Importance of Low Amplitude Positive Facial Nerve Stimulation Following CP Angle Tumor Surgery

Introduction: Preservation of facial nerve function is crucial to a successful CP angle tumor resection. Intraoperative facial nerve monitoring is a routine component of CP angle surgery. We sought to correlate short-term and long-term facial nerve function with respect to the minimum amplitude of stimulation required to obtain facial nerve identification at the conclusion of tumor resection.

Methods: We performed a review of a prospectively maintained database of all patients who underwent CP angle tumor surgery between September 2007 and March 2011. The minimum amplitude necessary to achieve positive facial nerve stimulation was noted in all cases. Facial nerve function outcomes (based on the House-Brackmann grading scale) were noted at three different postoperative times: immediately after surgery (postoperative day 1), 1 month postoperatively, and 6 months postoperatively.

Results: Twenty-six CP angle tumor/mass resections were performed in our study time span (18 acoustic neuromas, 5 meningiomas, 1 ganglioglioma, 1 juvenile pilocytic astrocytoma, and 1 mucormycosis mass). Positive facial nerve stimulation was achieved in all cases at the conclusion of the tumor resection. The minimum threshold to achieve this final positive stimulation ranged from 0.1 to 1 mA (mean, 0.26 mA). Immediate postoperative facial function varied from HB 1 to HB 4 (mean, 1.81). One month postoperatively, facial function varied from HB 1 to HB 3 (mean, HB 1.45). Six-month postoperative facial function varied from HB 1 to HB 2 (mean, 1.04). A trend was observed in which the higher the stimulation required to obtain final positive stimulation, the worse the initial facial nerve outcome and the longer the deficit took to return toward normal.

Conclusions: Our study showed that final facial nerve stimulation with low amplitude led to good facial nerve outcomes in the long term. We also noted that despite some suboptimal immediate postoperative facial nerve function, excellent long-term facial function was seen in all patients. Our data stress the importance of maintaining electrical (as well as anatomic) integrity of the facial nerve; we advocate doing this at all costs, even if a thin layer of tumor is left adherent to the facial nerve.