Preservation of labyrinthine structures while drilling the posterior wall of the internal auditory canal in surgery of vestibular schwannomas via the retrosigmoid suboccipital approach.

Abstract

To describe a new technique for safe drilling of the posterior wall of the internal auditory canal (IAC), in which the intact posterior lip of the internal auditory meatus is used as a fixed intraoperative reference point to preserve the integrity of labyrinthine structures. The retrosigmoid suboccipital approach was used to operate on 6 cases of vestibular schwannomas (Koos grade I and II), with preserved hearing. On a preoperative high-resolution computed tomography scan, a line starting 2 cm lateral to the lateral edge of the sigmoid sinus on the dura mater and directed tangential to the posterior semicircular canal or common crus was identified to intersect the posterior wall of the IAC. Drilling was carried out at a measured angle to the posterior petrous wall for meticulously measured distances, taking the intact posterior lip of the internal auditory meatus as a fixed bony point. In the 6 cases, the mean length of the posterior wall of the IAC measured on the preoperative high-resolution computed tomography scan was 10.79 mm±1.87. By using our technique, the percentage of total length of the posterior wall of IAC drilled was 75.3%±20.5%. No injury to the posterior semicircular canal or common crus was observed intraoperatively. Total excision was performed in 5 patients, and near-total excision was performed in 1 patient. Functional hearing was preserved in all patients; testing was done 1 month after surgery. Adequate drilling of the posterior wall of the IAC could be achieved, and tumor excision with hearing preservation was obtained by meticulous intraoperative planning and measurements based on preoperative computed tomography scanning and by keeping the intact posterior lip of the internal auditory meatus as a landmark for safe drilling.