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(Continued from page 115)
has been particularly effective for the facial nerve (VII) and the nerves that control the extraocular muscles, oculomotor (III), trochlear (IV), and abducens (VI). For these nerves, monitoring has been shown to decrease the incidence of reduced or lost function as a complication of such operations, mainly by making it possible to identify these nerves during the operation and detect when surgical manipulations are causing injuries to these nerves. These nerves are located by the surgeon probing tissue with an electrical nerve stimulator while EMG responses are recorded from each of the extraocular muscles. Some institutions have monitored intraoperatively the part of the vagal nerve (X) that innervates the laryngeal muscles, and other institutions have monitored the motor portion of the glossopharyngeal nerve (IX). The hypoglossal nerve (XII) has been monitored by recording EMG responses from the tongue, and the spinal accessory nerve (XI) has been monitored by recording EMG potentials from the trapezoid muscles.
Auditory function is most often monitored for the purpose of preserving function of cranial nerve VIII, which is at risk in operations to remove acoustic tumors or other skull base tumors and in MVD operations to treat such cranial nerve dysfunctions as trigeminal neuralgia, hemifacial spasm, glossopharyngeal neuralgia, disabling positional vertigo, or tinnitus. In some cases in which the brain stem is manipulated it is of value to monitor brain stem auditory evoked potent,ials (BSAERs) because such monitoring has been found to detect changes in the function of the nuclei and fiber tracts of the ascending auditory pathway.
Function of the vestibulocochlear nerve can be monitored either by recording BAEPs in response to transient stimuli such as click sounds, or by recording the compound action potentials (CAPs) by placing the recording electrode directly on the exposed vestibulocochlear nerve or in its vicinity.
For many years facial muscle contractions were identified by simple visual monitoring of the face by an assistant. Then 10 to 15 years ago, various devices came into use for recording facial muscle contractions electrically or using mechanotransducers. Sugita and Kobayashi introduced the use of accelerometers placed on the face, and another device that makes use of other mechanical recordings of movements of the facial musculature was described by Silverstein Delgado were among the first to use recordings of EMG potentials to monitor facial muscle contractions intraoperatively. Methods that make it possible to listen to the recorded EMG without disturbance from the stimulus artifact in addition to having the EMG displayed on an oscilloscope have been described. Recording of EMG is now the most common method for detecting facial muscle contractions, and making the EMG audible, in addition to oscillographic display, is now widely practiced. Several models of equipment are commercially available that make use of this principle.
Electrodes for recording facial EMG can he placed in individual muscle groups and the potentials can be amplified and displayed (Continued on page 117)
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