Electroencephalogram (EEG)

The history

The performance

The brain

Uses of the EEG

Resources

An electroencephalogram, usually abbreviated EEG and sometimes called Brain Wave Test, is a medical test that records electrical activity in the brain. During the test, the brain’s spontaneous electrical signals are traced onto paper or read onto a computer screen. The electroencephalograph is the machine that amplifies and records the electrical signals from the brain. The electroencephalogram is the actual information displayed on a computer screen or later printed out as a paper strip. The EEG changes with disease or brain disorder, such as epilepsy, so it can be a useful diagnostic tool, but usually must be accompanied by other diagnostic tests to be definitive. EEGs are also useful in diagnosing brain tumors, strokes, brain damage caused through head trauma, and other neurological conditions characterized by distinctive, abnormal brain wave patterns. They are also used in investigating psychiatric disorders such as schizophrenia, and in determining brain death—an important process for use in conjunction with organ donation for surgical transplant recipients.

The history

The founder of electroencephalography was Hans Berger (1873–1941), a German psychiatrist who made the first human EEG in 1924. Interested primarily in psychophysiology—that is, the relationship between the mind and the brain—Berger set about measuring the brain’s electrical activity in the hope that a physiological record of this kind would provide insight into mental processes. He found inspiration for his work in the electrocardiograph (ECG) invented by Dutch physiologist Willem Einthoven (1860–1927) in 1900, and in work done earlier on the brain waves of animals.

In 1875, Richard Caton (1842–1926), an English physiologist and surgeon, measured electrical activity in the exposed brains of rabbits and monkeys. Unfortunately, Caton had been unable to make a graphic recording; the first recording of this kind was made in 1913 by Russian scientist Vladimir Vladimirovich Pravdich-Neminskii (1879–1952), who used the Einthoven string galvanometer to record from the intact skulls of dogs. Using a galvanometer much like the one Pravdich-Neminskii had used a decade earlier; Berger began his search for the human EEG by experimenting with the exposed brains of dogs. He then started placing needle electrodes under the scalp of patients who had lost some of their skull bones in surgery. It was while working with one of these patients—a seventeen-year-old youth who had been operated on because of a suspected brain tumor—that Berger recorded the first human EEG in 1924. He was initially uncertain whether the electrical oscillations he recorded originated in the brain. It was not until after conducting many other experiments—including experiments on the intact skulls of healthy people and of people with brain disorders—that he published his first paper on the human electroencephalogram in 1929.

The initial reaction of other scientists to Berger’s work was one of disbelief; like Berger himself, the scientific world at first doubted whether the workings of an organ as complex as the brain could be recorded through the skull. Berger did not achieve an international reputation until 1934, when Edgar Douglas Adrian (1889–1977), a renowned English neurophysiologist, confirmed his findings. Even then, however, Berger remained unappreciated in his own country. In the late 1930s, the German Nazis forced him to retire from the University of Jena, where he had been professor and director of psychiatry since 1919. With his laboratory dismantled and no facilities to carry on his work, Berger fell into a depression and committed suicide in 1941.

Despite his reputation as a reserved and inflexible man, Berger would no doubt be pleased to know that, over the years, research scientists have used the EEG to identify the parts of the brain involved in the mental processes of reasoning, memory, and feeling. In addition, Berger would be interested in a system that has simplified EEG interpretation. Known as BEAM (brain electrical activity mapping), this system was invented by Frank Duffy of the Harvard Medical School in the early 1980s. It uses computer technology to combine the signals from the individual electrodes into an overall, color-coded map of the brain’s electrical activity.

BEAM can store large amounts of EEG data, compare healthy profiles with abnormal ones, and provide detailed analyses that have been used to accurately diagnose such conditions as dyslexia and schizophrenia, which are usually difficult to detect. Efforts are currently underway to use BEAM in matching EEG patterns to specific brain functions. For example, research scientists at Johns Hopkins University have used BEAM to map the electrical activity involved in the movement of a monkey’s arm; their studies have shown that when the monkey anticipates moving its arm, the pattern of electrical activity in its brain changes. If efforts like these are successful, it may one day be possible to use computers and the electrical activity of the brain not only to control artificial limbs but in many other revolutionary applications as well.

