electroencephalogram

n. (Abbr. EEG)
A graphic record of the electrical activity of the brain as recorded by an electroencephalograph. Also called encephalogram.

Electroencephalogram (EEG) Recording of electrical activity from the brains of animals was first reported by the British physiologist Caton in 1875. Berger, a German psychiatrist, described the human EEG in 1929, but it was only after a further description of ‘the Berger rhythm’, by Adrian and Matthews in Cambridge five years later, that it began to be used in research and diagnosis.

An early EEG. Effect of general excitatory states. Normal electrograms as varied by conditions of generalized excitation, relaxation, drowsiness, light sleep, and deep sleep. (From W. Penfield and T. C. Erickson (1941), Epilepsy and cerebral localization, Charles C. Thomas, Baltimore.)


Electroencephalography records, from electrodes placed on the scalp, the weak electrical activity generated by the brain, its voltage ten times smaller than that from the heart displayed by an electrocardiogram (ECG). This makes the EEG very sensitive to interference from muscle activity in the scalp or from electronic equipment in the vicinity. Moreover, whereas in the heart the origin of the activity is very well understood, the EEG records the collective activity of large populations of neurons in the cerebral cortex. For all the sophistication of modern recording equipment and computerized analysis of the records, it has to be accepted that the EEG remains a relatively crude measuring device, although it enables recognition of the different phases of normal sleep, and is of diagnostic value in some abnormal conditions.

A number of electrodes (typically about 20) are positioned on the scalp and connected in pairs, yielding 8-16 channels, each recording the potential between two electrodes. Each electrode receives signals from an area of cortex of 2-3 cm diameter. But a third of the cortex is inaccessible, in the depths of the indentations (the sulci), on the basal surface, or hidden within the larger folds of the brain. Some 10-15 min of recording results in 20-30 pages of paper that can be bound and read like a book — and often also analyzed by computer.

Information lies in the frequency and amplitude (voltage) of the waves recorded in different channels. At rest, relaxed and with the eyes closed, the frequency of these waves is 8-12 Hz (cycles/sec). This ‘alpha’ activity is believed to reflect the brain in ‘idling’ mode, because if the person then either opens the eyes, or does mental arithmetic with the eyes closed, these waves disappear, to be replaced by irregular patterns (so-called desynchronized activity). In normal sleep there is characteristic higher voltage activity, in patterns which vary according to the level of sleep.

When there is severe diffuse brain abnormality, such as encephalitis or conditions causing coma, there will be usually be no alpha activity, whilst in the vegetative state there may be alpha activity that fails to desynchronize on eye opening. Faster frequencies (beta, at >13 Hz) or slower (theta, at 4-8 Hz) can be normal in infancy and childhood. Even slower ‘delta’ waves (<4 Hz) can be normal in sleep and in infancy, but in awake adults indicate severe abnormality. When localized they may indicate pathology such as a tumour or abscess in the brain, causing the adjacent cortex to produce abnormal rhythms; however, modern imaging techniques have replaced EEG as a means of detecting and locating such lesions.

It is in the investigation of epilepsy that EEG has proved most useful — both in diagnosing epilepsy as the cause of abnormal behaviours and in localizing the site in the brain from which abnormalities are originating. During an epileptic seizure there are bursts of high voltage activity in the region of the brain affected. The problem is the low probability of a patient having an attack during routine recording. However, the record in a person with epilepsy is often abnormal between attacks, and these abnormalities are more likely to be found if the patient hyperventilates (producing temporary alkalosis in the brain), or is fasting (with temporary mild hypoglycaemia). But some patients who never have a clinical seizure may show such abnormalities on EEG, whilst a third of patients who do have seizures have a normal record at times between attacks. To increase the chance of securing a recording during an attack patients may be fitted with electrodes that transmit by radio to a receiver. Such telemetry allows 24-hour recording during normal activities — which may also be monitored on video in order to visualize any seizure that occurs.

When patients with epilepsy are being investigated with a view to possible surgery the location of the seizure-producing lesion may be identified by using electrodes placed to cover areas not available to those placed on the scalp. Electrodes introduced through the cheek to the base of the skull can detect activity from the under surface of the brain, whilst those inserted through a burr hole (a small opening in the skull) on to the surface of deeper parts of the brain, or even into the substance of the brain, bring other areas under surveillance.

A visual, auditory, or somatic stimulus will normally evoke an altered wave form from the appropriate area of the cortex (for example the occipital region for a visual input). It is therefore possible with the EEG to explore the integrity of sensory pathways by means of these evoked potentials. This method is also used during operative procedures to help surgeons to identify such sensory pathways by observing the effects of electrical stimulation on the EEG, so as to avoid damaging them.

An occasional use of the EEG is in confirming the diagnosis of brain death, when an isoelectric (flat) record may be obtained, but technical difficulties can lead to equivocal results. In some places, but not in the UK, EEG is mandatory after clinical tests have been completed.

