Abstract
The aim of this study was to investigate the role of trigeminal and facial nerve monitoring in the early identification of a superiorly (anterior and superior (AS)) displaced facial nerve. This prospective study included 24 patients operated for removal of large vestibular schwannomas (VS). The latencies of the electromyographic (EMG) events recorded from the trigeminal and facial nerve innervated muscles after mapping the superior surface of the tumor were analyzed. The mean latency of the recorded compound muscle action potential (CMAP) from the masseter muscle was 3.6 ± 0.5ms and of the peripherally transmitted responses by volume conduction from the frontalis, o. oculi, nasalis, o. oris, and mentalis muscles was 4.6 ± 0.9, 4.1 ± 0.7, 3.9 ± 0.4, 4.3 ± 0.8, and 4.5 ± 0.6ms, respectively, after trigeminal nerve stimulation in 24 (100%) patients (pattern I response). In 6 (25%) patients, the mean latency of CMAP on the masseter was 3.3 ± 0.3ms, and the latencies of the CMAP from the frontalis, o. oculi, nasalis, o. oris, and mentalis muscles were 6.5 ± 1.3, 5.0 ± 1.5, 7.5 ± 1.3, 7.4 ± 0.6, and 7.0 ± 1.5ms, respectively, longer than those of the peripherally transmitted responses (p = 0.002, p = 0.001, p < 0.001, and p = 0.015, respectively) indicating simultaneous stimulation of both nerves (pattern II response). All patients with this response were later confirmed to have an AS-displaced facial nerve. Recognizing the response resulting from simultaneous stimulation of both the facial and trigeminal nerves is important to help early identification of an AS-displaced facial nerve before it is visible in the surgical field and to avoid misleading information by confusing this pattern for a pure trigeminal nerve response.
Highlights
Understanding 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 compound muscle action potential (CMAP) at the end of surgery
Intraoperative monitoring of the trigeminal nerve during vestibular schwannoma (VS) surgery is relatively understated, as it is generally less likely to be injured during tumor resection than the facial nerve [10]
Stimulation of the facial nerve yields a response from the facial nerve monitoring channels, with the possibility of recording simultaneous volume conducted responses from the masseter and/or the temporalis muscles, because of the proximity of the contracting facial nerve innervated muscles to those innervated by the trigeminal nerve [17]
Summary
Intraoperative monitoring of the trigeminal nerve during vestibular schwannoma (VS) surgery is relatively understated, as it is generally less likely to be injured during tumor resection than the facial nerve [10]. Using a multichannel recording setup, stimulation of the motor axons of the trigeminal nerve produces a compound muscle action potential (CMAP) on the masseter and/or temporalis muscle and often simultaneously recorded volume conducted responses from one or more facial nerve monitoring channels. Such volume conducted responses are due to peripheral spread of activity from the contracting trigeminal nerve innervated, to the nearby facial nerve innervated muscles [23]. One exception to this rule occurs in some patients with large tumors and facial neve displacement
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