Hearing Loss After Activation of Hearing Preservation Cochlear Implants Might Be Related to Afferent Cochlear Innervation Injury

Objective: Characterize hearing loss (HL) after hearing preservation cochlear implantation and determine the association between high charge electrical stimulation (ES) and late loss of acoustic hearing. Methods: A retrospective cohort analysis of all hearing preservation implantees at our center (n = 42) assayed HL as a function of maximum charge. We analyzed serial audiometry from 85 patients enrolled in the multicenter Hybrid S8 trial to detail the hearing loss greater than 1 month after implantation. Cochleotypic explant cultures were used to assess susceptibility to high levels of ES. Results: Early HL after implantation tends to be mild and averages 12.2 dB. After activation of the Hybrid S8 device, 17 (20%) of 85 patients experienced acceleration of HL. Compared with the majority of patients who did not lose significant hearing after activation, these patients experienced more severe HL at 1 year. Five patients implanted at our center experienced acceleration of HL after high charge exposure. In patients implanted at our center, high charge was associated with late HL (Pearson 0.366, p = 0.016). In rat cochleotypic explants, high voltage ES damaged afferent nerve fibers, reflected by blebbing and a 50% reduction in the number of fibers innervating the organ of Corti. In contrast, hair cells displayed only minor differences in cell number and morphology. Conclusions: Based on clinical and in vitro data, we theorize that the combination of acoustic amplification and ES in the setting of intact hair cells and neural architecture may contribute, in part, to cochlear toxicity, perhaps by damaging the afferent innervation.