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(Continued from page 114)
important to note that a sharply cut nerve may produce only a brief burst of activity; thus monitoring cannot be expected to replace extreme caution when working near the cranial nerves. Besides these signals of interest, various electrical artifacts must be identified and ignored. These include high-frequency electrode pops, activity related to electrode manipulation, static from surgical instruments, and activity produced by irrigation.
In addition to monitoring the ongoing EMG activity related to the various cranial nerves, the various cranial nerves may also be electrically stimulated. This is usually done to determine the location of the nerve in the operative field, since many times the nerve is enveloped by tumor and may not be directly observable, or to determine the functional integrity of the nerve. The most common example of this procedure is the direct stimulation of CN VII. We use Grass 544 or 588 stimulators to control the stimulus rate, pulse duration, stimulus intensity, and switching of the sound system. A stimulus isolation transformer is utilized to drive a monopolar stimulating electrode. The stimulating electrode is a low impedance (1000 ohms) electrode, with the shaft insulated to, but not including, the tip. These devices are capable of producing currents as high as 150 mA, requiring care in their use. The return path for the stimulating current is provided by a metal electrode inserted into the adjacent muscle mass. The stimulus used is a constant voltage, with a pulse frequency of 10 Hz, and a pulse width of 100 msec. The voltage amplitude is typically varied between 0.1 and 1 volt. In some situations, where very precise localization of the nerve is required, bipolar stimulating electrodes are used. However, the great majority of the time, the question being asked is: Is the nerve there?
When CN VII is stimulated, usually all three branches are observed on the oscilloscope. The parameters typically measured for the stimulated VIIth nerve EMGs are the voltage threshold required to produce the evoked response of 0.3-0.5 volts, the latency to onset of 8-10 msec, and a peak-to-peak voltage of 1.0-2.0 mV. These parameters are measured at the beginning and end of the case, and whenever appropriate during the case. In addition, we have the capability to integrate the evoked EMG and use the area of the evoked EMG response as a criteria of change. For example, intraoperative recordings of the abnormal muscle response characteristic for patients with hemifacial spasm can directly aid the surgeon in achieving the therapeutic goal of a microvascular decompression (MVD) operation, i.e. to relieve the spasm. Techniques to monitor sensory cranial nerves have been most successful for monitoring the vestibulocochlear nerve (VIII) to reduce the incidence of impairment of hearing problems associated with presentation of efficient stimuli have so far made it impossible to make optimal use of monitoring the vestibular portion of the vestibulocochlear nerve or the optic nerve (II).
Intraoperative neurophysiologic monitoring of cranial motor nerves (Continued on page 116)
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