Acoustic (loudspeaker) facial electromyographic monitoring: Part 1. Evoked electromyographic activity during acoustic neuroma resection.

A modification of the technique of acoustic facial electromyographic (EMG) monitoring, involving the use of a bipolar wire electrode, was used to monitor facial EMG activity during 13 consecutive unselected acoustic neuroma resections. EMG activity was synchronously recorded on the audio channels of operative video tapes so that the patterns of evoked EMG activity could be analyzed in relation to specific intraoperative events. Despite a relatively wide variety of apparent eliciting mechanisms, evoked EMG activity occurred in only three general acoustic patterns; these were bursts, trains, and pulses. These respective patterns are described in detail and related to specific etiological mechanisms. The possible clinical significance of various patterns of evoked EMG activity is discussed.

Waveform morphology and amplitude variability in facial-nerve electroneurography.

This study assesses the effect of using bipolar recording electrodes in the nasolabial crease versus using monopolar recording electrodes in the crease and on the nasal alae. Seven normal patients had three trials of facial-nerve electroneurography on three separate test days. Peak-to-peak amplitudes of combined motor action potentials were measured; amplitude variability was not affected by the measurement site. A biphasic waveform shape was most consistently obtained by using the nasal alae as the recording point rather than the nasolabial crease. We hypothesize that alar electrodes primarily detect combined motor action potential activity from relatively synchronous, fast-conducting facial-nerve fibers, while the combined motor action potential detected by electrodes in the nasolabial crease reflect a relatively complex summation of both fast and slow fibers.

Transtragal, transtympanic electrode placement for intraoperative electrocochleographic monitoring.

Transtragal, transtympanic electrode placement was performed for the purpose of intraoperative electrocochleographic (ECoG) monitoring during seven suboccipital acoustic neuroma resections. The promontory, the tragus, and two external sutures stabilized the electrode during surgery. The only noted otologic sequallae of electrode placement were small circular lesions at tympanic membrane puncture sites. Satisfactory ECoG recordings were obtained in five of seven cases. Peak-to-peak N1 amplitudes were (on average) 13.4-fold larger than the corresponding amplitudes of peak I of the surface-recorded brainstem auditory evoked potential (BAEP) recordings. During acoustic neuroma resection, ECoG and BAEP recordings changed relatively independently, which suggests multiple mechanisms and/or sites of injury to the cochlea or cochlear nerve. However, once they had become manifest, none of the observed changes exhibited a tendency to return to preoperative patterns. Hearing was preserved postoperatively in only two of seven patients, one of whom could not be monitored due to technical difficulty. Transtragal, transtympanic electrode placement provided a rapid, stable, and safe method of obtaining intraoperative ECoG recordings. Although combined intraoperative monitoring of ECoG and BAEP responses appeared to provide more precise documentation of injury to the cochlea and/or cochlear nerve, it was probably not influential in preservation of hearing in this series.

Improved preservation of facial nerve function in the infratemporal approach to the skull base.

Although the infratemporal approach described by Fisch provides excellent exposure of the jugular foramen, intrapetrous carotid artery, and lateral skull base, the anterior displacement of the seventh cranial nerve often results in temporary facial paralysis. The use of a modified technique for facial nerve mobilization resulted in significant improvement of both early and final facial function. Since that earlier report, continuous intraoperative electrical facial nerve monitoring has been used during the infratemporal approach in 20 additional cases. Immediate postoperative facial function was normal in 93% of the monitored cases and in 70% of the cases in the unmonitored group. More importantly, no patients in the monitored group developed grade V or VI weakness after surgery, whereas 48% of the unmonitored patients had grade V or VI weakness during the early postoperative period. This article will describe how intraoperative facial nerve monitoring is used during infratemporal surgery and will compare early facial function in 31 unmonitored patients with early facial function in 20 monitored procedures.

A model of waveform reliability in facial nerve electroneurography.

This study identifies four factors that should be considered when evaluating the reliability of new testing methods in facial nerve electroneurography (ENOG): (1) waveform peak-to-peak amplitude; (2) response variability; (3) statistical significance of day-to-day test/retest differences; and (4) waveform morphology. These factors were applied to data gathered on seven normal subjects who had three trials of facial-nerve ENOG on three separate test days in order to determine an optimal recording electrode montage. Both bipolar and monopolar recording-electrode montages were used on the nasal alae and on the nasolabial crease. Resultant peak-to-peak amplitudes of combined motor action potentials were recorded and measured using a clinical evoked-potential instrument. A biphasic waveform shape with optimal peak-to-peak point rather than the nasolabial crease. The contribution to the assessment of response reliability of each of the four factors is discussed.

Acoustic (loudspeaker) facial EMG monitoring: II. Use of evoked EMG activity during acoustic neuroma resection.

Facial electromyographic (EMG) activity was continuously monitored via loudspeaker during eleven translabyrinthine and nine suboccipital consecutive unselected acoustic neuroma resections. Ipsilateral facial EMG activity was synchronously recorded on the audio channels of operative videotapes, which were retrospectively reviewed in order to allow detailed evaluation of the potential benefit of various acoustic EMG patterns in the performance of specific aspects of acoustic neuroma resection. The use of evoked facial EMG activity was classified and described. Direct local mechanical (surgical) stimulation and direct electrical stimulation were of benefit in the localization and/or delineation of the facial nerve contour. Burst and train acoustic patterns of EMG activity appeared to indicate surgical trauma to the facial nerve that would not have been appreciated otherwise. Early results of postoperative facial function of monitored patients are presented, and the possible value of burst and train acoustic EMG activity patterns in the intraoperative assessment of facial nerve function is discussed. Acoustic facial EMG monitoring appears to provide a potentially powerful surgical tool for delineation of the facial nerve contour, the ongoing use of which may lead to continued improvement in facial nerve function preservation through modification of dissection strategy.

Iatrogenic facial nerve injury: the role of facial nerve monitoring.

Intraoperative facial nerve monitoring has emerged as a powerful tool for facilitation of surgery involving the facial nerve. This tool must be properly applied and maintained, however, in order to avoid untoward results and to maximize its potential benefits. Facial nerve monitoring is best suited for prevention of iatrogenic injury. Once injury has become established, its value may be severely limited, especially for moderate to severe injury, in which visual assessment of injury is still the most effective means to determine the need for repair. The most valuable derivative of facial nerve monitoring is enhanced awareness of the edge of the facial nerve contour. This allows strategic alterations in surgical technique for improved facial nerve functional preservation. Although facial nerve monitoring appears to have already had a favorable influence on facial nerve preservation, further improvement seems likely over time through an ongoing process of trial and error.