Near-infrared light protects residual hearing by preserving spiral ganglion neurons in an animal Model of CI Surgery

INTRODUCTION: Given that today's cochlear implant (CI) candidates frequently present with substantial residual low-frequency hearing, the preservation of residual hearing is becoming an increasingly crucial but as yet inadequately resolved aspect of CI surgery. A recent study has shown that a 15-minute treatment with light in the red to near-infrared range (NIR) before the insertion of a CI array resulted in a significant reduction of hearing loss, especially in the frequency range of residual hearing, accompanied by significant conservation of outer hair cells. Given that the total number of rescued hair cells was insufficient to account for the substantial reduction in hearing loss observed, the present study aims to investigate the potential protective effect of NIR on spiral ganglion neurons and ribbon synapses in the context of CI surgery. METHODS: Guinea pigs underwent initial auditory brainstem response (ABR) recordings separately for each ear. Subsequently, unilateral NIR irradiation of the cochlea was performed, followed by bilateral insertion of guinea pig-sized electrode arrays. After 4 weeks, the ABR measurement was repeated and the cochleae were collected for histological analysis of spiral ganglion neurons and ribbon synapses. RESULTS: A significant reduction of hearing loss was found on the NIR treated side across the entire frequency range tested. The number of ribbon synapses was not affected by the insertion of an electrode array, and could therefore, not be influenced by NIR treatment. Spiral ganglion neurons were significantly reduced 4 weeks after implantation. This effect was rescued by NIR treatment, specifically in the basal portion of the cochlea. CONCLUSION: These findings show a neuroprotective effect of NIR on SGNs, which may be crucial not only for the protection of residual hearing but also for the electrical stimulation of SGNs via a CI.