gms | German Medical Science

Objective: To describe topographic changes of petrous bone anatomy in presence of vestibular schwannomas and their implications for the microsurgical treatment via the retrosigmoid transmeatal approach.

Method: Patients harbouring a unilateral vestibular schwannoma without previous petrous bone surgery were included in the prospective study. Pre- and postoperatively we performed high-resolution CT scans. On reformatted axial and coronal bone window slices we took anatomic measurements on both the affected and the healthy side and compared the topographic changes around the inner auditory canal. Further the integrity of endolymphatic structures and semicircular canals was analyzed. Correlating clinical parameters of the patients were collected.

Results: 100 patients were included in the study. The diameter of the IAC was larger on the affected side (p=0.001) and increased with increasing tumor size (p=0.001). The horizontal distance between IAC and external aperture of the vestibular aqueduct (VA) was significantly reduced in presence of a VS (p=0.001). The vestibular aqueduct was opened in 41% of cases, the semicircular canals in 18% of cases. In the group of patients showing a postoperative lesion of the vestibular aqueduct, the length of the IAC was shorter (p<0.001), the distance between IAC and external aperture reduced (p<0.001), the IAC drilled more (p=0.013) and the distance between drilling line and sigmoid sinus significantly shorter (p<0.001). With increasing tumor size the incidence of VA injury rose (p<0.005). With increasing tumor size also the preoperative hearing function decreased (p=0.003). This clearly influenced the surgical strategy in terms of more extensive drilling. Thus, in the patient group showing pre- and postoperatively good hearing function a VA injury occurred in only 27%, while in the group showing deafness prior to surgery the VA was opened in 67% of cases.

Conclusions: A growing VS significantly changes the topographic relationship between IAC and surrounding inner ear structures. In order to preserve integrity of inner ear structures a high-resolution CT scan is necessary to evaluate the individual anatomy. In some cases anatomy is unfavorable and renders impossible a transmeatal exposure without harming the vestibular aqueduct. In these cases an endoscope-assisted technique may be indicated to achieve hearing preservation.