— Bryan Jennett

See also convulsions; epilepsy.

EEG

A graphical record of the electrical activity of the brain. Three types of brainwaves are associated with different levels of arousal: theta waves occur during sleep, alpha waves are associated with wakefulness, and beta waves with excitement. EEGs can be used to monitor the effects of exercise since there is a close correlation between certain EEG wave patterns and fatigue or overtraining. They are also used to determine the extent of injuries inflicted to the head (for example, after a knockout in boxing).

Definition

An electroencephalogram (EEG), also called a brain wave test, is a diagnostic test which measures the electrical activity of the brain (brain waves) using highly sensitive recording equipment attached to the scalp by fine electrodes.

Purpose

EEG is performed to detect abnormalities in the electrical activity of the brain which may help diagnose the presence and type of various brain disorders, to look for causes of confusion, and to evaluate head injuries, tumors, infections, degenerative diseases, and other disturbances that affect the brain. The test is also used to investigate periods of unconsciousness. EEG may also confirm brain death in someone who is in a coma. EEG cannot be used to measure intelligence or diagnose mental illness. Specifically, EEG is used to diagnose the following:

• seizure disorders (such as epilepsy or convulsions)
• structural brain abnormality (such as a brain tumor or brain abscess)
• head injury, encephalitis (inflammation of the brain)
• hemorrhage (abnormal bleeding caused by a ruptured blood vessel)
• cerebral infarct (tissue that is dead because of a blockage of the blood supply)
• sleep disorders (such as narcolepsy)

Description

Brain cells communicate by producing tiny electrical impulses, also called brain waves. These electrical signals have certain rhythms and shapes, and EEG is a technique that measures, records, and analyzes these signals to help make a diagnosis. Electrodes are used to detect the electrical signals. They come in the shape of small discs that are applied to the head and connected to a recording device. The recording machine then converts the electrical signals into a series of wavy lines that are drawn onto a moving piece of graph paper. An EEG test causes no discomfort. Although having electrodes pasted on the skin may feel strange, they only record activity and do not produce any sensation. The patient needs to lie still with eyes closed because any movement can affect results. The patient may also be asked to do certain things during the EEG recording, such as breathing deeply and rapidly for several minutes or looking at a bright flickering light.

An EEG is performed by an EEG technician in a specially designed room that may be in the doctor's office or at a hospital. The patient is asked to lie on a bed or in a comfortable chair so that a relaxed EEG recording can be done. The technician either measures the scalp and marks the spots where small discs (electrodes) will be placed or fits the head with a special cap containing between 16 and 25 of these discs. The scalp is then rubbed with a mild, scratchy cleanser that may cause mild discomfort for a short while. The discs are attached to the body with a cream or gel. Alternatively, the technician may secure the discs to the skin with an adhesive. The heart may also be monitored during the procedure.

Precautions

Before an EEG, care should be taken to avoid washing hair with an oily scalp product 24 hours before the test. Doctors usually recommend that patients eat a meal or light snack some four hours before the test. Caffeinated drinks should be avoided for eight hours before the test. Sometimes, the EEG gives better results when the patient has had less than the usual amount of sleep. The doctor may ask that the child be kept awake for all or part of the night before the EEG. The healthcare provider may also discontinue some medications before the test.

Preparation

The physical and psychological preparation required for this test depends on the child's age, interests, previous experiences, and level of trust. For older children, research has shown that preparing ahead can reduce crying or resisting the test. In addition, children report less pain and show less distress when prepared. Proper preparation for the test can reduce a child's anxiety, encourage cooperation, and help develop coping skills.

Some general guidelines for preparing a toddler or preschooler for an EEG include the following:

• Explain the EEG procedure in words that the child understands, avoiding abstract terminology.
• Ensure that the child understands the exact body part involved and that the procedure will be limited to that area.
• Describe how the test is likely to feel.
• Give the child permission to yell, cry, or otherwise express any pain or discomfort verbally.
• Stress the benefits of the EEG procedure and list things that the child may find pleasurable after the test, such as feeling better or going home.

The above guidelines also apply to school age children. Additionally, for older children, parents can try the following:

• Suggest ways to keep calm and reduce anxiety such as counting, deep breathing, or thinking pleasant thoughts.
• Include the child in the decision-making process, such as the time of day where the EEG is performed.
• Suggest that the child hold the hand of the technician or someone else helping with the procedure.

As for adolescents, detailed information about the EEG should be provided and the reasons for the procedure should be explained in correct medical terminology. When the EEG is required for a seizure disorder, there is the potential risk that the test will trigger a seizure. This possibility should be openly discussed. Adolescents commonly have high concerns about risks and the best way to prepare them is to fully inform them. The healthcare provider could also be asked to limit the number of strangers entering and leaving the room during the EEG procedure, since they can raise the patient's anxiety level.

