An analysis of the spread of electric field within the cochlea for different devices including custom-made electrodes for subtotal cochleoectomy

PLOS ONE RESEARCH ARTICLE An analysis of the spread of electric field within the cochlea for different devices including custom-made electrodes for subtotal cochleoectomy Luise Wagner ID*, Stefan K. Plontke, Torsten Rahne Department of Otorhinolaryngology and Halle Hearing and Implant Center, Martin-Luther-University HalleWittenberg, Halle (Saale), Germany * a1111111111 a1111111111 a1111111111 a1111111111 a1111111111 OPEN ACCESS Citation: Wagner L, Plontke SK, Rahne T (2023) An analysis of the spread of electric field within the cochlea for different devices including custommade electrodes for subtotal cochleoectomy. PLoS ONE 18(9): e0287216. https://doi.org/10.1371/ journal.pone.0287216 Editor: Sameh Attia, Justus Liebig University Giessen, GERMANY Received: December 6, 2022 Accepted: June 1, 2023 Abstract Objective Cochlear implants (CIs) can restore hearing not only in patients with profound hearing loss and deafness, but also in patients following tumour removal of intra-cochlear schwannomas. In such cases, design and placement differ from conventional electrode insertion, in which the cochlea remains filled with fluid. Despite these technical and surgical differences, previous studies have tended to show positive results in speech perception in tumour patients. The purpose of this study is to retrospectively evaluate the ability to predict speech recognition outcomes using individual electric field spreads and to investigate worldwide unique tumour cases. Study design In a retrospective analysis in a university tertiary center electric field spreads were compared between two groups of electrode designs implanted between 2009 and 2020 i.e., between lateral wall electrodes and custom-made perimodiolar electrode carriers from the same company. The voltage gradients were analysed and grouped with speech recognition results. Published: September 8, 2023 Copyright: © 2023 Wagner et al. This is an open access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. Results Data Availability Statement: Data relevant to this study are available at https://www.kaggle.com/ datasets/luiwag/cochlear-implant-impedance. Conclusion Funding: The author(s) received no specific funding for this work. Competing interests: The authors have declared that no competing interests exist. Differences in electrical field spreads were found between lateral wall electrodes and the custom-made perimodiolar electrodes, whereas a significant influence of electric fields on scores in speech recognition cannot be demonstrated. Prediction of speech recognition outcome based on electric field propagation results seems not feasible. Significant differences in field spread between electrode arrays can be clearly demonstrated. This observation and its relevance to hearing treatment and speech recognition should therefore be further investigated in upcoming studies. PLOS ONE | https://doi.org/10.1371/journal.pone.0287216 September 8, 2023 1 / 12 PLOS ONE SEF cochleoectomy Introduction Cochlear implants and their limitations Cochlear implants (CIs) can restore hearing not only in patients with severe to profound hearing loss and deafness, but also in patients after tumour removal of intra-cochlear schwannomas, that are benign. These tumours originate in the peripheral branches of the eighth cranial nerve and often require surgical excision, resulting in partial or subtotal cochlear removal [1]. Interestingly, affected patients who were treated with CIs can potentially demonstrate comparable outcome performance as single-sided deafened CI users [1, 2] and sometimes even better. To evoke a sound perception in CI users, cochlear nerve fibres are stimulated by electrical current pulses along the electrode array. Electrode contacts close to the modiolus as provided by so-called perimodiolar electrode arrays achieve a more precise stimulation of a smaller amount of nerve fibers compared to straight electrode carriers that deliver the pulses with greater distance to the modiolus [REF]. Apical electrodes of longer electrode carriers achieve more apical nerve fibers and therefore are supposed to achieve a better low-frequency hearing than shorter electrodes [REF]. The electrical impedance of the electrode-tissue interface including the cochlear fluids determines the absolute values of current flow within the cochlea. Thus, even in cochleae with regular anatomy the current spread can vary between a focussed stimulation of a certain array and an excitation of nerve fibres that can span about one third of the electrode array or even more [3, 4]. Unfortunately, this undifferentiated spread leads to channel interaction, restricts spectral resolution and all together leads to inaccurate auditory representation—still a limiting factor in current CI technology [5]. Therefore, the enhancement of speech and music perception based on improved spatial resolution in CIs is widely discussed [6, 7]. Theories of influences on the intra-cochlear current flow The CI induced current spread and flow within the cochlea was modelled in several studies (see review [8]). This is based on different influencing factors, e.g., electrical resistivity of the bone and tissue [9, 10], electrical conductivity of the modiolus encapsulation [9], and changes in electrode-electrolyte interface by charge injection [8, 11]. Other like Choi et al. [12] used finite elements to model electrical impedances across the electrode array based on the assumption that the distance between electrode array and modiolus stays constant along its progression. However, the validity of this assumption has been questioned in other studies [13, 14]. Nevertheless, using these insights, Mens [15] was able to show strong gradient related field spreads, where the apical to basal voltage propagation shows an increase in amplitude of about three times. These results strongly emphasize the necessity of further investigations into such mechanisms and effects. In addition to electrochemical and physiological factors one can hypothesize that the actual design of the CI electrode array influences the spread of electric field. During the last decades CI manufacturers developed and offered a variety of different electrode designs, e.g., long lateral wall arrays with the aim to cover the complete cochlear length and nerve fibres (MED-EL, Austria), shorter perimodiolar electrode arrays to decrease distance between carrier and nerve fibres (Cochlear, Australia), and others. There are study results comparing the difference in word recognition between perimodiolar and lateral wall electrode designs finally base on the different electrical conditions within the cochlea showing facilitated word recognition in perimodiolar arrays [7]. To measure electrical impedances and voltage spreads in vivo CI manufacturers offer (...truncated)