Measuring the Electrical Status of the Bionic Ear. Re-thinking the Impedance in Cochlear Implants

Abstract

As in any biophysical electrode-tissue environment, impedance measurement shows a complex relationship which reflects the electrical characteristics of the medium. In cochlear implants (CIs), which is mostly a stimulation-oriented device, the actual clinical approach only considers one arbitrary time-measure of the impedance. However, to determine the main electrical properties of the cochlear medium, the overall impedance and its subcomponents (i.e., access resistance and polarization impedance) should be described. We here characterized, validated and discussed a novel method to calculate impedance subcomponents based on CI measurement capabilities. With an electronic circuit of the cochlear electrode-tissue interface and its computational simulation, the access resistance and polarization impedance were modeled. Values of each electrical component were estimated through a custom-made pulse delivery routine and the acquisition of multiple data points. Using CI hardware, results fell within the electronic components nominal errors (± 10%). Considering the method's accuracy and reliability, it is readily available to be applied in research-clinical use. In the man-machine nature of the CI, this represents the basis to optimize the communication between a CI electrode and the spiral ganglion cells.

Early Elevation and Normalization of Electrode Impedance in Patients With Enlarged Vestibular Aqueduct Undergoing Cochlear Implantation

“…Not all patients had genetic testing performed and therefore may have had subtle differences in phenotype. To address this only patients with otherwise normal cochleae phenotype (i.e., not incomplete partition/mondini malformations) were In addition, the clinical impedance measurements used in CI provides some, but incomplete information about the actual electrode-tissue interface (21). Without raw data for both access resistance and polarization impedance (not collected and modeled) any conclusions about the relative contributions of endocochlear environment versus surface quality of the electrode are theoretical (22).…”

“…(In addition, such would also indicate minimal, if any, effect from test/retest variation in impedances seen). In addition, the clinical impedance measurements used in CI provides some, but incomplete information about the actual electrode-tissue interface (21). Without raw data for both access resistance and polarization impedance (not collected and modeled) any conclusions about the relative contributions of endocochlear environment versus surface quality of the electrode are theoretical (22).…”

Biological Response Dynamics to Cochlear Implantation: Modeling and Modulating the Electrode–Tissue Interface

“…with R a the access resistance (in Ω), R p the polarization resistance (in Ω) and C p the polarization capacitance (in F). Although R p was computed, it is not further analyzed due to its small variation and very poor informative use (see Di Lella et al 2020). For reference and comparison, we also record the clinical impedance ( Z c ) implementing the method from Custom Sound fitting software.…”

“…The complexity and biological composition of the inner ear medium makes its electrical impedance nonlinear, meaning that the opposition to the current flow is time dependent. Many studies have modeled its electrical behavior and defined an equivalent circuit with known electrical subcomponents, facilitating its analysis and model’s equation (Vanpoucke et al 2004; Tykocinski et al 2005; Newbold et al 2010a,b; Giardina et al 2018; Di Lella et al 2019; Mesnildrey et al 2019; Di Lella et al 2020; Aebischer et al 2021). The overall impedance Z t is therefore represented with an access impedance Z a and a polarization impedance Z p , being Z t = Z a + Z p .…”

Long-term hearing preservation in cochlear implant patients

“…While it is impossible to directly assess impedance, its values can be obtained by measuring voltage, as provided by Ohm's law. In CIs, this measurement is performed by using a protocol known as "voltage telemetry" [65,66].…”