Impact of Petromeatal Bone Angle on Degree of Tumor Resection and Facial Nerve Injury for Vestibular Schwannomas During the Translabyrinthine Approach

Background: The translabyrinthine approach offers a trajectory of view into the posterior fossa that is parallel to the internal acoustic canal (IAC). Visualization and work anterior to the IAC is affected by variations in the angle formed by the IAC and the posteromedial aspect of the petrous pyramid, which we will refer to as “petromeatal angle.” We postulate that varying degrees of the petromeatal angle could affect the degree of tumor resection and facial nerve outcome in vestibular schwannoma (VS) surgery. Preoperative measurement of the petromeatal angle could then be used as a prognostic factor and guide surgical expectations in VS patients.

Methods: We conducted a retrospective review of the medical records of 39 patients who underwent a translabyrinthine approach for resection of VSs, confirmed by postoperative tissue diagnosis. The pertinent, collected data included ipsilateral petromeatal angle, petroclival angle, petrous-petrous (petrous Cobb) angle, tumor size, degree of tumor resection (gross/near-total versus sub-total), immediate postoperative cranial nerve (CN) VII deficit, and change in House–Brackmann score, calculated by subtracting the preoperative score from the postoperative score at discharge.

The petromeatal angle is formed at the junction of two lines that are parallel to the IAM anterior edge and the posteromedial surface of the petrous bone (Fig. 1). Petrous Cobb and petroclival angles were calculated as described by Desai et al.[1] Statistical analysis was performed using the R Statistical Software.

Fig. 1

Results:

Table 1 Lists the mean values and 95% confidence intervals (CIs) of the pertinent data

Using the Logit statistical model to estimate the odds, our data show that every additional degree of ipsilateral petromeatal angle increases the odds of being able to accomplish gross/near-total resection by 6.63% (p = 0.0375). Using the same statistical model, we also found that every additional cm in tumor size decreases the odds of gross/near-total resection by 79% (p = 0.0106). With tumor sizes below 2.25 cm, the percentage of patients with gross/near-total resection was >87%.

No statistically significant associations were found between the ipsilateral petromeatal angle and CN VII deficit (p = 0.366) or change in House–Brackmann score (p = 0.985). No significant associations were detected between the petroclival or petrous Cobb angles and any of the three dependent variables (degree of tumor resection, immediate postop CN VII deficit, change in House–Brackmann score).

Discussion/Conclusion: With a relatively small sample of patients, our results suggest that the petromeatal angle can be a reliable predictor of gross/near-total resection during the translabyrinthine approach for VSs.

We are currently reviewing additional medical records to improve the reliability of the results and the power of this study.

• Reference

• Desai SK, Baymon D, Sieloff E. , et al. Computed tomography morphometric analysis of the central clival depression and petroclival angle for application of the presigmoid approach in the pediatric population. J Pediatr Neurosci 2016; 11 (02) 109-111