Cochlear implant electrode impedance: a potential biomarker for clinical outcomes?

It has been suggested that inflammation affects the interface between the cochlear implant (CI) and the auditory system and can therefore impact patient outcomes. Studies show that changes in CI electrode impedance (EI) reflect tissue and chemical changes at this interface and can be associated with poor CI performance. Although impedance telemetry is performed routinely in the clinic the results are rarely analysed at group level, especially in the light of post-implantation hearing preservation. The objectives of this study were to establish a long-term temporal model of EI and investigate its relationship with hearing preservation. We analysed historical data from 180 MED-EL CI users seen at the University of Southampton Auditory Implant Service (USAIS). Results show an EI reduction across all channels following CI switch-on. In the following 2 years an increase in EI was seen in basal electrodes, whereas a reduction was observed in apical electrodes. Results of hearing preservation were pending final analysis at the time of writing.Previous research suggests the slow increase in EI is caused by the development of fibrotic tissue which is driven by an inflammatory response. Studies have shown that immune cell infiltration and fibrosis are greatest at the base, nearest to the site of surgery, which may explain the EI increase in basal electrodes observed here. Our long-term temporal model of EI establishes a norm that can be used by clinicians interpreting impedance telemetry results, especially in complex cases of sub-optimal CI performance.