AUDITORY BRAIN-STEM IMPLANT .1. ISSUES IN SURGICAL IMPLANTATION

Most patients with neurofibromatosis type 2 (NF2) are totally deaf aft er removal of their bilateral acoustic neuromas. Twenty-five patients with neurofibromatosis type 2 have been implanted with a brainstem ele ctrode during surgery to remove an acoustic neuroma. The electrode is positioned in the lateral recess of the fourth ventricle, adjacent to the cochlear nuclei. The present electrode consists of three platinum plates mounted on a Dacron mesh backing, a design that has been demons trated to be biocompatible and positionally stable in an animal model. Correct electrode placement depends on accurate identification of ana tomic landmarks from the translabyrinthine surgical approach and also on intrasurgical electrophysiologic monitoring. Some tumors and their removal can result in significant distortion of the brainstem and surr ounding structures. Even in the absence of identifiable anatomic landm arks, electrode location can be adjusted during surgical placement to find the location that maximizes the auditory evoked response and mini mizes activation of other monitored cranial nerves. Stimulation of the electrodes produces auditory sensations in most patients, with result s similar to those of single-channel cochlear implants. A coordinated multidisciplinary team is essential for successful application of an a uditory brainstem implant.