Nervus intermedius influences continuous facial nerve EMG-monitoring

Background:

In vestibular schwannoma surgery, facial nerve function may be monitored by quantification of A-trains, a specific pathological pattern, using the parameter traintime (1). However, false positive results with high traintime and comparatively mild postoperative functional deficits have been observed (2). This study tries to link this phenomenon to the presence of an anatomically and/or electrophysiologically identifiable Nervus intermedius (NI).

Methods:

In a consecutive group of 87 patients undergoing vestibular schwannoma surgery, the operative videotapes and electrophysiological data from intermittent electrical stimulation were analysed with respect to the presence/absence of a “split facial nerve”, in which the smaller portion represents the NI (3). The influence of this dichotomic criterion on facial nerve monitoring results, namely the correlation between traintime and postoperative facial nerve function, was scrutinized.

Results:

In the majority of patients (48 out of 87), the NI was identified successfully. In those cases in which the nerve reacted to electrical stimulation, EMG-responses were found mainly in the orbicular oris and the paranasal muscles. Statistical correlation between traintime and functional postoperative outcome was significantly better in the patient group without visible NI as compared to those patients in which the nerve was actually identified (Spearman's Rho 0.73 vs. 0.43). This inferior correlation resulted from additional pronounced A-train activity in EMG-channels covering the orbicular oris and paranasal muscles.

Discussion:

The NI contributes motor fibers to the facial muscles, targeting mainly the oricular oris and paranasal muscles (4,5). Its presence as an identifiable structure may lead to pronounced manipulation, which in turn seems to interfere with facial nerve monitoring, producing A-train activity without significant clinical correlate.

References:

[1] Prell J, Rampp S, Romstöck J, Fahlbusch R, Strauss C: Train time as a quantitative electromyographic parameter for facial nerve function in patients undergoing surgery for vestibular schwannoma. Journal of Neurosurgery 06/2007; 106(5):826 – 32.

[2] Prell J, Rachinger J, Scheller C, Alfieri A, Strauss C, Rampp S: A real-time monitoring system for the facial nerve. Neurosurgery 04/2010; 66(6):1064 – 73; discussion 1073.

[3] Strauss C, Prell J, Rampp S, Romstöck J: Split facial nerve course in vestibular schwannomas. Journal of Neurosurgery 12/2006; 105(5):698 – 705.

[4] Scheller C, Rachinger J, Prell J, et al. Schwannoma of the intermediate nerve. J Neurosurg 2008;109:144 – 48

[5] Ashram YA, Jackler RK, Pitts LH, et al. Intraoperative electrophysiologic identification of the nervus intermedius. Otol Neurotol 2005;26:274 – 79