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Three different types of electrodes are used to record the EMGs. These are: fine wire electrodes, which have the highest impedance and the narrowest field of view; subdermal needles, which have an intermediate impedance and a larger field of view; and disk surface electrodes, which have the lowest impedance and the largest field of view (by field of view is meant the integrated level of electrical activity). Our recording techniques are essentially the same for all cranial nerves and all muscle groups. Subdermal platinum needle electrodes are utilized in bipolar recording configurations; that is, all recordings are done between a pair of electrodes inserted into the same muscle group. There is one exception to the bipolar recording technique: we occasionally record transfacially between the orbicularis oculi and the mentalis muscles to reduce the number of channels allocated to monitoring CN VII. Bipolar recordings are used to minimize confusion regarding which cranial nerve or branch of a cranial nerve is producing the observed EMG. The electrodes are normally placed prior to the start of the procedure; however, occasionally electrodes are placed in a sterile field by the surgeons. The EMG electrodes are held in place with tape and benzoine. We favor the needle electrodes over the fine wire and disk electrodes, because of the signal characteristics that they provide and their ease of application and maintenance.
The amplifier bandpass is set from 10 to 1000 Hz, and a gain of 5000-20,000 is routinely used. The unstimulated EMG activity from up to eight channels is monitored continuously throughout the case. This ongoing activity is continuously monitored on an oscilloscope and periodic episodes of interesting activity may be saved into a computer file.
Most importantly, the sound from all channels of activity is monitored continuously. Our system has the capability of amplifying the activity on eight channels simultaneously and driving an audio system with this amplified signal. This system has gauges that measure the relative amounts of activity on each channel, allowing the channel with the most activity to be isolated and listened to by itself if so desired. In addition, the system has suppressor circuits, which suppress the artifactual sounds related to bipolaring and stimulating. The importance of the audio system in identifying the level of activity in the muscle groups cannot be stressed too much. These signals are listened to continuously for evaluation of nerve function both by the neurophysiologists and by the surgeons. Four categories of EMG activity are observed: (a) no activity, which in an intact nerve is the best situation, but which may also be the case in a nerve that has been sharply dissected; (b) irritation activity, which sounds like soft intermittent flutter and is consistent with working near the nerve; (c) injury activity, which sound like a continuous, nonaccelerating tapping and which can be an indicator of permanent injury to the cranial nerve; and (d) a "killed-end" response, which sounds like an accelerating firing pattern and is an unequivocal indicator of nerve injury. It is (Continued on page 115)
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