A new paradigm of hearing loss and preservation with cochlear implants: Learnings from fundamental studies and clinical research

Hearing preservation has shaped the cochlear implant (CI) industry for the greater part of the past two decades, being a topical and influential area of focus in research, surgical, audiological, and commercial sectors. Improvements in clinical and surgical aspects of the intervention and device technology have enabled expansion of candidacy to those with increasing levels of low-frequency acoustic hearing (LFH). For patients with functionally relevant acoustic hearing available post-operatively (Adunka et al., 2018), the additional benefits of combined electric & acoustic stimulation (EAS) may be accessed (Gantz et al., 2022; Gantz and Turner, 2003; Von Ilberg et al., 1999; Gifford et al., 2022; Plant and Babic, 2016).

Improvements in surgical practice and electrode array designs have resulted in moderate-to-high rates of preservation being commonly reported with both lateral wall and perimodiolar electrode arrays (Buchman et al., 2020; Helbig et al., 2016; Iso-Mustajärvi et al., 2020; Kant et al., 2022; Lenarz et al., 2013; Ramos-Macías et al., 2018; Roland et al., 2016; Shew et al., 2021; Woodson et al., 2021a). Large cohort studies from individual clinics have reported on high rates of long-term preservation of LFH (Jensen et al., 2021), with evidence for the rates of post-operative preservation also increasing over time, likely on account of continued improvements across the field (Perkins et al., 2021). Furthermore, in the subset of patients where LFH is maintained beyond 6-months post-activation there is evidence for long-term stability of LFH for 5 years post-activation, accompanied by continued use of EAS, and as long as 15 years in some cases (Roland et al., 2018; Gantz et al., 2018). Conversely, it is well established that in some patients, acoustic hearing decreases in the months to years following implantation (Kant et al., 2022; Nguyen et al., 2013; Snels et al., 2019; Wanna et al., 2018).

Preserving hearing with CI is a complex, multifaceted, and time-dependent endeavour. As such, there are three main phases from which preservation can be considered (Table 1), each of which warrant a combined effort across the multiple sectors within the CI industry to progress.

This paper reports on three major outcomes of the “Residual Hearing Research Program” (RHP). These outcomes correspond to efforts to identify and understand the underlying mechanisms of loss of LFH, i.e., Phase 1 of hearing preservation described in Table 1. Two of the outcomes to be described herein are the basis for the novel paradigm. This includes a systems approach to frame the many potential factors and assess the complex nature of post-operative acoustic hearing loss following CI. Then subsequently positions these factors along the timeline of CI and patient hearing history (CI timeline) which can then be evaluated in a stepwise manner against findings from the RHP to assess the potential role of the risk factors, identify gaps, and new hypotheses. The third outcome corresponds to key learnings about the role of the immune response, and more specifically, inflammation and the foreign body response (FBR) following CI surgery and stimulation. These outcomes will incorporate a review of findings in published work from the RHP, including those in this issue, and other relevant studies, and collectively used to test some fundamental questions; specifically, “what is associated with a measurable loss of acoustic hearing following CI surgery and use?”, “what is the causal relationship between factors and events along the CI timeline?” and most broadly “what induces a change from homeostasis within the cochlea?”.

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