Establishing reliable neural health measures in adult cochlear implant users

Following cochlear implantation, measures of auditory nerve function based on implant-driven electrical stimulation can be used to characterize neural health and support longitudinal monitoring and treatment outcome assessment, with particular relevance for advanced therapies targeting neural hearing loss. This study evaluated the reliability and measurement precision of neural-health measures derived from three complementary electrode-specific tests: panoramic electrically-evoked compound action potentials (PECAPs), interphase-gap (IPG) effects, and polarity effects. Six experienced adult cochlear implant users were assessed across five sessions by trained audiologists. PECAPs were measured across all electrodes, while IPG and polarity effects were assessed at three electrodes per participant. Test-retest reliability was quantified using intraclass correlation coefficients (ICCs), with measurements repeated over intervals ranging from 1 day to several weeks. Measurement error was further characterized using the minimum detectable change (MDC), defined as the smallest change required to exceed measurement error with 95% confidence. PECAPs neural-responsiveness measure and polarity effect on comfort levels demonstrated excellent reliability (ICC > 0.90), while IPG effects on ECAP amplitude growth function (AGF) slope showed good reliability (ICC > 0.80). Other variables showed moderate reliability, including polarity effect on threshold, and IPG effects on ECAP maximum amplitude, threshold, and AGF offset. MDC estimates indicated that relatively small within-electrode changes would be sufficient to exceed measurement error at the individual participant-electrode level. Exploratory analyses showed limited associations between measures, consistent with the idea that they may reflect distinct aspects of neural status. Together, these findings establish PECAPs, IPG effects and polarity effects as reproducible and complementary measures of auditory nerve health suitable for longitudinal assessment. Their combined use provides a robust physiological framework for monitoring neural status and detecting therapy-related changes in cochlear implant users, including in clinical studies of interventions targeting neural hearing loss. Further standardization of testing and analysis protocols, alongside expansion to larger multicentre cohorts will strengthen the generalizability and facilitate broader clinical and research adoption.