What is electroencephalography (EEG)?
Clinical Electroencephalography (EEG) uses from eight to sixteen pairs of electrodes called derivations. The “international 10-20" system of electrode placement provides coverage of the scalp at standard locations denoted by the letters F (frontal), C (central), P (parietal), T (temporal), and O (occipital). Subscripts of odd for left-sided placement, even for right-sided placements, and z for midline placement further define electrode location. During the procedure, the patient remains quiet, with eyes closed, and refrains from talking or moving. In some circumstances, however, prescribed activities such as hyperventilation may be requested. An EEG test is used to diagnose seizure disorders, brain-stem disorders, focal lesions, and impaired consciousness.
Electrical potentials caused by normal brain activity have atypical amplitudes of 30 to 100 millivolts and irregular, wavelike variations in time. The main generators of the EEG are probably postsynaptic potentials, with the largest contribution arising from pyramidal cells in the third cortical layer. The ongoing rhythms on an EEG background recording are classified according to the frequencies that they produce as delta (less than 3.5 hertz), theta (4.0 to 7.5 hertz), alpha (8.0 to 13.0 hertz), and beta (greater than 13.5 hertz). In awake but relaxed normal adults, the background consists primarily of alpha activity in occipital and parietal areas and beta activity in central and frontal areas. Variations in this activity can occur as a function of behavioral state and aging. Alpha waves disappear during sleep and are replaced by synchronous beta waves of higher frequency but lower voltage. Theta waves can occur during emotional stress, particularly during extreme disappointment and frustration. Delta waves occur in deep sleep and infancy and with serious organic brain disease.
During neurosurgery, electrodes can be applied directly to the surface of the brain (intracranial EEG) or placed within brain tissue (depth EEG) to detect lesions or tumors. Electrical activity of the cerebrum is detected through the skull in the same way that the electrical activity originating in the heart is detected by an electrocardiogram (ECG or EKG) through the chest wall. The amplitude of the EEG, however, is much smaller than that of the ECG because the EEG is generated by cells that are not synchronously activated and are not geometrically aligned, whereas the ECG is generated by cells that are synchronously activated and aligned. Variations in brain wave activity correlate with neurological conditions such as epilepsy, abnormal psychopathological states, and level of consciousness such as during different stages of sleep.
The two general categories of EEG abnormalities are alterations in background activity and paroxysmal activity. An EEG background with global abnormalities indicates diffuse brain dysfunction associated with developmental delay, metabolic disturbances, infections, and degenerative diseases. EEG background abnormalities are generally not specific enough to establish a diagnosis—for example, the “burst-suppression” pattern may indicate severe anoxic brain injury as well as a coma induced by barbiturates. Some disorders do have characteristic EEG features: An excess of beta activity suggests intoxication, whereas triphasic slow waves are typical of metabolic encephalopathies, particularly as a result of hepatic or renal dysfunction. Psychiatric illness is generally not associated with prominent EEG changes. Therefore, a normal EEG helps to distinguish psychogenic unresponsiveness from neurologic disease. EEG silence is an adjunctive test in the determination of brain death, but it is not a definitive one because it may be produced by reversible conditions such as hypothermia. Focal or lateralized EEG abnormalities in the background imply similarly localized disturbances in brain function and thus suggest the presence of lesions.
Paroxysmal EEG activity consisting of spikes and sharp waves reflects the pathologic synchronization of neurons. The location and character of paroxysmal activity in epileptic patients help clarify the disorder, guide rational anticonvulsant therapy, and assist in determining a prognosis. The diagnostic value of an EEG is often enhanced by activation procedures, such as hyperventilation, photic (light) stimulation, and prolonged ambulatory monitoring, or by using special recording sites, such as nasopharyngeal leads, anterior temporal leads, and surgically placed subdural and depth electrodes. During a seizure, paroxysmal EEG activity replaces normal background activity and becomes continuous and rhythmic. In partial seizures , paroxysmal activity begins in one brain region and spreads to uninvolved regions.
One of the most important uses of EEGs has been to diagnose certain types of epilepsy and to pinpoint the area in the brain causing the disturbance. Epilepsy is characterized by uncontrollable excessive activity in either all or part of the central nervous system and is classified into three types: grand mal epilepsy, petit mal epilepsy, and focal epilepsy. Additionally, EEGs are often used to localize tumors or other space-occupying lesions in the brain. Such abnormalities may be so large as to cause a complete or partial block in electrical activity in a certain portion of the cerebral cortex, resulting in reduced voltage. More frequently, however, a tumor compresses the surrounding nervous tissue and thereby causes abnormal electrical excitation in these areas.
Some researchers predict new uses of EEG technology in the future, although many of these applications appear dubious. Attempts to interpret thought patterns so that an EEG could serve as a lie detector or measurement of intellectual ability, for example, have proven unsuccessful.
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