**1. Introduction**

Cochlear implants (CIs) are the most successful sensory prosthetic devices developed to date and have revolutionized the world of audiology, offering hope to individuals with severe to profound hearing loss [1–3]. However, while CIs significantly improve auditory perception for many, the technology is not without its challenges. One such issue is the unwanted stimulation of the facial nerve (FNS), a side effect reported in an estimated 5.6% of CI users [4]. FNS can lead to involuntary facial twitching, vertigo, or indistinct pain and thereby impact quality of life. To unlock the benefits of CIs for these individuals, we need to gain a better understanding of the contributing factors that lead to FNS and circumvent them.

**Citation:** Gärtner, L.; Backus, B.C.; Le Goff, N.; Morgenstern, A.; Lenarz, T.; Büchner, A. Cochlear Implant Stimulation Parameters Play a Key Role in Reducing Facial Nerve Stimulation. *J. Clin. Med.* **2023**, *12*, 6194. https://doi.org/10.3390/ jcm12196194

Academic Editor: Christof Röösli

Received: 26 August 2023 Revised: 18 September 2023 Accepted: 21 September 2023 Published: 25 September 2023

**Copyright:** © 2023 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

Traditionally, FNS in CI users has been managed through the adjustment of stimulation parameters by the audiologist. The most common type of stimulation for cochlear implants consists of biphasic pulses with a leading cathodic phase—a method derived from animal studies, which suggested its superior effectiveness [5,6]. These biphasic pulses are then amplitude modulated based on the time-varying envelope output from the filter linked to the corresponding electrode. The majority of CI manufacturers employ this paradigm. For a more comprehensive overview of CI functionality, readers are directed to [3]. To prevent unpleasant facial nerve stimulation, clinicians often lower the pulse current (and expand the pulse duration) or deactivate troublesome electrodes altogether. However, these methods do not always work, and even when they do, they can compromise CI performance. In severe cases, the CI becomes unusable.

Some CI manufacturers, such as MED-EL (Innsbruck, Austria), offer a 'triphasic' stimulation mode. In this mode, there are 3 pulse phases: (1) a leading cathodic pulse phase; (2) an intermediate anodic phase presented with twice the phase duration; and (3) a final repeated cathodic phase. While this method has shown effectiveness in reducing unwanted FNS for some CI patients [5,6], it does not always work and usually results in reduced battery life.

Emerging evidence from case reports suggests a promising treatment for severe cases is re-implantation with the Oticon Medical (OM) (Smørumnedre, Denmark) Neuro-Zti implant. Re-implantation with this device has been shown to effectively address FNS. However, the reason for this is not yet fully understood. While surgical adjustments such as varying array geometries and positioning may play a role, there are also distinct implant-related attributes to consider. These include the unique stimulation parameters used by OM devices, such as anodic leading pulses, passive capacitive charge return, and their unconventional distributed all-polar (DAP) grounding scheme [7]. Both elements are distinct from other CI systems. Moreover, in the OM device, loudness is not coded by pulse current but rather by pulse duration. Understanding whether the reduced FNS seen in the literature stems from surgical factors or from these implant-related ones will provide valuable insight to further improve CI technology and enhance patient outcomes and quality of life for those suffering from FNS.

This analysis presents a novel follow-up study on two CI subjects who experienced FNS prior to re-implantation with OM Neuro-Zti implants [8] and reveals the reasons why this intervention helped them.
