The Influence of Cochlear Volume on Temporal Changes of Impedance among Cochlear Implant Patients

Background: There is evidence that the cochlear volume may influence audiometric thresholds and CI electrodes’ impedance. The aim of the present study was to evaluate the impedance changes over time and correlate them to the residual volume of the cochlea. Methods: An MRI scan was performed via 3-D reconstruction before every surgery to obtain a residual volume for each ear. We performed repeated assessments of electrode impedance, both intra-operatively and post-implant, at the following intervals: 3 months, 6 months, and one year. The same type of perimodiolar array was implanted for each. Results: Thirty-four patients (10 (29.41%) male patients and 24 (70.59%) female patients) were evaluated. Patients received the implants between 2008 and 2017. The mean age of implantation was 13 ± 17.17 years, and the average of hearing thresholds improved after one year of the surgery. The mean cochlear volumes of the implanted ears were 68.16 ± 10.74 mm3 (right ear) and 56.54 ± 13.75 mm3 (left ear). We observed an increase in the basal electrodes’ impedance at the 3rd month. Yet, for the apical electrodes’ impedance, there was a decrease in averaged values. Conclusions: Post-operative impedance measurements were increased when compared to the intraoperatively measured basal values. Newly formed connective tissue is thought to be the cause of the higher impedance values.