Topographic Changes in Petrous Bone Anatomy in the Presence of a Vestibular Schwannoma and Implications for the Retrosigmoid Transmeatal Approach

BACKGROUND: The maneuver of transmeatal drilling carries the risk of injuring inner ear structures, which may cause immediate or delayed hearing loss. OBJECTIVE: To describe the changes in petrous bone anatomy caused by the tumor and to analyze both the incidence and the risk pattern for violation of the endolymphatic system in a surgical series. METHODS: One hundred patients operated on for vestibular schwannoma were included in this prospective study. Thin-slice computed tomography was performed before and after surgery. We assessed topographic measurements on both the pathological and healthy sides. Postoperatively, we evaluated anatomic and functional values. RESULTS: The diameter of the internal auditory canal was significantly larger (P < .001) in the petrous bones of the affected sides than in the contralateral healthy sides. An average of 5.6 ± 1.8 mm of the internal auditory canal was drilled, and the distance from the medial border of the sigmoid sinus to the drilling line (tangential to the drilled surface of the posterior lip of the internal auditory canal) was 9.8 ± 2.9 mm. A postoperative violation of the vestibular aqueduct (VA) was detected in 41 cases; the VA was intact in 55 cases; and the VA could not be clearly defined in 4 cases. The incidence of VA injury increased with increasing tumor size. In the patient group with good preoperative and postoperative hearing function, a VA injury occurred in 26% of cases, whereas the incidence increased to 67% in preoperatively deaf patients. CONCLUSION: Vestibular schwannomas cause significant distortion of the petrous bone anatomy. Detailed preoperative knowledge of the topography is necessary for the preservation of function. ABBREVIATIONS: IAC, internal auditory canal VA, vestibular aqueduct VS, vestibular schwannoma