Comparative spatiotemporal dynamics of access resistance, polarization impedance, and nested bipolar impedance in...

Intracochlear injury during surgical placement of a cochlear implant (CI) electrode array (EA) and the immune response to a foreign body can result in fibrotic scar formation and neoossification that may compromise device performance. Dexamethasone-eluting electrode arrays (Dex-EAs) have been developed to reduce the inflammatory response. Electrode impedance measurements provide a noninvasive biomarker of the intracochlear environment. Nested bipolar (BP) impedance, which uses only intracochlear electrodes for both stimulation and recording, may be particularly sensitive to local tissue changes. This study compared the spatial and temporal profiles of nested bipolar impedances with the profiles of access resistance (Ra) and polarization impedance (Zp), which are derived from measurements that use intra- and extracochlear electrodes, in recipients of standard EAs or Dex-EAs.

Thirty-six adult CI recipients were included (17 control 632 EAs; 19 experimental 632D Dex-EAs). Impedance measurements were obtained at initial activation (IA) and at 2 weeks, 1 month, 3 months, 6 months and 12 months after initial activation. Ra and Zp were derived from transimpedance matrix measurements using a monopolar configuration, while nested bipolar impedance was obtained using a four-electrode intracochlear configuration. Impedance profiles were analyzed across electrodes and time to assess spatial and temporal trends.

Across all 3 impedance measures, all 22 electrodes, and all 6 post-operative time points, Dex-EAs consistently exhibit lower impedance values compared to controls. Even so, Zp decreases over time in both groups (though to a lesser extent for Dex-EAs) as does Ra for apical sites. When focusing on the region of highest interest - the base - different patterns are evident. In control subjects, Ra and nested bipolar impedance progressively increase over the 12 months, first noticeable at the most extreme basal sites and involving an expanding number of more apical sites with time. The changes are significantly more pronounced for nested BP impedance than Ra (Paired T-test: p = 0.001 at 6 months, p < 0.001 at 12 months). This particular pattern is specific to control ears.

Dex-EAs are associated with lower and more stable impedance profiles over time. Nested bipolar impedance demonstrates greater sensitivity to intracochlear changes compared to Ra or Zp, supporting its use as a focused biomarker of the intracochlear environment in future studies.