Novel Impedance Measures as Biomarker for Intracochlear Fibrosis

Abstract

Measurement of the complex electrical impedance of the electrode contacts can provide new insights into the factors playing a role in the preservation of residual hearing with cochlear implants (CIs). However, unraveling the contributions related to the different phenomena from impedance data necessitates more advanced measurement and analysis techniques. The present study explores a new impedance measurement option recently included into the cochlear-implant programming software and aims to contribute to a more solid basis for the clinical use of impedance measures as a biomarker for fibrous tissue formation. Twenty adult CI-recipients were followed from surgery until 1 year after implantation by means of Electrode Voltage Telemetry (EVT), also called Electric Field Imaging or TransImpedance-Matrix measurement, and a 4-point technique for probing the voltage between adjacent electrode contacts. The data were compared to the electrode location derived from computed tomography, and to the device usage log. Using our impedance model for electrical stimulation of the cochlea, the polarization impedance related the electrode-tissue interface was determined, and the bulk impedance (access resistance) was split into a near-field and a far-field component. On average, the polarization impedance increased abruptly after surgery, indicating a strong passivation of the electrode contacts before cochlear-implant initiation. Its initial rise resolved almost completely soon after device switchon (2-4 weeks). The gradual increase of the access resistance mainly happened during the first 40 days on a time scale very similar to that observed in a guinea-pig study correlating impedance changes to fibrous tissue growth. The higher increase towards the round window is consistent with the higher amount of tissue observed in histological animal studies close to the electrode entry point. While the initial changes were due to the near-field resistance, the far-field resistance began to rise only after one month for half of the study group, once the near-field component had reached its critical value. This suggests indeed fibrosis initiating near the electrode contacts and spreading thereafter farther away. The near-field resistance positively correlated to device usage. EVT data allow for a further decomposition of the impedance at a cochlear-implant electrode, yielding a more detailed description of the postoperative intracochlear phenomena, such as fibrosis.

Keywords: Cochlear implants; Electrical impedance; Electrical stimulation; Electrode voltage telemetry; Fibrosis; Transimpedance matrix.