The performance

To perform an EEG, electrodes, which are wires designed to detect electrical signals, are placed on the cranium either by inserting a needle into the scalp or by attaching the wire with a special adhesive. The electrodes are placed in pairs so that the difference in electric potential between them can be measured. The wires are connected to the electroencephalograph, where the signal is amplified and directed into pens that record the waves on a moving paper chart. The tracing appears as a series of peaks and troughs drawn as lines by the recording pens.

Basic alpha waves, which originate in the cortex, can be recorded if the subject closes his eyes and puts his brain “at rest” as much as possible. Of course, the brain is never still, so some brain activity is going on and is recorded in waves of about six to 12 per second, with an average of about 10 per second. The voltage of these waves is from five to 100 microvolts. A microvolt is one-one millionth of a volt. Thus, a considerable amount of amplification is required to raise the voltage to a discernable level.

The rate of the waves, that is, the number that occur per second, appears to be a better diagnostic indicator than does the amplitude, or strength. Changes in the rate indicating a slowing or speeding up are significant, and unconsciousness occurs at either extreme. Sleep, stupor, and deep anesthesia are associated with slow waves and grand mal seizures cause an elevated rate of brain waves. The only time the EEG line is straight and without any wave indication is at death. A person who is brain dead has a straight, flat EEG line.

The rates of alpha waves are intermediate compared with other waves recorded on the EEG. Faster waves, 14 to 50 waves per second, are lower in voltage than alpha waves are called beta waves. Very slow waves, averaging 0.5–5 per second, are delta waves. The slowest brain waves are associated with an area of localized brain damage such as may occur from a stroke or blow on the head.

The individual at rest and generating a fairly steady pattern of alpha waves can be distracted by a sound or touch. The alpha waves then flatten somewhat, that is their voltage is less and their pattern becomes more irregular when the individual’s attention is focused. Any difficult mental effort such as multiplying two four-digit numbers will decrease the amplitude of the waves, and any pronounced emotional excitement will flatten the pattern. The brain wave pattern will change to one of very slow waves, about three per second, in deep sleep.

Though the basic EEG pattern remains a standard one from person to person, each individual has his own unique EEG pattern. The same individual given two separate EEG tests weeks or months apart will generate the same alpha wave pattern, assuming the same conditions of the tests. Identical twins will both have the same pattern. One twin will virtually match the second twin to the extent that the two tracings appear to be from the same individual on two separate occasions.

Though the EEG is a useful diagnostic tool, its use in brain research is limited. The electrodes detect the activity of only a few neurons in the cortex out of the billions that are present. Electrode placement is standardized so the EEG can be interpreted by any trained neurologist. Also, the electrical activity being measured is from the surface of the cortex and not from the deeper areas of the brain.

The brain

The brain is the center of all human thought, feeling, emotion, movement, and touch, among other facilities. It consists of the prominent cerebrum, the cerebellum, and the medulla oblongata. The cerebral cortex, or outer layer, has specialized areas for sight, hearing, touch, smell, taste, and so on.

The basic cell of the brain is the neuron, which monitors information coming in to it and directs an appropriate response to a muscle or to another neuron. Each neuron is connected to other neurons through axons, which carry information away from a neuron, and dendrites, which carry information to the neuron. Thus, an axon from one neuron will end at a dendrite of another. The very tiny space between the two nerve endings is a synapse. The message is passed across the synapse by the release of certain chemical messengers, from the axon that cross the space and occupy receptor areas in the dendrite. These chemicals are called neuro-transmitters. Thus neurons are in constant electrical contact with other neurons, receiving and passing on information at the rate of billions of reactions a second.

Neuronal connections are established early in life and remain intact throughout one’s lifetime. An interruption of those connections because of a stroke or accident results in their permanent loss. Sometimes, with great effort, alternative pathways or connections can be established to restore function to that area, but the original connection will remain lost.

Uses of the EEG

The electroencephalogram is a means to assess the degree of damage to the brain in cases of trauma, or to measure the potential for seizure activity. It is used also in sleep studies to determine whether an individual has a sleep disorder and to study brain wave patterns during dreaming or upon sudden awakening.

