Similar instruments are used to insonate the basal vessels of the circle of Willis in adults (“transcranial Doppler”). The transcranial Doppler uses a 2-MHz pulsed signal that is able to pass through the calvarial bone and then receives a frequency-shifted signal from the blood flowing in the lumen of the basal vessels. This allows the detection of vascular stenoses and the greatly increased blood flow velocity caused by vasospasm from subarachnoid hemorrhage.
Ultrasound has several advantages, notably that it is noninvasive, harmless (hence can be used repeatedly but caution is required in applying it to the globe), convenient because of the portability of the instrument, and inexpensive. More specific applications of this technique are discussed in Chap. 37, on developmental diseases of the nervous system, and in Chap. 33, on stroke. The related technique of echocardiography has also assumed a central role in the evaluation of stroke, as indicated in Chap. 33.
The electroencephalographic examination, for many years a standard laboratory procedure in the study of all forms of cerebral disease, has been supplanted by CT and MRI for the purposes of localization of structural lesions. It continues to be an essential part of the assessment of patients with seizures and those suspected of having seizures, as well as in brain death, and for the study of sleep (polysomnography) as described in the American Electroencephalographic Society Guideliness. It is also used in evaluating the encephalopathy of many systemic metabolic diseases and in the operating room to monitor cerebral activity in anesthetized patients. For a few diseases, such as Creutzfeldt-Jakob (prion) disease, it is a useful confirmatory laboratory test. The technique is described here in some detail, as its general use in neurology cannot suitably be assigned to any other single chapter.
The electroencephalograph records spontaneous electrical activity generated in the cerebral cortex. This activity reflects the electrical currents that flow in the extracellular spaces of the brain that are the summated effects of innumerable excitatory and inhibitory synaptic potentials upon cortical neurons. This spontaneous activity of cortical neurons is highly influenced and synchronized by subcortical structures, particularly the thalamus and high brainstem reticular formation. Efferent impulses from these deep structures are probably responsible for entraining cortical neurons to produce characteristic rhythmic brain-wave patterns, such as alpha rhythm and sleep spindles (see further on).
Electrodes, which are typically silver or silver-silver chloride discs 0.5 cm in diameter, are placed on the scalp using a conductive medium. The electroencephalograph has 8 to 32 or more amplifying units capable of recording from many areas of the scalp at the same time. The amplified brain rhythms are seen as waveforms of brain activity in the frequency range of 0.5 to 30 Hz (cycles per second) on a standard display that runs at 3 cm/s. In the past, the amplified signals were recorded on paper by a bank of pens but now, a digital format of the rhythms can be displayed on a computer screen and stored electronically.
The favored configuration of electrode pairs, or montage, is the “International 10-20” system, which uses 10 electrodes on each side of the cranium and emphasizes contiguous regions of the brain for ease of visual inspection of the record (Fig. 2-7A).
The resulting electroencephalogram (EEG), essentially a voltage-versus-time graph, consists of a number of simultaneous parallel wavy lines, or “channels” (Fig. 2-7B). Each channel represents the difference in electrical potential between two electrodes (a common or ground electrode may be used as one recording site, but the channel still represents a bipolar recording). A positive voltage potential deflects the signal downward, and a negative one, upward by convention. The channels are arranged for viewing into standard montages that generally allow comparison of the activity from one region of the cerebral cortex to others, and particularly to the corresponding region of the opposite side.