Advantages of the retrosigmoid approach in auditory brain stem implantation

From April 1997 to December 1999, six patients (five men and one woman), ranging in age from 22 to 37 years with neurofibromatosis type 2 (NF2) were operated on via the classic retrosigmoid-transmental (RS-TM) approach for removal of a vestibular schwannoma (VS) (tumor size from 12 to 40 mm) and for auditory brain stem implantation (ABI). After tumor removal, the floor of the lateral recess of the fourth ventricle and the convolution of the dorsal cochlear nucleos were reached, and the ABI was inserted. More recently, an ABI was implanted via the retrosigmoid approach in a 4-year-old boy with a cochlear malformation (common cavity) associated with cochlear nerve aplasia. Electrically evoked auditory brain stem responses (EABRs) and neural response telemetry (NRT) were performed to verify the correct positioning of the inserted electrodes. No major complications related to ABI were observed. ABI has been activated to date in five of the NF2 patients. Auditory sensations with various numbers of electrodes were evoked in all patients. We consider the RS-TM approach the route of choice for ABI insertion in patients with NF2 and good hearing, offering a chance of hearing preservation, and in patients with complete cochlear ossification, severe head trauma and cochlear fracture, or nerve disruption, or a combination of these. A new indication for ABI implantation via the RS approach is presented by patients with bilateral cochlear nerve aplasia.