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separately so that contractions of different muscle groups can be detected. However, since the purpose of recording facial EMG is to identify when any portion of the facial nerve has been stimulated, there is no obvious reason to discriminate between EMGs from different muscle groups. Recording separately from different muscle groups only adds complexity to the recording, so recording the EMG potentials from the entire face on only one channel is recommended. To do this, place one of two differential recording electrodes in the upper face and the other in the lower face.
Needle electrodes, such as subdermal platinum needles, are suitable for recording EMG potentials and are more practical to use in the operating room than surface electrodes. Needle electrodes can he placed in less time than can surface electrodes, and needle electrodes tend to stay in place better than do surface electrodes, when they are secured by a good quality adhesive tape.
A handheld, monopolar stimulating electrode that is electrically insulated except for about 1 mm at the tip is suitable for probing the tumor and surrounding tissue for the purpose of finding the facial nerve in operations for acoustic rumors. Bipolar electrodes are more specific stimulators of the nerve and may therefore help to identify the nerve more accurately, but since it is only the negative phase of the stimulation impulse that is effective, the orientation of a bipolar electrode is critical to its ability to stimulate a nerve. Checking for proper orientation of a bipolar stimulating electrode may thus add unnecessary complexity to detecting the facial nerve, which increases the risk that the nerve may be damaged because the surgeon cannot detect the nerve's presence.
Short (100-200 us) negative (cathodal) rectangular pulses are suitable for driving a monopolar stimulating electrode. Constant-voltage (or nearly constant-voltage) impulses, rather than the more often used constant-current impulses, are preferable for probing in the cerebellopontine angle because constant-voltage stimulators deliver almost the same electrical current to a nerve regardless of the degree of electrical shunting that occurs. Shunting of current is variable in the cerebellopontine angle because the amount of cerebrospinal fluid present at the location of stimulation varies. If the stimulating electrode is driven by a constant current instead of constant voltage, the amount of current that passes through tissues will he directly dependent on the degree of shunting, which is uncontrollable.
A monopolar electrode can also be used conveniently to probe a large tumor to identify areas of the tumor where no facial nerve is present, so such portions can be removed safely without risk of injuring the facial nerve. When electrical stimulation is used for that purpose, the current delivered must be as independent as possible of such factors as electrical shunting. Monopolar electrodes driven by nearly constant voltage, as were just described, have proven to be effective for that purpose. Using a nerve stimulator to identify portions of a tumor that can be removed without risk of injury to the (Continued on page 118)
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