Facial nerve monitoring parameters - prognostic value of the postoperative facial nerve outcomes after cerebello pontine angle surgery

Abstract

Introduction: Facial nerve (FN) paralysis is a frequently encountered complication in the surgical management of cerebello-pontine (CP) angle surgery. Its extent varies from barely visible to disfiguring paralysis, affecting the quality of the patient s life. Complete removal of the tumor with functional preservation of the FN remains the goal of the surgical procedure. The introduction of electromyographic monitoring of FN has improved the rate of FN preservation. We report the technique, outcome and complications seen in 5 cases of CP angle tumor surgery performed with intraoperative neurophysiologic monitoring of the FN function. Material and Methods: Five patients with CP angle tumors, including 4 vestibular schwannomas and one meningioma, were operated in our institution by retrosigmoid approach, during the period from December 2010 to April 2011. The ISIS intraoperative neuromonitoring system (Inomed, Germany) was used to perform the FN free running electromyography (EMG), triggered compound muscle action potentials (CMAP) and brainstem auditory evoked potentials (BAEP). Data was collected prospectively, and included the minimal stimulus intensity (mA), electromyographic response (mV), the proximal-todistal ratio of the stimulation threshold and the “A-train time” on free running facial EMG (sec). Facial nerve assessment was done by House&Brackmann grading system criteria before surgery, after the oper­ative procedure and after 2 weeks. All patients had a good FN function (grade I or II House-Brackmann) before surgery. Results: Four patients (80%) had a good FN function first day after surgery, expressed by HouseBrackmanngrade I or II. All these patients had a low stimulation threshold below 0,05 mA, a proximal to distal stimulation ratio equal to 1,0 and an A-train time below 5 seconds. One patient had a HouseBrackmann grade V FN function, although the nerve anatomical continuity was preserved during surgery, but with the increase of the stimulation threshold from 0,05 mA to 0,7 mA at the end of surgery, and a train time more than 5 seconds (6,8 sec). Conclusions: The intraoperative neuromonitoring of the FN allows a more efficient CP angle tumor removal with a good preservation of the FN function. Additionally, the direct nerve stimulation parameters and the overall train time on free running EMG can predict the FN outcome with useful accuracy.