Cochlear implant electrode impedance: a retrospective study of MED-EL CI users

It has been suggested that inflammation can affect the interface between the cochlear implant (CI) and the auditory system and therefore impact patient outcomes. This is supported by reports that anti-inflammatory treatment using steroids improves speech recognition performance and reduces undesirable symptoms. In-vitro and in-vivo investigations show that CI electrode impedance measurements reflect tissue changes around the implanted electrode and therefore provides an objective view of inflammatory activity within the cochlea. Here we review historical impedance data from 200 MED-EL CI users (40 paediatric, 160 Adult) seen at the University of Southampton Auditory Implant Service (USAIS). The main aim was to establish a normative model of the evolution of electrode impedance over several months and years for MED-EL 12 channel electrode arrays. Preliminary results show a reduction in electrode impedance for all electrodes between device activation and the next appointment one week later. The basal electrodes show a steady increase in impedance over several months before plateauing and subsequently decreasing. Results show an ongoing decrease in impedance for the mid and apical electrodes which highlights a difference in the long-term evolution of impedance for different cochlear regions. Previous research suggests that the slow increase in electrode impedance is caused by the development of fibrotic tissue which is driven by an inflammatory response to the electrode array. This mechanism may explain the increase observed in the basal electrodes, especially as the trauma from surgery might further stimulate the inflammatory process. The drop in electrode impedance following device activation is likely to be caused by electrolytic changes at the electrode-cochlea interface. The slow reduction in impedance seen in the apical electrodes may be caused by molecular changes to the electrode that increase their effective surface area. The next stage of analysis will test for differences between the adult and paediatric CI users in this sample group as well as grouping activated and deactivated electrodes to test the effect of chronic stimulation on electrode impedance. In future work the impedance results will be analysed for correlation with hearing preservation measures. This will allow us to test whether the localised changes in impedance, which are thought to be inflammatory mediated, have a relationship with an indicator of cochlear health