The EEG is also a useful second-level diagnostic tool to follow-up a computerized tomogram (CT) scan to assist in finding the exact location of a damaged area in the brain. The EEG is one of a battery of brain tests available and is seldom used alone to make a

KEY TERMS

Electrode— A wire with a special terminal on it to attach to a part of the body to measure certain signals or transmit stimuli.

diagnosis. The EEG tracing can detect an abnormality but cannot distinguish between, for example, a tumor and a thrombosis (site of deposit of a blood clot in an artery).

Although persons with frequent seizures are more likely to have an abnormal EEG than are those who have infrequent seizures, EEGs cannot be solely used to diagnose epilepsy. Approximately 10% of epilepsy patients will have a normal EEG. A normal EEG, therefore, does not eliminate brain damage or seizure potential, nor does an abnormal tracing indicate that a person has epilepsy. Something as simple as visual stimulation or rapid breathing (hyperventilation) may initiate abnormal electrical patterns in some patients.

If the EEG is taken at the time the patient has a seizure, the pattern will change. A grand mal seizure will result in sharp spikes of higher voltage and greater frequency (25 to 30 per second). A petit mal seizure also is accompanied by sharp spikes, but at a rate of only three waves per second.

Also, the EEG is not diagnostic of mental illness. The individual who is diagnosed with schizophrenia or paranoia may have an EEG tracing interpreted as normal. Most mental illness is considered to be a chemical imbalance of some sort, which does not create abnormal electrical activity. However, an EEG may be taken of an individual who exhibits bizarre, abnormal behavior to rule out an organic source such as thrombosis as the cause.

Patients being diagnosed for a brain disorder can be monitored on a 24-hour basis by a portable EEG unit. A special cap with electrodes is fitted onto the head where it will remain during the time the test is being run. The electroencephalograph is worn on the belt. A special attachment on the machine enables the patient to communicate with the physician and transmit the data the machine has accumulated.

Resources

BOOKS

Germann, William J. Principles of Human Physiology. San Francisco, CA: Pearson Benjamin Cummings, 2005.

Mader, Sylvia S. Understanding Human Anatomy and Physiology. Boston, MA: McGraw-Hill Higher Education, 2005.

Niedermeyer, Ernst. Electroencephalography: Basic Principles, Clinical Applications, and Related Fields. Philadelphia, PA: Lippincott William & Wilkins, 2005.

Nunez, Paul L. Electric Fields of the Brain: The Neurophysics of EEG. Oxford, UK, and New York: Oxford University Press, 2006.

Randall, David J. Eckert Animal Physiology: Mechanisms and Adaptations. New York: W.H. Freeman & Company, 2002.

Van De Graaff, Kent M., and R. Ward Rhees, eds. Human Anatomy and Physiology: Based on Schaum’s Outline of Theory and Problems of Human Anatomy and Physiology. New York: McGraw-Hill, 2001.

Larry Blaser

Electroencephalography

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. The results of the test can distinguish psychiatric conditions such as schizophrenia, paranoia, and depression from degenerative mental disorders such as Alzheimer's and Parkinson's diseases. 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.

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 of 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 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, multifocal 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.

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 edition. 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.

harden, c. l., f. t. burgut, and a. m. kanner. "the diagnostic significance of video-eeg monitoring findings on pseudoseizure patients differs between neurologists and psychiatrists." epilepsia 44, no. 3 (2003): 453–456.

stepien, r. a. "testing for non-linearity in eeg signal of healthy subjects." acta experimental neurobiology 62, no. 4 (2002): 277–281.

vanhatalo, s., m. d. holmes, p. tallgren, j. voipio, k. kaila, and j. w. miller. "very slow eeg responses lateralize temporal lobe seizures: an evaluation of non-invasive dc-eeg." neurology 60, no. 7 (2003): 1098–1104.

organizations

american association of electrodiagnostic medicine. 421 first avenue sw, suite 300 east, rochester, mn 55902. (507) 288–0100, fax: (507) 288–1225. [email protected]. <http://www.aaem.net/>.

american board of registration for electroencephalographic technologists. p.o. box 916633, longwood, fl 32791-6633 .

american board of registration of eeg and ep technologists. po box 891663, longwood, fl 32791. (407) 788–6308. <http://www.abret.org/index.htm>.

