Minimally traumatic surgical techniques and advances in cochlear implant (CI) electrode array designs have allowed acoustic hearing present in a CI candidate prior to surgery to be preserved post-operatively. As a result, these patients benefit from combined electric-acoustic stimulation (EAS) post-operatively. However, 30-40% of EAS CI users experience a partial loss of hearing up to 30 dB after surgery. In the present study, electrocochleography (ECoG) was used to study cochlear microphonic (hair cell response) and auditory nerve neurophonic (neural response) in patients with preserved hearing and patients with loss of hearing. These measures were obtained longitudinally over the course of CI use. At each test session, ECoG amplitude growth functions for several low-frequency stimuli were obtained. The threshold, slope, and suprathreshold amplitude at a fixed stimulation level was obtained from each growth function at each time point. Subjects were categorized as having stable hearing or loss of hearing. Longitudinal linear mixed effects models were used study trends in ECoG thresholds, slopes, and amplitudes for these two categories of subjects. Results showed that CM and ANN thresholds and amplitudes were stable in CI users with preserved residual hearing. CM and ANN thresholds increased (worsened) while CM and ANN amplitudes decreased (worsened) for those with delayed hearing loss. The slope did not distinguish between subjects with stable hearing and subjects with delayed loss of hearing. These results provide a new application of post-operative ECoG as an objective tool to monitor residual hearing and understand the pathophysiology of delayed hearing loss.

BJG is a consultant for Cochlear Ltd. MRH is co-founder and Chief Medical Officer for iotaMotion, Inc. VDT is a consultant for iotaMotion, Inc. All other authors declare that they have no conflict of interest.

National Institute on Deafness and Other Communicative Disorders (P50 DC 000242, PI: Gantz; 1R01 DC017846, PI: He; R21 DC019458, PI: He) National Institute of General Medical Sciences (1R01 DC016038, PI: He)

I confirm all relevant ethical guidelines have been followed, and any necessary IRB and/or ethics committee approvals have been obtained.

Yes

The details of the IRB/oversight body that provided approval or exemption for the research described are given below:

The IRB of University of Iowa gave ethical approval of this work.

I confirm that all necessary patient/participant consent has been obtained and the appropriate institutional forms have been archived, and that any patient/participant/sample identifiers included were not known to anyone (e.g., hospital staff, patients or participants themselves) outside the research group so cannot be used to identify individuals.

Yes

I understand that all clinical trials and any other prospective interventional studies must be registered with an ICMJE-approved registry, such as ClinicalTrials.gov. I confirm that any such study reported in the manuscript has been registered and the trial registration ID is provided (note: if posting a prospective study registered retrospectively, please provide a statement in the trial ID field explaining why the study was not registered in advance).

Yes

I have followed all appropriate research reporting guidelines and uploaded the relevant EQUATOR Network research reporting checklist(s) and other pertinent material as supplementary files, if applicable.

Yes

All data produced in the present study are available upon reasonable request to the authors