Cochlear implants (CIs) have become the standard of care in the treatment of severe-to-profound hearing loss and allow for speech discrimination without lip reading in the majority of implanted patients. Excellent results warrant continuous expansion of the implantation criteria with more and more patients with residual acoustic hearing receiving CIs. Preservation of acoustic functions is an important surgical goal, but it depends not only on atraumaticity of the surgical act but can also be influenced by physiologic processes occurring during the healing process. For example, excessive fibrotic reactions can lead to delayed loss of residual hearing, modification of the current paths and decreased performance.

Characterization of the conditions that can affect the efficacy of CI-outcome is important in order to gain a better understanding of the interpatient variability in performance that still exists. It may not only help to optimize personal treatment and postoperative CI-programming, but also assist in developing improvements in electrode design and stimulation strategies (Mens, 2007). One of the basic factors in cochlear implant outcome is the electric field within the cochlea, which is affected by a number of extrinsic aspects, such as the electrode design or the electrode location within the cochlea, as well as a multitude of physiological factors such as the composition of the perilymph, inflammatory reactions and the tissue growth around the electrode array and new bone formation.

The CI-electrode stimulation thresholds, dynamic range, spatial channel interaction as well as the hearing outcome and the potential loss of residual hearing depend on the electrode design (Lee et al., 2019; Wilson and Dorman, 2008) and the precise location of its contacts with respect to the neural structures of the cochlea (Chakravorti et al., 2019; Davis et al., 2016; Finley et al., 2008; Holden et al., 2013; Noble et al., 2013; Wanna et al., 2015). Trauma caused by creating the surgical access and by electrode insertion into the cochlea provoke acute inflammatory reactions accompanied with elevated concentrations of proteins in the perilymph, which may easily adsorb onto the electrode surface, whereas chronic inflammatory response occurs as a result of a foreign body reaction, despite biocompatibility of the materials, and initiates the development of a fibrous tissue sheath around the electrode array (Foggia et al., 2019; Jia et al., 2016, 2013). New tissue formation after cochlear implantation affects T/C levels and negatively correlates with dynamic range (Kawano et al., 1998). Although no correlation with CNC word recognition scores was found, inflammation and fibrous tissue growth was demonstrated to be negatively correlated with survival of sensory cells and the preservation of residual hearing (Kamakura and Nadol, 2016; Li et al., 2007). In addition, they may result in increased power consumption of the cochlear implant device through increased electrical impedance.

Monitoring the impedance at the electrode contacts of a cochlear implant allows to evaluate the influence of these phenomena on the electrical stimulation field. In human temporal-bone and in vitro studies the relation between the electrical impedance and the location of the electrode contacts in the cochlea has been used to determine modiolar distance after insertion (Pile et al., 2016) and even in real-time during insertion, potentially providing a means for optimization (Aebischer et al., 2021; Giardina et al., 2018; Tan et al., 2013). Several animal studies have been performed to correlate the impedance measured at the contacts of an implanted electrode array to inflammatory effects due to insertion trauma and to the presence of inflammatory cells, fibrous tissue and new bone formation in the cochlea (Grill and Mortimer, 1994; Huang et al., 2007; Tykocinski et al., 2001; Wilk et al., 2016). This relation suggests that the impedance at the electrode contacts may be used as a measure for the effects of implantation-induced trauma (Clark et al., 1995; Xu et al., 1997), electrical stimulation (Tykocinski et al., 2001; Wilk et al., 2016) and medication eluting electrode arrays on these phenomena (Huang et al., 2007; Needham et al., 2020; Wilk et al., 2016).

