gms | German Medical Science

Objective: Restoration of hearing in patients with lesions of the cochlear nerve, especially in Neurofibromatosis 2, is since several years technically possible. The generally acknowledged method is the implantation of an auditory brainstem implant (ABI) via the retrosigmoid or translab approach. By this treatment patients may regain a useful hearing and in some cases an open speech understanding. However, there is still a big difference in performance compared to cochlea implant (CI) patients. They even show an increased hearing ability by bilateral stimulation of the cochlea. The surgical and technical feasibility of bilateral stimulation in ABI candidates was to be evaluated.

Methods: Three patients with midline tumors in the posterior fossa were investigated intraoperatively. One suffered from NF-2, had a neurinoma in the right cerebello-pontine angle and was planned to be implanted with an ABI. Prior a big meningioma at the cranio-cervical junction has to be resected. The others had a tumor in the area of the vermis cerebelli.

All cases were operated in semi sitting position. After tumor resection two placing electrodes of the Med-El ABI system were inserted via the forth ventricle on the auditory tubercle. Stimulation parameter settings were identical with those used in ABI surgeries. The EABR signal was recorded between Cz and A1 (left ear lobe). Recordings were triggered at a rate of 13 Hz, 1000 responses were averaged. The electrode impedances for simultaneous stimulation were similar.

Results: In all cases it was safely possible to insert two placing electrodes into the forth ventricle and on the auditory tubercle of each side. The suboccipital midline approach proofed to be ideal for this attempt. Both cochlear nuclei could be identified and stimulated either singular or simultaneously. The morphology of the EABR recordings were similar to those identified in previous ABI implantations. With bilateral simultaneous stimulation an EABR signal could be recorded with increasing amplitude by higher stimulation currents. No significant difference in unilateral compared to bilateral stimulation mode in terms of latencies or amplitudes was recorded, but a tendency to higher amplitudes in bilateral mode.

Conclusions: Bilateral simultaneous stimulation of the cochlear nuclei is technically possible and the midline suboccipital approach is appropriate for safe placement of the electrodes via the forth ventricle to the area of the cochlear nuclei. The possible benefit for the patients in terms of hearing performance has still to be evaluated.