Internal Acoustic Canal Drilling in Retrosigmoid Approach for Vestibular Schwannomas Excision: Univariate & Multivariate Analyses for Outcome Prediction

Introduction: The effect of internal acoustic canal (IAC) drilling during retrosigmoid resection of vestibular schwannoma (VS) has been understudied previously. This study aimed to review the outcome of retrosigmoid VS excision with and without IAC drilled, and to identify predictors of recurrence and functional outcomes.

Methods: Patients with VS excision via retrosigmoid approach in a single centre from 2004 to 2024 were reviewed retrospectively. Uni- and multivariate analyses were performed to determine predictors of outcomes.

Results: 127 patients were identified after excluding patients with neurofibromatosis, other cranial nerve schwannomas, excision by other approaches and prior radiation/operations. Post-operative stereotactic radiotherapy/radiosurgery for residual tumor was given to 6.3% of patients. Mean clinic & imaging follow-up time was 7.86 and 6.86 years respectively.

IAC drilling was performed in 77.2% (n = 98) of patients, while 22.8% (n = 29) did not have IAC drilled.
There were comparable patient demographics, tumor maximum diameter and Koos grade. Gross or near total excision was achieved in 87.8% in the IAC drilling group and 55.2% in the no IAC drilling group. There was significantly higher degree of excision (p < 0.001) and longer operation time (p < 0.001) in the IAC drilling group. There was no significant difference in cerebrospinal fluid leakage, early/late reoperations, recurrence, and hearing deterioration. However, there was significant higher rate of persistent deterioration in facial nerve function (p = 0.049).

Among all patients, 7.8% of patients had early reoperation. None had injury of jugular bulb/labyrinthine structures. Total recurrence rate was 8.7%. Upon uni- and multivariate analyses, smaller extent of excision predicts higher likelihood of recurrence (OR = 4.058, p = 0.011). Longer operation time predicts facial nerve function deterioration (OR = 1.007, p = 0.004). No significant predictor of hearing deterioration was identified.

Conclusion : IAC drilling in retrosigmoid approach for vestibular schwannoma excision allows higher degree of excision which may predict lower tumour recurrence rate. Nonetheless, this might come at the cost of longer operation time and higher rate of facial nerve function deterioration.