The retrosigmoid approach for auditory brainstem implantation

Objective: To describe our experience with the retrosigmoid-transmeatal (RS -TM) approach in auditory brainstem implantation (ABI) as well as the anato mosurgical guidelines for this route. Study Design: Retrospective case review. Setting: Ear, Nose, and Throat Department of the University of Verona. Patients: Five patients with neurofibromatosis type 2 (NF2) were operated o n for vestibular schwannoma removal with ABI implantation from April 1997 t o June 1999. The patients were four men and one woman, whose ages ranged fr om 22 to 37 years. The tumor sizes ranged from 12 to 30 mm. The records of a total of 179 patients operated on for vestibular schwannoma (VS) removal via the RS-TM approach from January 1990 to June 1999 were also evaluated. Their ages ranged from 18 to 88 years (average 54 years). The tumor sizes r anged from 4 to 50 mm. Five patients had a solitary VS in the only hearing ear. Intervention: The classic RS-TM approach was used in all patients. After tu mor excision, for ABI implantation, the landmarks (seventh, eighth, and nin th cranial nerves, choroid plexus) for the foramen of Luschka were carefull y identified. The choroid plexus was then partially removed, and the tela c horoidea was divided and bent back. The floor of the lateral recess of the fourth ventricle and the convolution of the dorsal cochlear nucleus became visible. The electrode array was then inserted into the lateral recess and correctly positioned with the aid of electrically evoked auditory brainstem responses (EABRs). Main Outcome Measures: Intraoperative EABR and postoperative speech percept ion evaluation. Results: Auditory sensations were induced in all patients with various numb ers of electrodes. Different pitch sensations could be identified with diff erent electrode stimulation. Conclusions: In the authors' experience, the RS-TM approach is the route of choice for patients who are candidates for ABI when there is a chance of h earing preservation during surgery. If auditory function is lost during sur gery, anatomical preservation of the cochlear nerve may allow hearing resto ration with a cochlear implant. Direct intraoperative recording of cochlear nerve action potentials (CNAPs) and round window electrical stimulation ar e mandatory for these purposes. in addition, decompression of the intrameat al portion of the vestibular schwannoma and planned partial tumor resection with hearing preservation are also possible with the RS-TM approach.