Intracochlear Electrocochleography: Response Patterns During Cochlear Implantation and Hearing Preservation

Objectives: Electrocochleography (ECochG) obtained through a cochlear implant (CI) is increasingly being tested as an intraoperative monitor during implantation with the goal of reducing surgical trauma. Reducing trauma should aid in preserving residual hearing and improve speech perception overall. The purpose of this study was to characterize intracochlear ECochG responses throughout insertion in a range of array types and, when applicable, relate these measures to hearing preservation. The ECochG signal in cochlear implant subjects is complex, consisting of hair cell and neural generators with differing distributions depending on the etiology and history of hearing loss. Consequently, a focus was to observe and characterize response changes as an electrode advances. Design: In 36 human subjects, responses to 90 dB nHL tone bursts were recorded both at the round window (RW) and then through the apical contact of the CI as the array advanced into the cochlea. The specific setup used a sterile clip in the surgical field, attached to the ground of the implant with a software-controlled short to the apical contact. The end of the clip was then connected to standard audiometric recording equipment. The stimuli were 500 Hz tone bursts at 90 dB nHL. Audiometry for cases with intended hearing preservation (12/36 subjects) was correlated with intraoperative recordings. Results: Successful intracochlear recordings were obtained in 28 subjects. For the eight unsuccessful cases, the clip introduced excessive line noise, which saturated the amplifier. Among the successful subjects, the initial intracochlear response was a median 5.8 dB larger than the response at the RW. Throughout insertion, modiolar arrays showed median response drops after stylet removal while in lateral wall arrays the maximal median response magnitude was typically at the deepest insertion depth. Four main patterns of response magnitude were seen: increases > 5 dB (12/28), steady responses within 5 dB (4/28), drops > 5 dB (from the initial response) at shallow insertion depths ( 5 dB occurring at deeper depths (5/28). Hearing preservation, defined as 0.57, maximum of 0.80 for the maximal response). Conclusions: Monitoring the cochlea with intracochlear ECochG during cochlear implantation is feasible, and patterns of response vary by device type. Changes in magnitude alone did not account for hearing preservation rates, but considerations of phase, latency, and neural contribution can help to interpret the changes seen and improve sensitivity and specificity. The correlation between the absolute magnitude obtained either before or during insertion of the ECochG and the hearing threshold changes suggest that cochlear health, which varies by subject, plays an important role. ACKNOWLEDGMENTS: This project was funded by the NIH through NIDCD (F30 DC015168). The senior author D.F. has or has had research projects with MED-EL, Cochlear Corporation and Advanced Bionics. C.G., K.B., K.H., and H.P. declare that their involvement in research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest. C.B. is a consultant for Advanced Bionics and Cochlear Corporation, and O.A. is a consultant for MED-EL and Advanced Bionics. O.A. and C.B. have equity stakes in Advanced Cochlear Diagnostics. The authors have no conflicts of interest to declare. C.G. and D.F. designed experiments. Data collection occurred with surgeons K.B., O.A., C.B., and H.P. while at UNC-Chapel Hill. K.H. helped with analysis. C.G. and D.F. wrote the article, and all authors contributed significantly to analysis and revisions leading to its final form. Received June 5, 2018; accepted August 6, 2018. Address for correspondence: Christopher K. Giardina, 101 Mason Farm Rd, Glaxo Bldg 142, Chapel Hill, NC 27599, USA. E-mail: christopher_giardina@med.unc.edu Copyright © 2018 Wolters Kluwer Health, Inc. All rights reserved.

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