a series of neurological evaluations. Rarely does the EEG form the sole basis for a particular diagnosis.

Full instructions should be given to individuals receiving an EEG when they schedule their test. Typically, individuals taking medications that affect the central nervous system, such as anticonvulsants, stimulants, or antidepressants, are told to discontinue their prescription for a short time prior to the test (usually one to two days). However, such requests should be cleared with the treating physician. EEG test candidates may be asked to avoid food and beverages that contain caffeine, a central nervous system stimulant. They may also be asked to arrive for the test with clean hair that is free styling products to make attachment of the electrodes easier.

Individuals undergoing a sleep EEG may be asked to remain awake the night before their test. They may be given a sedative prior to the test to induce sleep.

Aftercare

If an individual has suspended regular medication for the test, the EEG nurse or technician should advise as to when to begin taking it again.

Risks

Being off certain medications for one to two days may trigger seizures. Certain procedures used during EEG may trigger seizures in persons with epilepsy. Those procedures include flashing lights and deep breathing. If the EEG is being used as a diagnostic for epilepsy (i.e., to determine the type of seizures an individual is experiencing) this may be a desired effect, although the person needs to be monitored closely so that the seizure can be aborted if necessary. This type of test is known as an ictal EEG.

Normal results

In reading and interpreting brain wave patterns, a neurologist or other physician will evaluate the type of brain waves and the symmetry, location, and consistency of brain wave patterns. Brain wave response to certain stimuli presented during the EEG test (such as flashing lights or noise) will also be evaluated.

The four basic types of brain waves are alpha, beta, theta, and delta, with the type distinguished by frequency. Alpha waves fall between 8 and 13 Hertz (Hz), beta are above 13 Hz, theta between 4 and 7 Hz, and delta are less than 4 Hz. Alpha waves are usually the dominant rhythm seen in the posterior region of the brain in older children and adults, when awake and relaxed. Beta waves are normal in sleep, particularly for infants and young children. Theta waves are normally found during drowsiness and sleep and are normal in wakefulness in children, while delta waves are the most prominent feature of the sleeping EEG. Spikes and sharp waves are generally abnormal; however, they are common in the EEG of normal newborns.

Different types of brain waves are seen as abnormal only in the context of the location of the waves, a person’s age, and one’s conscious state. In general, disease typically increases slow activity, such as theta or delta waves, but decreases fast activity, such as alpha and beta waves.

Not all decreases in wave activity are abnormal. The normal alpha waves seen in the posterior region of the brain are suppressed merely if a person is tense. Sometimes the addition of a wave is abnormal. For example, alpha rhythms seen in a newborn can signify seizure activity. Finally, the area where the rhythm is seen can be telling. The alpha coma is characterized by alpha rhythms produced diffusely, or, in other words, by all regions of the brain.

Some abnormal beta rhythms include frontal beta waves that are induced by sedative drugs. Marked asymmetry in beta rhythms suggests a structural lesion on the side lacking the beta waves. Beta waves are also commonly measured over skull lesions, such as fractures or burr holes, in activity known as a breach rhythm.

Usually seen only during sleep in adults, the presence of theta waves in the temporal region of awake, older adults has been tentatively correlated with vascular disease. Another rhythm normal in sleep, delta rhythms, may be recorded in the awake state over localized regions of cerebral damage. Intermittent delta rhythms are also an indication of damage of the relays between the deep gray matter and the cortex of

WHO PERFORMS THE PROCEDURE AND WHERE IS IT PERFORMED?

Electroencephalography is often administered by specially trained technicians who are supervised by a neurologist or other physician with specialized training in administering and interpreting the test. Because of the equipment involved, an EEG is usually administered in a hospital setting. It may be conducted in a professional office.

the brain. In adults, this intermittent activity is found in the frontal region whereas in children, it is in the occipital region.

The EEG readings of persons with epilepsy or other seizure disorders display bursts, or spikes, of electrical activity. In focal epilepsy, spikes are restricted to one hemisphere of the brain. If spikes are generalized to both hemispheres of the brain, mul-tifocal epilepsy may be present. The EEG can be used to localize the region of the brain where the abnormal electrical activity is occurring. This is most easily accomplished using a recording method, or montage, called an average reference montage. With this type of recording, the signal from each electrode is compared to the average signal from all the electrodes. The negative amplitude (upward movement, by convention) of the spike is observed for the different channels, or inputs, from the various electrodes. The negative deflection will be greatest as recorded by the electrode that is closest in location to the origin of the abnormal activity. The spike will be present but of reduced amplitude as the electrodes move farther away from the site producing the spike. Electrodes distant from the site will not record the spike occurrence.

A final variety of abnormal result is the presence of slower-than-normal wave activity, which can either be a slow background rhythm or slow waves superimposed on a normal background. A posterior dominant rhythm of 7 Hz or less in an adult is abnormal and consistent with encephalopathy (brain disease). In contrast, localized theta or delta rhythms found in conjunction with normal background rhythms suggest a structural lesion.

Morbidity and mortality rates

There are few adverse conditions associated with an EEG test. Persons with seizure disorders may induce seizures during the test in reaction to flashing lights or by deep breathing. Mortality from an EEG has not been reported.

QUESTIONS TO ASK THE DOCTOR

• How many EEG procedures has the technician performed?
• What preparations are being made to treat an induced seizure?
• Is the supervising physician appropriately certified to interpret an EEG?

Alternatives

There are no equivalent tests that provide the same information as an EEG.

Resources

BOOKS

Chin, W. C., and T. C. Head. Essentials of Clinical Neurophysiology, 3rd ed. London: Butterworth-Heinemann, 2002.

Daube, J. R. Clinical Neurophysiology, 2nd ed. New York: Oxford University Press, 2002.

Ebersole, J. S., and T. A. Pedley. Current Practice of Clinical Electroencephalography, 3rd ed. Philadelphia: Lippincott Williams & Wilkins, 2002.

Rowan, A. J., and E. Tolunsky. Primer of EEG. London: Butterworth-Heinemann, 2003.

PERIODICALS

De Clercq, W., P. Lemmerling, S. Van Huffel, and W. Van Paesschen. “Anticipation of epileptic seizures from standard EEG recordings.” Lancet 361, no. 9361 (2003): 971–972.

ORGANIZATIONS

American Association of Neuromuscular and Electrodiag-nostic Medicine (AANEM).421 First Avenue SW, Suite 300 East, Rochester, MN 55902. (507) 288-0100. Fax: (507) 288-1225. Email: [email protected]. http://www.aanem.org/index.cfm.

American Board of Registration for Electroencephalographic Technologists. P.O. Box 916633, Longwood, FL 32791–6633.

American Board of Registration of Electroencephalographic and Evoked Potential Technologists, Inc http://www.abret.org.

American Society of Electroneurodiagnostic Technologists Inc, 204 W. 7th Carroll, IA 51401. (712) 792-2978. http://www.aset.org/.

Epilepsy Foundation. 4351 Garden City Drive, Landover, MD 20785-7223. (800) 332-1000 or (301) 459-3700. http://www.efa.org.

OTHER

Dowshen, Steven, M.D. “EEG (Electroencephalography).” Kid’s Health For Parents. September 2007. http://kidshealth.org/parent/system/medical/eeg.html [Accessed April 11, 2008].

National Society for Epilepsy. “Epilepsy Information: Electroencephalography.” October 2006. http://www.epilepsynse.org.uk/pages/info/leaflets/eeg.cfm [Accessed April 11, 2008].

L. Fleming Fallon, Jr., MD, DrPH

Laura Jean Cataldo, RN, EdD