Aftercare

There are no side effects or special procedures required after an EEG. The technician simply removes the gel with water and the adhesive, if used, with a special cleanser. Shampooing will rid the hair of any other material. A few patients are mildly sensitive to the gel or may get irritation from the rubbing of their scalps.

Risks

The EEG test is very safe. However, if a patient has a seizure disorder, a seizure may be triggered by the flashing lights or hyperventilation. The healthcare provider performing the EEG is trained to take care of the patient if this happens.

Normal Results

An EEG returns normal results when brain waves have normal frequency and amplitude and other characteristics are typical.

Parental Concerns

Before the test, parents should know that the child probably will cry, and restraints may be used. The most important way to help a child through an EEG procedure is by being there and caring. Crying is a normal response to the strange environment, unfamiliar people, restraints, and separation from the parent. Infants and young children will cry more for these reasons than because the test or procedure is uncomfortable. Knowing this from the onset may help parents feel less anxiety about what to expect. Having specific information about the test may further reduce anxiety.

See also Encephalitis; Narcolepsy; Sleep disorders.

Resources

Books

Electroencephalogram: A Medical Dictionary, Bibliography, and Annotated Research Guide to Internet. San Diego, CA: Icon Health Publications, 2004.

Shaw, John C. The Brain's Alpha Rhythms and the Mind. New York: Elsevier Science, 2003.

Periodicals

Foley, C. M., et al. "Long-term Computer-assisted Outpatient Electroencephalogram Monitoring in Children and Adolescents." Journal of Child Neurology 15, no. 1 (January 2000): 49–55.

Jenny, O. G., and M. A. Carskadon. "Spectral Analysis of the Sleep Electroencephalogram during Adolescence." Sleep 27, no. 4 (June 2004): 774–83.

Nasr, J. T., et al. "The Electroencephalogram in Children with Developmental Dysphasia." Epilepsy Behavior 2, no. 2 (April 2001): 115–18.

Wassmer, E., et al. "Melatonin as a Sleep Inductor for Electroencephalogram Recordings in Children." Clinical Neurophysiology 112, no. 4 (April 2001): 683–85.

Organizations

American Academy of Neurology Foundation. 1080 Montreal Avenue, St. Paul, MN 55116. Web site: www.neurofoundation.com.

American Society of Neurophysiological Monitoring. PO Box 60487, Chicago, IL 60660–0487. Web site: www.asnm.org.

National Institute of Neurological Disorders and Stroke (NINDS). PO Box 5801, Bethesda, MD 20824. Web site: www.ninds.nih.gov.

Web Sites

"EEG." Medline Plus. Available online at www.nlm.nih.gov/medlineplus/ency/article/003931.htm (accessed November 17, 2004).

"The '10–20 System' of Electrode Placement." Available online at (accessed November 17, 2004).

[Article by: Monique Laberge, Ph.D.]

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EEG

A graphical recording of the electrical activity of the brain made during electroencephalography.

(i-lek-troh-en-sef-uh- loh-gram)

A written recording of the electrical activity of the brain. Electroencephalograms are useful in studying and detecting brain disorders.

The record produced by electroencephalography; a tracing of the electric impulses of the brain. Called also EEG.

The human body is an electrochemical organism, although the electrical activities of the body are relatively weak in contrast to electrical appliances. Although weak, the electrical activity of the human body, particularly the nerves, can be measured. The brain is especially active in this regard, and electrical variations can be measured by means of electrodes taped to the head and face. The machines used for this purpose produce what is referred to as an electroencephalogram, or EEG. When recorded on a roll of paper, EEGs produce what looks like a wave pattern, hence the expression brain wave. Different stages of sleep are characterized by different brain wave patterns, and contemporary laboratory sleep research relies heavily on EEG measurements.

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Electroencephalogram

Dansk (Danish)
n. - elektroencefalogram, EEG

Français (French)
n. - électroencéphalogramme

Deutsch (German)
n. - Elektroenzephalogramm

Ελληνική (Greek)
n. - (ιατρ.) ηλεκτροεγκεφαλογράφημα

Italiano (Italian)
elettroencefalogramma

Português (Portuguese)
n. - eletroencefalograma (m) (Med.)

Русский (Russian)
электроэнце- фалограмма

Español (Spanish)
n. - electroencefalograma

Svenska (Swedish)
n. - elektroencefalogram

中文（简体）(Chinese (Simplified))
脑电图, 脑动电流图

中文（繁體）(Chinese (Traditional))
n. - 腦電圖, 腦動電流圖

한국어 (Korean)
n. - 뇌전도

日本語 (Japanese)
n. - 脳波図

עברית (Hebrew)
n. - ‮תרשים הפעילות החשמלית של המוח, אא"ג‬

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