Value of intraoperative monitoring of the trigeminal nerve in detection of a superiorly displaced facial nerve during surgery for large vestibular schwannomas

Abstract

ObjectiveTo investigate the role of trigeminal and facial nerve monitoring in the early identi cation of a superiorly displaced facial nerve. Patients and MethodsThis prospective study included 24 patients operated for removal of large vestibular schwannomas (VS). Electromyographic (EMG) events recorded after mapping the superior surface of the tumor were evaluated by analyzing the latencies of the responses from the masseter and facial nerve innervated muscles. ResultsThe latency of the recorded compound muscle action potential (CMAP) from the masseter muscle was 3.6 ±0.5 msec, and of the simultaneously recorded volume conducted responses from the frontalis, o.oculi, nasalis, o.oris and mentalis muscles were 4.6 ±0.9, 4.1 ±0.7, 3.9 ±0.4, 4.3 ±0.8 and 4.5 ±0.6 msec respectively after trigeminal nerve stimulation in 24 (100%) patients. In 6 (25%) patients, the mean latency of CMAP on the masseter was 3.6 ±0.5 msec, and the latencies of the CMAP from the frontalis, nasalis, o.oris and mentalis muscles were longer than those of the volume conduced responses (p=0.002; p=0.001; p< 0.001; and p=0.015 respectively) indicating stimulation of both nerves (trigemino-facial EMG response). All patients with this response were later con rmed anatomically to have an AS displaced facial nerve. ConclusionUnderstanding the trigemino-facial EMG response is of value in identifying an AS displaced facial nerve; in preventing electrophysiological confusion between the trigeminal and the facial nerves; and in detecting the presence of volume conducted contributions in the measured facial nerve CMAP at the end of surgery.

Impact of Latency Variations on the Predictive Value of Facial Nerve Proximal-to-Distal Amplitude Ratio during Vestibular Schwannoma Surgery

“…This relation may persist even after tumor excision as some strands of the FN may retain their superior position at the end of dissection due to the presence of adhesions. 20 It is worth noting, however, that not all patients identified intraoperatively as having an AS FN course belonged to the short latency group. A possible reason could be a change in the spatial relation between the facial and trigeminal nerves postresection in some patients, thus limiting the simultaneous stimulation of both nerves.…”

“…Such findings support the hypothesis that when the latency of the FN CMAP falls within the normal range, the calculated amplitudes are purely due to the stimulation of the FN, and therefore are more predictive of outcome. 20 However, a problem presented within the groups with latencies less than 6 ms, and more than 8 ms.…”

“…If the FN is displaced in an anterior and superior (AS) direction, bringing it near the trigeminal nerve, stimulation of the FN may result in simultaneous activation of the trigeminal nerve. 20 Consequently, the amplitude of the recorded CMAP from the facial muscles may have peripheral volume conducted contributions from the contracting trigeminal nerve innervated muscles, confounding the predictive ability of the amplitude ratio measurement. A shorter CMAP latency after proximal FN stimulation may be a clue to the existence of such peripheral volume conducted contributions.…”

“…A shorter CMAP latency after proximal FN stimulation may be a clue to the existence of such peripheral volume conducted contributions. 20 The normal latency of the recorded CMAP after stimulation of the FN near its root exit zone (REZ) has been reported to be 6 to 8 ms. 8,21,22 A shorter latency may point to the existence of associated peripheral volume conduction; while a longer latency may point to injury of some FN fibers, possibly the fast-conducting ones. 19,20 This study was designed to highlight the impact of CMAP latency variations on the predictive ability of P/D amplitude ratio measurements for a more accurate interpretation of this test.…”

An Artificial Neural Network Model for Predicting Postoperative Facial Nerve Outcomes After Vestibular Schwannoma Surgery

“…Shorter latencies are, in turn, associated with variability in the predictive ability of P/D. 9,25 Although in recent years, the use of multichannel FN monitoring became more common because of its ability to reliably detect free-run EMG activity, 19,27 many practices still rely on the o. oris and o. oculi muscles. Our results confirm the importance of the addition of more muscles, such as the mentalis, to standard practice and the substitution of the o. oculi with other muscles when assessing the postoperative outcome using the P/D.…”