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>.

joint review committee on electroneurodiagnostic technology. 3350 south 198th rd., goodson, mo 65659-9110. (417) 253–5810. <http://www.caahep.org/about/coas.htm>.

other

hofstra university. "electroencephalography." [cited april 27, 2003]. <http://people.hofstra.edu/faculty/sina_y_rabbany/engg181/eeg.html>.

bergey, gregory k., and piotr j. franaszczuk. "epileptic seizures are characterized by changing signal complexity." [cited april 17, 2003]. <http://erl.neuro.jhmi.edu/pfranasz/cn00/cn00.pdf>.

rutherford, kim, m.d. "eeg (electroencephalography)." kid's health for parents. june 2001 [cited april 27, 2003]. <http://kidshealth.org/parent/system/medical/eeg.html>.

national society for epilepsy. "epilepsy information: electroencephalography." september 2002 [cited april 27, 2003]. <http://www.epilepsynse.org.uk/pages/info/leaflets/eeg.cfm>.

L. Fleming Fallon, Jr., MD, DrPH

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.

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?

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

Electroencephalography

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. The results of the test can distinguish psychiatric conditions such as schizophrenia , paranoia, and depression from degenerative mental disorders such as Alzheimer's and Parkinson's diseases. An EEG may also be used to monitor brain activity during surgery to assess the effects of anesthesia. Additionally, it is used to determine brain status and brain death.

Precautions

There are few adverse conditions associated with an EEG test. Persons with seizure disorders may experience seizures during the test in reaction to flashing lights or by deep breathing.

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.

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 of 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).

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 or 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 of spray or other 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 tool 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 they are 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 state of consciousness. 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 an 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 a wakeful 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 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, multifocal 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 (an upward movement) 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.

Resources

BOOKS

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

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

Ebersole, J. S., and T. A. Pedley. Current Practice of Clinical Electroencephalography, 3rd Edition. 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.

Harden, C. L., F. T. Burgut, and A. M. Kanner. "The Diagnostic Significance of Video-EEG Monitoring Findings on Pseudoseizure Patients Differs between Neurologists and Psychiatrists." Epilepsia 44, no. 3 (2003): 453–456.

Stepien, R. A. "Testing for Non-linearity in EEG Signal of Healthy Subjects." Acta Experimental Neurobiology 62, no. 4 (2002): 277–281.

Vanhatalo, S., M. D. Holmes, P. Tallgren, J. Voipio, K. Kaila, and J. W. Miller. "Very Slow EEG Responses Lateralize Temporal Lobe Seizures: An Evaluation of Non-invasive DC-EEG." Neurology 60, no. 7 (2003): 1098–1104.

ORGANIZATIONS

American Association of Electrodiagnostic Medicine. 421 First Avenue SW, Suite 300 East, Rochester, MN 55902. (507) 288-0100; Fax: (507) 288-1225. [email protected]. <http://www.aaem.net/>.

American Board of Registration of EEG and EP Technologists. PO Box 891663, Longwood, FL 32791. (407) 788-6308. <http://www.abret.org/index.htm>.

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>.

Joint Review Committee on Electroneurodiagnostic Technology. 3350 South 198th Rd., Goodson, MO 65659-9110. (417) 253-5810. <http://www.caahep.org>.

OTHER

Electroencephalography. Hofstra University. April 27, 2003 (February 18, 2004). <http://people.hofstra.edu/faculty/sina_y_rabbany/>.

Bergey, Gregory K., and Piotr J. Franaszczuk. "Epileptic Seizures Are Characterized by Changing Signal Complexity." April 17, 2003 (February 18, 2004). <http://erl.neuro.jhmi.edu/pfranasz/CN00/cn00.pdf>.

Rutherford, Kim, M.D. "EEG (Electroencephalography)." Kid's Health For Parents. June 2001 (February 18, 2004). <http://kidshealth.org/parent/system/medical/eeg.html>.

Epilepsy Information: Electroencephalography. National Society for Epilepsy. September 2002 (February 18, 2004). <http://www.epilepsynse.org.uk/pages/info/leaflets/eeg.cfm>.

