Facial nerve outcome score: a new score to predict long-term facial nerve function after vestibular schwannoma surgery

Abstract

Introduction: Patients' quality of life (QoL), facial nerve (FN), and cochlear nerve (CN) (if conserved) functions should be pursued as final outcomes of vestibular schwannoma (VS) surgery. In regard to FN function, different morphologic and neurophysiological factors have been related to postoperative outcomes. The aim of the current retrospective study was to investigate the impact of these factors on the short- and long-term FN function after VS resection. The combination of preoperative and intraoperative factors resulted in designing and validating a multiparametric score to predict short- and long-term FN function.

Methods: A single-center retrospective analysis was performed for patients harboring non-syndromic VS who underwent surgical resection in the period 2015-2020. A minimum follow-up period of 12 months was considered among the inclusion criteria. Morphological tumor characteristics, intraoperative neurophysiological parameters, and postoperative clinical factors, namely, House-Brackmann (HB) scale, were retrieved in the study. A statistical analysis was conducted to investigate any relationships with FN outcome and to assess the reliability of the score.

Results: Seventy-two patients with solitary primary VS were treated in the period of the study. A total of 59.8% of patients showed an HB value < 3 in the immediate postoperative period (T1), reaching to 76.4% at the last follow-up evaluation. A multiparametric score, Facial Nerve Outcome Score (FNOS), was built. The totality of patients with FNOS grade A showed an HB value < 3 at 12 months, decreasing to 70% for those with FNOS grade B, whereas 100% of patients with FNOS grade C showed an HB value ≥ 3. The ordinal logistic regression showed three times increasing probability to see an HB value ≥ 3 at 3-month follow-up for each worsening point in FNOS score [Exp(B), 2,999; p < 0.001] that was even more probable [Exp(B), 5.486; p < 0.001] at 12 months.

Conclusion: The FNOS score resulted to be a reliable score, showing high associations with FN function both at short- and long-term follow-up. Although multicenter studies would be able to increase its reproducibility, it could be used to predict the FN damage after surgery and the potential of restoring its function on the long-term period.

Keywords: facial nerve; facial nerve function; intraoperative neuromonitoring; outcome score; restrosigmoid approach; vestibular schwannoma.

Figure 1

Distribution of postoperative FN function (House–Brackmann scale) over time. T1, IV postoperative day; T2, 3-month follow-up; T3, 1-year follow-up; T4, last follow-up.

Figure 2

Facial Nerve Outcome Score (FNOS). Each item of the score is showed: dimensions, Samii grade, morphology, facial nerve threshold T1 (FT-1) (the minimum amplitude (mA) required to obtain an EMG response of the nerve at its proximal emergence at brainstem level after tumor removal), and delta threshold (DT) (the difference in absolute value between FT-1 and FT-0). The final score is the result of dimensions + Samii grade + morphology + (FT-1 * DT).

Figure 3

Three exemplary cases of patients with vestibular schwannoma were reported illustrating the “Facial Nerve Outcome Score” calculation. In all cases, a 12-month postoperative brain MRI (axial cut of T1-weigthed contrast enhanced MRI) showed the extent of resection.

Figure 4

Association between FNOS and FN function (HB grade) at different time (T1-T4, A–D). Patients are divided in "good" and "poor" HB outcome < 3 and ≥ 3, respectively, considering the result of the score FNOS (FNOS-A, -B and -C).

Figure 5

Association between Samii grade and FN function at different time (T1–T4, A–D). Patients are divided, considering the Samii grade (T2–T3a vs. T3b–T4), in “good” and “poor” HB outcome, < 3 and ≥ 3, respectively.