Effects of Implantation and Reimplantation of Cochlear Implant Electrodes in an In Vivo Animal Experimental Model (Macaca fascicularis)

Objectives: The objectives of this study were to evaluate the effect of reimplanting a cochlear implant electrode in animal normal-hearing cochlea to propose measures that may prevent cochlear injury and, given its close phylogenetic proximity to humans, to evaluate the macaque as a model for electroacoustic stimulation. Design: Simultaneous, bilateral surgical procedures in a group of 5 normal-hearing specimens (Macaca fascicularis) took place in a total of 10 ears. Periodic bilateral auditory testing (distortion product otoacoustic emissions and auditory brainstem evoked responses [ABR]) took place during a 6-month follow-up period. Subsequently, unilateral explantation and reimplantation was performed. Auditory follow-up continued up to 12 months, after which animals were sacrificed and both temporal bones extracted for histological analysis. Results: Implantation and reimplantation surgeries were performed without complications in 9 of 10 cases. Full insertion depth was achieved at reimplantation in four of five ears. Auditory evaluation: Statistically significant differences between implanted and reimplanted were observed for the frequencies 2000 and 11,000 Hz, the remaining frequencies showed no differences for distortion product otoacoustic emission. Before the procedure, average thresholds with click-stimuli ABR of the five animals were 40 dB SPL (implanted group) and 40 dB SPL (reimplanted group). One week after first implantation, average thresholds were 55 dB SPL and 60 dB, respectively. After 12 months of follow-up, the average thresholds were 72.5 dB SPL (implanted group) and 65 dB SPL (reimplanted group). Hearing loss appeared during the first weeks after the first implantation and no deterioration was observed thereafter. Differences for ABR under click stimulus were not significant between the two ear groups. Similar results were observed with tone-burst ABR. A 15 dB shift was observed for the implanted group preoperatively versus 1-week post surgery and an additional 17.5 dB shift was seen after 12-month follow-up. For the reimplanted group, a 20 dB shift was observed within the first week post reimplantation surgery and an additional 5 dB after 6 months follow-up. Statistical analysis revealed significant differences between the implanted and reimplanted ear groups for frequencies 4000 Hz (p = 0.034), 12000 Hz (p = 0.031), and 16,000 Hz (p = 0.031). The histological analysis revealed that the electrode insertion was minimally traumatic for the cochlea, mainly indicating rupture of the basilar membrane in the transition area between the basal turn and the first cochlear turn only in Mf1 left ear. Conclusions: With application of minimally traumatic surgical techniques, it is possible to maintain high rates of hearing preservation after implantation and even after reimplantation. Partial impairment of auditory thresholds may occur during the first weeks after surgery, which remains stable. Considering the tonotopic distribution of the cochlea, we found a correlation between the histological lesions sites and the auditory findings, suggesting that a rupture of the basilar membrane may impact hearing levels. The macaque was observed to be a functionally and anatomically an excellent animal model for cochlear implantation.

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