In addition to using a patient's history, physical examination, and imaging results, the clinician can obtain information about the functional status of various parts of the nervous system by monitoring its electrical activity, via a variety of electrodiagnostic tests.
Electroencephalography provides a noninvasive method for studying the ongoing or spontaneous electrical activity of the brain. The potentials of the brain are recorded in an electroencephalogram (EEG); they appear as periodic waves, with frequencies ranging from 0.5 to 40 cycles per second (cps or hertz [Hz]) and with an amplitude that ranges from five to several hundred microvolts. Because the amplitude of cerebral electrical activity is much smaller than that obtained from the heart in an electrocardiogram (ECG), sensitive (but stable) amplification is necessary to produce an undistorted record of brain activity; this requires proper grounding and electrical shielding.
Electroencephalography can provide useful information in patients with structural disease of the brain, especially when seizures occur or are suspected. Electroencephalograms can be very useful in classifying seizure disorders, and because optimal drug therapy varies for different types of seizures, the EEG findings may have important implications for treatment. Electroencephalography is also useful in evaluating cerebral abnormalities in a number of systemic disorders and in performing workups on patients with sleep disorders.
Because computed tomography (CT) scanning and magnetic resonance imaging (MRI) have higher spatial resolution and can localize lesions in three dimensions, these imaging techniques are usually used in preference to EEG for the localization of destructive lesions in the brain. When other tests are not available, an EEG can furnish help in determining the area of cerebral damage. Electroencephalography has its limitations, however, and normal-appearing records can be obtained despite clinical evidence of severe organic brain disease. The use of depth electrography—the localization of a focus by recording from electrodes implanted within the brain—may be advisable in certain cases.
The activity recorded in the EEG originates mainly from the superficial layers of the cerebral cortex. Current is believed to flow between cortical cell dendrites and cell bodies. (The dendrites are oriented perpendicular to the cortical surface.) As a result of the synchronous activation of axodendritic synapses on many neurons, summed electrical currents flow through the extracellular space, creating the waves recorded as the EEG. The pattern of activation of cortical neurons, and thus the EEG, is modulated by inputs from the thalamus and reticular formation.
To detect changes in activity that may be of diagnostic importance, simultaneous recordings are obtained, when possible, from multiple areas on both the left and right sides of the brain. Electrodes are ordinarily attached to the scalp over the frontal, parietal, occipital, and temporal areas; they are also attached to the ears (Fig 23–1).