P 19 Auditory brainstem implants: Intraoperative electrophysiological measures and hearing outcome

Abstract

Introduction: Auditory brainstem implants (ABI) are primarily designed for tumour related deafness in neurofibromatosis type 2 (NF2) patients. These neuro-prosthetic devices bypass the auditory nerve and produce hearing sensations by direct stimulation of the cochlear nuclei (CN). This study investigates the importance of intraoperative electrically evoked auditory brainstem responses (EABR) with regards to the auditory outcome. Patients & Methods: Out of a prospectively collected series of ABI implantations from 2005 to 2019, 22 patients (11 male, 11 female) fulfilled inclusion criteria (min. age of 15 y, NF2 diagnosis) and were analysed retrospectively for EABR and hearing outcome. EABR analysis relied on the presence and number of vertex positive peaks P1, P2 and P3 at brainstem stimulation. For post-operative hearing outcome a new Clinical ABI Outcome Classification was developed and applied at 6 to 12 months containing 4 categories: Category 1, Star Performer, with >80% speech understanding in auditory only MTP (mono- to polysyllabic) test and ability for continuous spoken conversation without any lip reading; Category 2, Good Performer, with >40 to 80% in auditory only MTP test and some speech understanding combined with lip reading; Category 3, Useful Performance, communication with some additional measures possible (hearing, lip-reading and written notes); Category 4, Non-useful Performance, no or only scarce sound reception. Results: In 22 patients, 146 EABR recordings at various sites of the implant were evaluated: A three-peak-formation was present in 7, a two-peek-formation in 115 (78,8%), and one-peak in 13 EABRs, while 11 remained without any reproducible responses. EABR waveforms showed some variation: Peak P1 developed just out of or after the stimulus artefact while peaks P2 and P3 sometimes showed melting and larger latency differences. P1 appears to correspond to wave III of the acoustically evoked ABR. Overall auditory outcome was useful (Categories 1, 2 or 3) in 95,5% of cases, with Star or Good Performance in about 77% of patients. EABR presence predicted auditory rehabilitation correctly in 95,5%. Conclusion: False positive EABR are rare and a matter of open discussion such as on lead dislocation or secondary brainstem nuclei degeneration. Overall, intra-operative reproducible EABR are highly predictive of adequate brainstem activation and useful hearing rehabilitation with ABI in deaf NF2 patients and appear indispensable for implant positioning. The most reliable peak P1 of EABR may represent immediate activity of cochlear nuclei. The importance of further peaks P2 and P3 and their anatomic correlation still need further evaluation and possibly correlation with more long-term auditory development. The presented ABI hearing classification includes the internationally accepted MTP test but goes beyond this and proves to be a universal tool to elucidate the patient's capacity for speech communication in real life.