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facial nerve can markedly decrease the time necessary for removal of particularly large tumors. A filter setting of 10-3000 Hz is suitable. The recorded potentials should he made audible by connecting the output of the amplifier to an audioamplifier and a speaker. It is useful to have as part of this system a circuit that silences the speaker when the stimulus artifact occurs.
The recording electrodes shown in Figure 7 will record not only the EMG activity from all facial muscles but also the EMG potentials from the masseter and temporalis muscles, which are innervated by the motor portion (portio minor) of the trigeminal nerve. Stimulation of the trigeminal nerve will therefore also give rise to recorded EMG potentials when the electrode configuration is used. When the recorded potentials are displayed on an oscilloscope in addition to being made audible, it is easy to distinguish between the potentials from stimulation of the facial nerve and those that originate from stimulation of the trigeminal nerve, because the latency of the EMG potentials that are generated by facial muscles is 5-6 ms whereas the EMG response from the muscles innervated by the trigeminal nerve is only 1.5-2 ms.
There are other reasons why it is useful to observe facial EMG responses on an oscilloscope. The amplitude (or, rather, the area) of the recorded EMG potentials when the facial nerve is stimulated supramaximally is a measure of how many nerve fibers are activated. A reduction in the amplitude of the EMG potentials indicates that there is a conduction block in part of the facial nerve. Should the amplitude of EMG potentials remain low, the patient is likely to have facial weakness postoperatively. Recording of facial EMG is also effective in detecting injuries to the facial nerve from surgical manipulations such as those that occur when tumor is scraped off the exposed facial nerve. If such manipulations injure the facial nerve, the nerve usually gives rise to characteristic and more-or-less sustained EMG. Caution should he exercised when electrical stimulation is used on nerves that innervate large muscles. Supramaximal stimulus strength may cause injury because it overactivates the feedback loop. During MVD operations for hemifacial spasm, it can be of benefit to monitor the abnormal muscle contraction that seems to be specific to patients with hemifacial spasm.
This abnormal (or delayed) muscle contraction can be elicited by electrical stimulation of a branch of the peripheral portion of the facial nerve while recording EMG potentials from muscles innervated by other branches of the facial nerve. When recorded from the mentalis muscle as the temporal or zygomatic branch of the facial nerve is stimulated electrically, this abnormal muscle contraction appears as brief EMG activity that may be followed by a long burst of irregular EMG activity. While the initial component of the response has a relatively constant latency of about 10msec . The use of intraoperative monitoring of the abnormal (or delayed) muscle response is based on the finding that the muscle response (Continued on page 119)
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