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Related to Electro-encephalography: magnetic resonance imaging, positron emission tomography




Electroencephalography, or EEG, is a neurological test that uses an electronic monitoring device to measure and record electrical activity in the brain.


The EEG 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 disease (e.g., stroke, tumors, encephalitis), mental retardation, sleep disorders, degenerative diseases such as Alzheimer's disease and Parkinson's disease, and certain mental disorders (e.g., alcoholism, schizophrenia, autism).
An EEG may also be used to monitor brain activity during surgery and to determine brain death.


Electroencephalography should be administered and interpreted by a trained medical professional only. Data from an EEG is only one element of a complete medical and/or psychological patient assessment, and should never be used alone as the sole basis for a diagnosis.


Before the EEG begins, a nurse or technician attaches approximately 16-20 electrodes to the patient's scalp with a conductive, washable paste. 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; and depth electrodes, which are surgically-implanted into the brain. The EEG electrodes are painless, and are used to measure the electrical activity in various regions of the brain.
For the test, the patient lies on a bed, padded table, or comfortable chair and is asked to relax and remain still during the EEG testing period. An EEG usually takes no more than one hour. During the test procedure, the patient 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 electroencephalograph machine makes a continuous graphic record of the patient's brain activity, or brainwaves, on a long strip of recording paper or on a computer screen. This graphic record is called an electroencephalogram.
The sleep EEG uses the same equipment and procedures as a regular EEG. Patients 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.
In an ambulatory EEG, patients are hooked up to a portable cassette recorder. They then go about their normal activities, and take their normal rest and sleep for a period of up to 24 hours. During this period, the patient and patient's family record any symptoms or abnormal behaviors, which can later be correlated with the EEG to see if they represent seizures.
Many insurance plans provide reimbursement for EEG testing. Costs for an EEG range from $100 to more than $500, depending on the purpose and type of test (i.e., asleep or awake, and invasive or non-invasive electrodes). Because coverage may be dependent on the disorder or illness the EEG is evaluating, patients should check with their individual insurance plan.


Full instructions should be given to EEG patients when they schedule their test. Typically, individuals on 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). Patients may be asked to avoid food and beverages that contain caffeine, a central nervous system stimulant. However, any such request should be cleared by the treating physician. Patients may also be asked to arrive for the test with clean hair free of spray or other styling products.
Patients 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.


If the patient has suspended regular medication for the test, the EEG nurse or technician should advise him when he can begin taking it again.


Being off medication for one-two days may trigger seizures. Certain procedures used during EEG may trigger seizures in patients 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 suffering from), this may be a desired effect, although the patient 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 brainwave patterns, a neurologist or other physician will evaluate the type of brainwaves and the symmetry, location, and consistency of brainwave patterns. He will also look at the brainwave response to certain stimuli presented during the EEG test (such as flashing lights or noise). There are four basic types of brainwaves: alpha, beta, theta, and delta. "Normal" brainwave patterns vary widely, depending on factors of age and activity. For example, awake and relaxed individuals typically register an alpha wave pattern of eight to 13 cycles per second. Young children and sleeping adults may have a delta wave pattern of under four cycles per second.

Abnormal results

The EEG readings of patients 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, multifocal epilepsy may be present.
The diagnostic brainwave patterns of other disorders varies widely. The appearance of excess theta waves (four to eight cycles per second) may indicate brain injury. Brain wave patterns in patients with brain disease, mental retardation, and brain injury show overall slowing. A trained medical specialist should interpret EEG results in the context of the patient's medical history, and other pertinent medical test results.



Restak, Richard M. Brainscapes: An Introduction to What Neuroscience Has Learned About the Structure, Function, and Abilities of the Brain. NewYork: Hyperion, 1995.

Key terms

Epilepsy — A neurological disorder characterized by recurrent seizures with or without a loss of consciousness.
Ictal EEG — Used to measure brain activity during a seizure. May be useful in learning more about patients who aren't responding to conventional treatments.
Gale Encyclopedia of Medicine. Copyright 2008 The Gale Group, Inc. All rights reserved.


the recording of changes in electric potentials in various areas of the brain by means of electrodes placed on the scalp, on the brain surface, or within the brain itself, and connected to a vacuum tube radio amplifier, which amplifies the impulses more than a million times. The impulses are of sufficient magnitude to move an electromagnetic pen that records the brain waves. adj., adj electroencephalograph´ic.

The rate, height, and length of the waves vary in different parts of the brain, and each individual has a unique and characteristic pattern. Age and degree of consciousness also cause the wave patterns to differ. Most of the recorded waves in a normal adult's EEG are the occipital alpha waves, which are best obtained from the back of the head (occipital region) when the subject is resting quietly, but not asleep, with the eyes closed. These waves are blocked by excitement or by opening the eyes.

