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Changes in Electrode Impedance and Cochlear Microphonic in Cochlear Implant Recipients in the First Year After
Imogen A M L van Beurden1,2, Frank T Hartong1,2, Ilse Y Haan1,2
1Department of Otorhinolaryngology and Head and Neck Surgery, University Medical Center Utrecht, Utrecht University, Utrecht, The Netherlands.
Objective:
We examined changes in electrode impedance and cochlear microphonic (CM) during the first year after implantation and investigated the extent to which changes in impedance related to changes in CM. In addition, we explored whether residual hearing was related to impedance and/or CM.
Study Design:
Prospective longitudinal design.
Setting:
Tertiary referral center in Utrecht, the Netherlands.
Patients:
Twenty-one adults underwent cochlear implantation surgery between January 2020 and January 2025.
Interventions:
Impedance and CM responses to pure-tone stimuli (125, 250, and 500 Hz; sound levels 100 to 115 dB HL) at all 16 electrodes were measured at 5 timepoints: intraoperatively, postoperatively after 6 weeks, 3 months, 6 months, and 12 months. Residual hearing was measured by pure-tone average (PTA) to 125 to 500 Hz (70 to 115 dB HL) at 12 months.
Main Outcome Measures:
Longitudinal analyses of impedance and CM amplitude were performed per segment of the electrode array (apex, middle, and base). Impedance was averaged across the electrodes of the segment. The CM amplitudes were derived by averaging the mean amplitudes of electrodes per segment and then averaging across the 3 frequencies. Correlation analyses were performed (a) between absolute impedance and CM; (b) between impedance differences and CM amplitude ratio; (c) between PTA and impedance; and (d) between PTA and CM.
Results:
Changes in impedance and in CM over time differed between segments. The most robust changes in impedance were seen in the middle segment between surgery and the visit after 6 weeks. CM decreased postoperatively, especially in the apical and middle segments between surgery and the visit after 6 weeks. Impedance and CM were uncorrelated for absolute and relative measures. Twelve-month postoperative residual hearing was poorer when 6-week impedance was higher and when 3-month CM amplitude was lower.
Conclusions:
CM and impedance were not correlated, although both metrics can be affected by tissue growth triggered by the electrode array. Larger clinical studies are needed to investigate the predictive value of impedance and CM for postoperative residual hearing.