The experience from animal experiments was extrapolated to human CI-recipients, interpreting electrode impedance changes during the first few days, weeks or months after surgery in terms of inflammatory reactions and tissue growth (Busby et al., 2002; Jia et al., 2011; Tykocinski et al., 2005). Using it as a measure of fibrous tissue growth following cochlear implantation, the role of the surgical technique on tissue growth and its consequences for the loss of residual hearing was investigated (Jia et al., 2011). Similarly, lower postoperative impedances with intraoperative steroid application were associated to reduction in tissue sheath formation around the electrode, whereas the evolution of the impedance at iridium-coated Contour electrodes provided an indication of faster tissue growth (Paasche et al., 2006). Electrode impedances were proposed as a biomarker for inner ear pathology (Shaul et al., 2019; Tejani et al., 2022), which could possibly predict future residual hearing loss (Choi et al., 2017). Also, a relation was observed between increased electrical impedances measured in a few cochlear implant patients with signs of upper respiratory infections, which were accompanied by increased subjective loudness sensations (Zarowski et al., 2020). Recently, the advantage of device switch-on within 24 h after surgery was argued on the basis of the postoperative evolution of electrode impedances measured at Slim Modiolar electrodes and the evolution of fibrous tissue presumably correlated with it (Sunwoo et al., 2021). Other work on (predominantly) Slim Straight electrodes, however, showed no influence of early device switch- on long-term impedances in a clinically meaningful manner (Saoji et al., 2021).

Although originally integrated into the CI-programming software as a means for device-integrity monitoring, the clinically available telemetry can also be used to study and monitor the electrical stimulation field within the cochlea. It measures the impedance at a single instance at the end of the first pulse phase of a 25-μs biphasic stimulation pulse, but it has the disadvantage that several phenomena are lumped into a single parameter per electrode contact, which makes it difficult to disentangle them. Several of the aforementioned studies, specifically those on animals, have therefore determined more than one voltage value during the stimulation pulse. Tykocinski et al. (2005) pointed out that measuring three points along the voltage waveform allows one to separate the polarization impedance (ZP), describing the electrochemical processes at the electrode–electrolyte interface and modelled by a resistor parallel to a capacitor, and the impedance of the cochlear tissues, modelled by an access resistance (RA). Newbold et al. (2010) have shown in an in vitro model of the electrode–tissue interface that protein adsorption was only reflected in ZP polarization impedance, whereas cell coverage of the electrode mainly contributes to RA. Still, RA accumulates the resistive effects along the current path from the vicinity of the electrode contact up to the reference electrode.

The impedance model of the cochlea proposed by Vanpoucke et al. (2004b) offers the possibility to further decompose RA into a component representing the near-field effect in the vicinity of the stimulation contact and a far-field component by using a lumped leaky-tube model for the electrical conduction throughout the cochlea. The input of the model consists of Electrode Voltage Telemetry (EVT) data, also called TransImpedance-Matrix measurements (TIM) or Electric Field Imaging (EFI), which measures the voltage at several instances along the stimulation waveform and not only at the stimulation electrode itself, but also at all the other contacts of the electrode array. EVT/TIM/EFI has proven successful in detecting electrode tip foldovers (Klabbers et al., 2021; Leblans et al., 2021; Vanpoucke et al., 2012; Zuniga et al., 2017). Also, it has been used to examine spatial channel interaction at a limited number of electrode contacts (Tang et al., 2011), to determine the importance of the facial nerve canal as a conduction path to the reference electrode (Vanpoucke et al., 2004a) and to show that increased access resistance rather than polarization resistance appears to drive the increase in the total impedances observed with delayed sudden loss of residual hearing (Tejani et al., 2022).

In spite of the available literature, a broader application of the more advanced impedance measurement options in the clinical setting was hindered for a long time because validated measurement protocols were not available in the CI-device programming software (Di Lella et al., 2020; Tykocinski et al., 2005). Di Lella et al. (2020) published a technical validation of a custom-made software for research in the clinical environment that allows to separate ZP and RA, but it did not include the measurement of the transimpedance matrix. Leblans et al. (2016) presented a clinical validation of the EVT research software for the Nucleus implant developed at CTC Cochlear Belgium, which has been implemented into Custom Sound–EP as the TransImpedance-Matrix (TIM) measurement option in the meantime.

The present article reports on clinical results of a longitudinal study using Cochlear's EVT software tool. The analysis explores the advantage of the decomposition of RA into a near- and far-field component for a better characterization of the postoperative impedance changes. As an additional tool to determine the location and origin of the changes, data were also collected using a 4-point probe technique for voltage measurements (Di Lella et al., 2020; Grill and Mortimer, 1994; Needham et al., 2020). The aim of the study is to contribute to a more solid basis for the use of impedance measurements as a biomarker for post-operative fibrous tissue formation in the clinical environment, considering also the effect of electrical stimulation on the impedance at CI-electrode contacts.