The beta waves, obtained from the central and front parts of the head, are more closely related to the sensory-motor parts of the brain. These waves are blocked in the same way as are alpha waves, by opening the eyes. In a normal EEG the frequencies are predominately within the range of alpha and beta rhythms at the rate of 8 to 30 hertz (cycles per second). During sleep the brain cells generate higher voltage electrical waves, but the rhythm is slowed down to 2 or 3 hertz, sometimes with short “sleep” spindles of about 15 hertz. One should use the word “normal” with caution when speaking of EEG readings. Some persons with mild deviations from normal may have no evidence of cerebral disease, while others with readings within normal ranges may be suffering from a serious disorder.

Irregular slow waves of 2 to 3 hertz, called delta waves or the delta pattern, are normally found in deep sleep and in infants and young children, but they indicate an abnormality in the awake adult. Rhythmic slow waves of 4 to 7 hertz are called theta waves. “Electrical silence,” or no evidence of brain activity, when demonstrated once and then again in 24 hours, has been taken as one of the criteria of death.

Electroencephalography is widely used in studying brain function and in tracing the connections between the parts of the central nervous system. It is particularly valuable in diagnosing epilepsy, brain tumor, and other diseases of and injury to the brain.
Patient Care. The electroencephalograph is an extremely sensitive instrument, and readings can be greatly influenced by the actions and physiologic status of the subject. It is apparent, then, that the cooperation of the patient is needed, and that the patient should be properly prepared physically and psychologically in order to obtain an accurate and useful record of brain activity. Patients are more likely to be cooperative if they have had adequate preparation in the purpose of the test, how the procedure will be carried out, and what will be expected of them during the testing. Their fears about electricity and how it will be used must be allayed. They should know that the electrodes lead minute amounts of electrical charge from the body and that there is no danger of electric shock and no relationship of this procedure to electroshock therapy. In most instances the test is painless because the electrodes are attached to the scalp with collodion. If needle electrodes are to be used, however, patients should be told there will be mild discomfort because the needles are extremely small.ƒ

A sleep recording usually is taken when a seizure disorder is suspected. The patient will be expected to go to sleep during the test. Some EEG technicians encourage the patient to stay up later than usual the evening before the test and to awaken early in order to be more likely to fall asleep during testing. Medications to produce sleep are given only as a last resort because these drugs alter brain wave patterns. Infants and small children are not allowed to nap before the test.

Other aspects of physical preparation include withholding all anticonvulsants, tranquilizers, and stimulants for at least 24 to 48 hours prior to testing. This includes coffee, tea, cola drinks, and alcohol. Hypoglycemia affects the brain wave patterns and so the patient is told not to skip any meals.

At the beginning of the test a baseline EEG reading is obtained by having the patient lie quietly with the eyes closed in a dimly lit room. The patient is cautioned to avoid movement of the eyelids, mouth, or tongue because these activities can be particularly disruptive. Provocative or “stressing” techniques are sometimes used during the EEG testing. These are particularly useful in the diagnosis of epilepsy because they can evoke seizure potentials on the EEG. The two techniques most often used are hyperventilation and “photic” stimulation, which employs flickering lights to stimulate the brain. When these or any other techniques are anticipated, patients should be informed so they will not become unduly apprehensive before and during the testing.
Miller-Keane Encyclopedia and Dictionary of Medicine, Nursing, and Allied Health, Seventh Edition. © 2003 by Saunders, an imprint of Elsevier, Inc. All rights reserved.

e·lec·tro·en·ceph·a·log·ra·phy (EEG),

Registration of the electrical potentials recorded by an electroencephalograph.
Farlex Partner Medical Dictionary © Farlex 2012


Neurology A technique in which the brain's electrical activity is measured by electrodes, and sent to a device that records impulses–brain waves on a continuous feed sheet of paper; EEG is used to evaluate seizure disorders, diminished consciousness, intracranial masses or vascular lesions. See Sleep encephalogram.
McGraw-Hill Concise Dictionary of Modern Medicine. © 2002 by The McGraw-Hill Companies, Inc.


(EEG) (ĕ-lek'trō-en-sef'ă-log'ră-fē)
Registration of the electrical potentials recorded by an electroencephalograph.
Medical Dictionary for the Health Professions and Nursing © Farlex 2012


The process of making a multiple tracing, by voltmeter-operated pens, of the electrical activity of the brain. The multiple readings are of the constantly varying voltage differences occurring between pairs of points on the scalp of the subject. The electroencephalograph (EEG) is affected by sleep, HYPERVENTILATION, drugs, concussion, brain injury, brain tumours, bleeding within the brain (cerebral haemorrhage), brain inflammation (ENCEPHALITIS), EPILEPSY and various psychiatric conditions. It also assists in the determination of brain death.
Collins Dictionary of Medicine © Robert M. Youngson 2004, 2005


(EEG) (ĕ-lek'trō-en-sef'ă-log'ră-fē)
Registration of electrical potentials recorded by an electroencephalograph.
Medical Dictionary for the Dental Professions © Farlex 2012
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