Among 24 patients who underwent surgical resection of VS tumors 13 were females (54.2%) and 11 were males (45.8%) . The age of the patients ranged from 26 to 65 years with a mean of 46.8 ± 12.7 years. The selected patients had tumor sizes ranging from 31 mm up to 58 mm with a mean of 39 ± 7 mm. Tumors were right sided in 12 patients (50%) and left sided in 12 patients (50%). Tumors were approached through the retrosigmoid (RS) approach in all patients (100%).
Intraoperative trigeminal and facial nerve monitoring
The trigeminal nerve was stimulated at the superior surface of the tumor in 24 (100%) patients with a current intensity between 0.2 and 0.4 mA. The mean latency of the recorded CMAP from the masseter muscle was 3.6 ± 0.5 msec, while the mean latencies of the simultaneously recorded responses from the frontalis, o.oculi, nasalis, o.oris and mentalis recording channels were 4.6 ± 0.9, 4.1 ± 0.7, 3.9 ± 0.4, 4.3 ± 0.8 and 4.5 ± 0.6 msec respectively. The latencies of the recorded responses from the various facial nerve monitoring channels were all less than 6 msec in duration and were therefore considered volume conducted responses associated with trigeminal nerve stimulation (Table 1, Figure 1-A). The frequency of the simultaneously recorded volume conducted responses associated with trigeminal nerve stimulation varied among facial nerve innervated muscles, and were observed on the frontalis, o.oculi, nasalis, o.oris and mentalis channels in 12 (50%), 17 (70.8%), 5 (20.8%), 7 (29.2%) and 4 (16.7%) patients respectively (Table 1).
Further mapping of the superior surface of the tumor with the same current intensity in 6 (25%) patients resulted in a different recorded pattern that consisted of a response recorded from the masseter muscle having a mean latency of 3.3 ± 0.3 msec which was not different from the previously response from the same muscle (p= 0.214) and was defined as a CMAP due to trigeminal nerve stimulation. In addition, there were simultaneously recorded responses from the facial nerve monitoring channels having mean latencies of 6.5 ± 1.3, 5.0 ± 1.5, 7.5 ± 1.3, 7.4 ± 0.6 and 7.0 ± 1.5 msec from the frontalis, o.oculi, nasalis, o.oris and mentalis muscles respectively. These responses were significantly longer in latency from the frontalis, nasalis, o.oris and mentalis muscles than the previously recorded volume conducted responses from the same muscles (p= 0.002; p= 0.001; p< 0.001; and p= 0.015 respectively) (Table 1). Nevertheless, the mean latency of the response from the o.oculi in this pattern was not statistically different from the volume conducted response recorded from the same muscle (p= 0.244). It is worth noting that the o.oculi muscle was observed to have the highest number of volume conduction responses when the trigeminal nerve was stimulated.
Detailed analysis of the recorded CMAPs recorded from the facial nerve innervated muscles in those 6 patients showed variability in latency measurement between muscles. Therefore, to confirm that the facial nerve was simultaneously stimulated with the trigeminal nerve and to define the resulting pattern, the number of muscles having a CMAP with a latency of 7 msec or more (confirming definite facial nerve stimulation) were assessed in each patient (Table 2: patients 1-6). When a response from the masseter muscle having a latency of less than 5 msec (due to trigeminal nerve stimulation), was simultaneously recorded with a response from at least one of the five facial nerve monitoring channels having a latency of 7 msec or more (due to facial nerve stimulation) the resulting pattern was named “trigemino-facial EMG response” (Figure 1-B).
During surgery the facial nerve course was anatomically identified as AC in 10 (41.7%) and AI in 6 (25%), AS in 8 (33.3%), patients. The facial nerve in all 6 patients showing a trigemino-facial EMG response was anatomically confirmed later during the course of surgery to be displaced in an AS direction. However, among the 8 patients displaying an AS course of the facial nerve anatomically, 2 patients did not show an electrophysiologic trigemino-facial EMG response (Table 3). Mapping of the superior surface of the tumor in those 2 patients resulted in CMAPs having latencies of 7 msec or more from facial nerve monitoring channels (4 out of 5 and 5 out of 5 facial muscles), and was not associated with a CMAP recorded from the masseter muscle, a pattern indicating direct facial nerve stimulation at the superior surface of the tumor (Table 2: patients 7&8) (Figure 1-C). The facial nerve was found anatomically to be displaced at a higher plane that separated it from the trigeminal nerve and was in closer proximity to the stimulator in those 2 patients.
The facial nerve was stimulated at the end of surgery at the root exit zone in all patients at 2 mA. The latency of the responses from the frontalis, o.oculi, nasalis and o.oris muscles were significantly shorter in the 6 patients who displayed a trigemino-facial EMG response compared to the latencies from the same muscles recorded from other patients (p=< 0.001; p= 0.001; p= 0.019; and p= 0.010 respectively). Results indicated that the CMAP measured at the end of surgery in these patients is not purely due to facial nerve stimulation but has volume conducted contributions. In addition, the latency of the responses from the mentalis muscle were shorter in the patients displaying the same pattern than the rest of the patients however, results did not reach statistical significance (p= 0.171) (Table 4). It is worth noting that the mentalis muscle had the least number of volume conducted responses when the trigeminal nerve was stimulated.
Surgical results and relation to postoperative outcome
The degree of adhesion between the tumor capsule and the facial nerve just medial to the porus was described as weak in 4 (16.7%), intermediate in 9 (37.5%) and strong in 11(45.8%) patients . Gross total tumor removal was achieved in 18 (75%) patients; near total removal (a maximum 2 mm layer of tumor capsule was left behind at the site of maximal adhesion to the facial nerve ) was achieved in 4 (16.7%) patients; and a subtotal removal was performed in 2 (8.3%) patients . The relation between facial nerve course, the degree of adhesion and degree of tumor removal are shown in Table 5. The immediate postoperative facial nerve outcome was grade I & II in 12 (50%) patients; grade III & IV in 7 (29.2%) patients; and grade V & VI in 5 (20.8%) patients. The long-term postoperative outcome was grade I & II in 17 (70.8%) patients; grade III & IV in 7 (29.2%) patients; and none of the patients had grade V & VI long term postoperative outcome. The relation between facial nerve course, immediate and long term post-operative outcomes are shown in Table 5.