L. Fleming Fallon, Jr., MD, DrPH

Electroencephalography

Definition

Electroencephalography, or EEG, is a neurological test that involves attaching electrodes to the head of a patient 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. The results of the test can distinguish psychiatric conditions (schizophrenia, paranoia, depression) from degenerative mental disorders such as Alzheimer's and Parkinson's diseases. An EEG may also be used to monitor brain activity during surgery to assess the effects of anesthesia, and also to determine brain death.

Precautions

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.

Description

Before the EEG begins, a nurse or technologist attaches approximately 16 to 21 electrodes to the patient's scalp with a 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, the patient 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, 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 electroence-phalography unit makes a continuous graphic record of the patient's brain activity, or brainwaves, 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.

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, or up to eight or nine hours if it is a night sleep.

In an ambulatory EEG, patients 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, 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.

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 of 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 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).

Preparation

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). However, such requests should be cleared with the treating physician. Patients may be asked to avoid food and beverages that contain caffeine, a central nervous system stimulant. Patients may also be asked to arrive for the test with clean hair free of spray or other styling products to make attachment of the electrodes easier.

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.

Aftercare

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

Complications

Being off medication for one to 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 experiencing) 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.

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. Brainwave response to certain stimuli presented during the EEG test (such as flashing lights or noise) will also be evaluated.

The four basic types of brainwaves 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 posterior rhythm 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, the patient's age, and the patient's conscious state. Overall, pathology typically increases slow activity, such as theta or delta waves, but decreases fast activity, such as alpha and beta waves.

Not all decrease in wave activity is abnormal, however. The normal alpha waves seen in the posterior region of the brain are suppressed merely if the patient 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, that is, 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, 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 the brain. In adults, this intermittent activity is found in the frontal region while in children it is in the occipital region.

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 EEG can be used to localize the region of the brain where the abnormal electrical activity is occurring. This is most easily done 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 kind 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. In contrast, localized theta or delta rhythms found in conjunction with normal background rhythms suggest a structural lesion.

Health care team roles

Electroencephalograpy is often performed by specially trained electrodiagnostic technologists. Training for such a position can be on the job but often involves study at a one to two-year college or vocational program. A typical program would include:

• human anatomy and physiology
• neurology and neuroanatomy
• neurophysiology
• medical terminology
• computer technology and instrumentation

Certification of electrodiagnostic technologists specializing in electroencephalography and the related area of evoked potentials is available through the American Board of Registration of Electroencephalographic and Evoked Potential Technologists.

A physician such as neurologist, neurosurgeon, or internist does the final review and diagnosis based on the results of the EEG. The doctor can be present for the testing or may review saved tracings. Other health care professionals, such as nurses, aid in patient education concerning this procedure.

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); used to locate foci of epileptic seizures prior to epilepsy surgery.

Zygomatic arch— Cheekbone; a quadrilateral bone forming the prominence of the cheek; articulates with the frontal, sphenoid, and maxillary, and temporal bone.

Resources

BOOKS

Misulis, Karl E. "Electroencephalography Basics." In Essentials of Clinical Neurophysiology Boston: Butterworth-Heinemann. 2002.

U.S. Department of Labor, Bureau of Labor Statistics. "Electroneurodiagnostic Technologist" In Occupational Outlook Handbook 2000–01 Edition. Washington, DC: The Bureau. 2000.

PERIODICALS

Shpritz, D.W. "Neurodiagnostic Studies." Nursing Clinics of North America 34 (September 1999): 593-606.

Wallace, Brian, et al. "A History and Review of Quantitative Electroencephalograpy in Traumatic Brain Injury." Journal of Head Trauma Rehabilitation 16 (April 2001): 165.

ORGANIZATIONS

American Board of Registration of Electroencephalographic and Evoked Potential Technologists. P.O. Box 916633, Longwood, FL 32791-6633. (407) 788-6308. 〈http://www.abret.org〉.

OTHER

Duffy, Frank H. "Introduction to EEG and qEEG." Clinical Neurophysiology Laboratory. Boston Children's Hospital. 〈http://fhdno2.tch.harvard.edu/www/qeeg/qeegintro.html#EEG〉 (June 17, 2001).