Measurement and analysis of access resistance and polarization impedance in cochlear implant recipients

Background: Impedance measurements are commonly performed at the end of cochlear implant surgery, not only to confirm that all electrodes are working but also to monitor the impedances of the newly implanted electrodes. The current method of testing allows the determination of only the overall electrode impedance but not its components, access resistance and polarization impedance. To determine whether any longitudinal change in the electrode impedance is caused by a change in the endocochlear environment or rather caused by a change in the surface quality of the electrode, it is necessary to extract access resistance and polarization impedance. Methods: We applied an impedance model that enabled us to calculate access resistance and polarization impedance after measurement of electrode impedance at three points along the voltage waveform. Results: The results show that the value of the components of electrode impedance varied with time after surgery: access resistance increased slowly over time, whereas polarization impedance increased up to Week 2 but decreased after commencement of electrical stimulation at that stage. These results are consistent with the hypothesis that a layer of fibrous tissue forms around the electrode within the cochlear canal, resulting in a slow increase of access resistance, whereas a layer of proteins forms on the surface of the electrode in the early phase after implantation. Electrical stimulation appears to disperse this surface layer, thereby reducing both the polarization impedance and electrode impedance. Conclusion: The method presented enables the extraction of more detailed information about the longitudinal changes in the intracochlear environment after cochlear implantation.