Predicting Cochlear Implant Electrode Placement Using Monopolar, Three-Point and Four-Point Impedance Measurements

Abstract

This study aimed to investigate the relationship between cochlear implant (CI) electrode distances to the cochlea's inner wall (the modiolus) and electrical impedance measurements made at the CI's electrode contacts. We introduced a protocol for "three-point impedances" in which we recorded bipolar impedances in response to monopolar stimulation at a neighboring electrode. We aimed to assess the usability of three-point impedances and two existing CI impedance measurement methods (monopolar and four-point impedances) for predicting electrode positioning during CI insertion. Methods: Impedances were recorded during stepwise CI electrode array insertions in cadaveric human temporal bones. The positioning of the electrodes with respect to the modiolus was assessed at each step using cone beam computed tomography. Linear mixed regression analysis was performed to assess the relationship between the impedances and electrode-modiolar distances. The experimental results were compared to clinical impedance data and to an existing lumped-element model of an implanted CI. Results: Three-point and four-point impedances strongly correlated with electrode-modiolar distance. In contrast, monopolar impedances were only minimally affected by changes in electrode positioning with respect to the modiolus. An overall model specificity of 62% was achieved when incorporating all impedance parameters. This specificity could be increased beyond 73% when prior expectations of electrode positioning were incorporated in the model. Conclusion: Three-point and four-point impedances are promising measures to predict electrode-modiolar distance in real-time during CI insertion. Significance: This work shows how electrical impedance measurements can be used to predict the CI's electrode positioning in a biologically realistic model.

Electrocochleographic Patterns Predicting Increased Impedances and Hearing Loss after Cochlear Implantation

“…Four-point impedance measurements are a relatively new method to assess the local impedance within the cochlea (Bester et al 2020a; Sijgers et al 2022). This impedance measurement is conducted by passing current between two electrodes whilst the potential is recorded between two electrodes lying between the stimulating electrodes.…”

Human cochlear microstructures at risk of electrode insertion trauma, elucidated in 3D with contrast-enhanced microCT

“…Contrary to standard microCT imaging 28 , 31 , CECT enables clear visualization of cochlear soft tissues structures in their original shape, without the need for removal of the intracochlear fluid, which enables more accurate assessment of soft-tissue electrode insertion trauma. In addition, CECT imaging of the human cochlea has the potential to enable highly accurate modelling and testing of novel techniques for atraumatic electrode insertion, such as impedance measurements 82 , 83 and robotics-assisted insertion 31 . Finally, CECT holds a great promise for nondestructive, quantitative 3D study of cochlear pathologies, underlying hearing loss, which are currently mainly being investigated with classical histology 84 .…”

P11 - Detecting Cell Growth on Cochlear Implant Stimulation Electrodes by Impedance Spectrometry

“…Specifically, a change in the geometry of the stimulation electrodes [8], e.g. due to curvature of the CI in the cochlea or the distance to the wall of the cochlea [6], can influence the impedance. Therefore, measurements should be made over a certain frequency range in order to differentiate between these effects [9].…”

“…Thus, stimulation efficiency of the CI decreases and the patient's ability to hear deteriorates. However, the degradation of a patient's hearing can also have other reasons, such as an incorrect position of the CI inside the cochlea or a tip fold-over during insertion [6][7][8]. In case of a degradation of the hearing ability of a patient with CIs, it is important to know the cause at an early stage in order to intervene with an individual treatment.…”