Restoration of hearing with an auditory brainstem implant in a patient with neurofibromatosis type 2--case report

Hearing Restoration with Auditory Brainstem Implant

Auditory brainstem implant (ABI) technology attempts to restore hearing in deaf patients caused by bilateral cochlear nerve injury through the direct stimulation of the brainstem, but many aspects of the related mechanisms remain unknown. The unresolved issues can be grouped into three topics: which patients are the best candidates; which type of electrode should be used; and how to improve restored hearing. We evaluated our experience with 11 cases of ABI placement. We found that if at least seven of eleven electrodes of the MED-EL ABI are effectively placed in a patient with no deformation of the fourth ventricle, open set sentence recognition of approximately 20% and closed set word recognition of approximately 65% can be achieved only with the ABI. Appropriate selection of patients for ABI placement can lead to good outcomes. Further investigation is required regarding patient selection criteria and methods of surgery for effective ABI placement.

Auditory brainstem implants: current state and future directions with special reference to the subtonsillar approach for implantation

In this article, the authors describe the current state of the auditory brainstem implant (ABI), comparing it to that of the cochlear implant (CI). The CI restores hearing by stimulating the cochlear nerve in the cochlea in patients whose deafness has been caused by inner ear disease; the ABI restores hearing by stimulating the cochlear nucleus of the brainstem in patients who are deaf because of bilateral cochlear nerve dysfunction. Up to now, about 500 patients worldwide have undergone ABI and had their hearing restored, most of whom suffer from neurofibromatosis type 2. Hearing performance, however, is not as good as that offered by the cochlear implant. To improve the quality of hearing, new techniques such as advanced coding strategies and penetrating electrodes, are now being introduced.

Microsurgical anatomy of the cochlear nuclei

OBJECTIVE

The dorsal and ventral cochlear nuclei are exposed when treating lesions involving the fourth ventricle and the cerebellopontine angle. Precisely locating the nuclei is important in the placement of the auditory brainstem implantation and in intraoperative monitoring of auditory evoked potentials. This study examined the microsurgical anatomy of the cochlear nuclei.

METHODS

The relationships and the surgical approaches to the nuclei were examined in 20 formalin-fixed specimens with the aid of an operating microscope.

RESULTS

The dorsal cochlear nucleus is located in the floor of the lateral recess of the fourth ventricle on the posterior surface of the inferior cerebellar peduncle where it produces a prominence, the auditory tubercle. The ventral cochlear nucleus is positioned on the lateral surface of the inferior cerebellar peduncle where it often straddles the anterior edge of the foramen of Luschka at the junction of the lateral recess and cerebellopontine angle and produces less of a prominence on the surface of the brainstem than on the dorsal nucleus.

CONCLUSION

Both the dorsal and ventral cochlear nuclei can be exposed after opening the foramen of Luschka and the lateral recess of the fourth ventricle by the retrosigmoid or translabyrinthine approach. The dorsal cochlear nucleus is positioned so that it would be damaged by a tumor within or approaches to the floor of the lateral recess. The ventral cochlear nucleus is positioned so that it would be vulnerable to damage by tumors within or operative approaches to the cerebellopontine angle. The dorsal cochlear nucleus and the adjacent intraventricular part of the ventral cochlear nucleus have anatomic characteristics that make them the preferable site of placement for the auditory brain stem implant.