CLINICAL STUDIES 1064 | VOLUME 66 | NUMBER 6 | JUNE 2010 www.neurosurgery-online.com Julian Prell, MD Department of Neurosurgery, University of Halle, Halle, Germany Jens Rachinger, MD Department of Neurosurgery, University of Halle, Halle, Germany Christian Scheller, MD Department of Neurosurgery, University of Halle, Halle, Germany Alex Alfieri, MD Department of Neurosurgery, University of Halle, Halle, Germany Christian Strauss, MD Department of Neurosurgery, University of Halle, Halle, Germany Stefan Rampp, MD Department of Neurosurgery, University of Halle, Halle, Germany Reprint requests: Julian Prell, MD, Department of Neurosurgery, University of Halle, Ernst-Grube-Str 40, 06097 Halle, Germany. E-mail: julian.prell@medizin.uni-halle.de Received, February 3, 2009. Accepted, December 5, 2009. Copyright © 2010 by the Congress of Neurological Surgeons R elevant facial nerve paresis is seen frequently in the immediate postoperative period, especially in patients with large vestibular schwannomas. The cosmetic and social implica- tions are of critical importance for the patients. 1-4 Consequently, the prevention of facial paresis is one of the main goals in vestibular schwannoma surgery. Although anatomic integrity of the nerve is preserved in most cases, 2,5-7 functional impair- ment ranging from slight postoperative paresis to total paralysis is still seen in up to 70% of all patients. 1,3,8-11 As a consequence, neurosurgeons have always desired reliable methods capable of foreseeing paresis and ultimately its extent. A multitude of techniques for intraoperative facial nerve monitoring have been developed to reach this goal. 12-21 Apart from methods that are based on stimulation techniques, providing informa- tion about the functional status of the facial nerve at distinct and discrete points of time during sur- gery, the free-running electromyogram (EMG) as introduced by Prass and Lüders 17 in 1986 has been the technique of choice for >2 decades. Visual analysis of all recorded EMG activity 20 has allowed differentiation between various pat- terns of prolonged, surgery-related activity com- monly referred to as “neurotonic discharges.” 22 One of these patterns, the A-train, has been shown to be pathognomonic for postoperative facial paresis. 2,20,21,23,24 The A-train is a pattern composed of similar elements with high interpeak frequency. Additional characteristics are sudden onset and sudden termination. Its duration varies between milliseconds and several seconds. The normal amplitudes of A-trains are comparatively low, usually presenting with 100 to 200 μV (Figure 1). Subsequently, an algorithm for automated detection and quantification of A-trains by a com- puter program has been developed. With this algorithm, all episodes of the facial nerve EMG ABBREVIATIONS: EMG, electromyogram; HB, House and Brackmann A Real-Time Monitoring System for the Facial Nerve OBJECTIVE: Damage to the facial nerve during surgery in the cerebellopontine angle is indi- cated by A-trains, a specific electromyogram pattern. These A-trains can be quantified by the parameter “traintime,” which is reliably correlated with postoperative functional out- come. The system presented was designed to monitor traintime in real-time. METHODS: A dedicated hardware and software platform for automated continuous analy- sis of the intraoperative facial nerve electromyogram was specifically designed. The auto- matic detection of A-trains is performed by a software algorithm for real-time analysis of nonstationary biosignals. The system was evaluated in a series of 30 patients operated on for vestibular schwannoma. RESULTS: A-trains can be detected and measured automatically by the described method for real-time analysis. Traintime is monitored continuously via a graphic display and is shown as an absolute numeric value during the operation. It is an expression of overall, cumulated length of A-trains in a given channel; a high correlation between traintime as measured by real-time analysis and functional outcome immediately after the operation (Spearman correlation coefficient [ρ] = 0.664, P < .001) and in long-term outcome (ρ = 0.631, P < .001) was observed. CONCLUSION: Automated real-time analysis of the intraoperative facial nerve electromyo- gram is the first technique capable of reliable continuous real-time monitoring. It can crit- ically contribute to the estimation of functional outcome during the course of the operative procedure. KEY WORDS: A-train, Electromyogram, Facial nerve, Monitoring, Real-time, Vestibular schwannoma Neurosurgery 66:1064-1073, 2010 DOI: 10.1227/01.NEU.0000369605.79765.3E www.neurosurgery-online.com Downloaded from https://academic.oup.com/neurosurgery/article-abstract/66/6/1064/2556787 by guest on 12 June 2020