title: Factors associated with hearing loss in a normal-hearing guinea pig model of Hybrid cochlear implants.
abstract: The Hybrid cochlear implant (CI), also known as Electro-Acoustic Stimulation (EAS), is a new type of CI that preserves residual acoustic hearing and enables combined cochlear implant and hearing aid use in the same ear. However, 30-55% of patients experience acoustic hearing loss within days to months after activation, suggesting that both surgical trauma and electrical stimulation may cause hearing loss. The goals of this study were to: 1) determine the contributions of both implantation surgery and EAS to hearing loss in a normal-hearing guinea pig model; 2) determine which cochlear structural changes are associated with hearing loss after surgery and EAS. Two groups of animals were implanted (n = 6 per group), with one group receiving chronic acoustic and electric stimulation for 10 weeks, and the other group receiving no direct acoustic or electric stimulation during this time frame. A third group (n = 6) was not implanted, but received chronic acoustic stimulation. Auditory brainstem response thresholds were followed over time at 1, 2, 6, and 16 kHz. At the end of the study, the following cochlear measures were quantified: hair cells, spiral ganglion neuron density, fibrous tissue density, and stria vascularis blood vessel density; the presence or absence of ossification around the electrode entry was also noted. After surgery, implanted animals experienced a range of 0-55 dB of threshold shifts in the vicinity of the electrode at 6 and 16 kHz. The degree of hearing loss was significantly correlated with reduced stria vascularis vessel density and with the presence of ossification, but not with hair cell counts, spiral ganglion neuron density, or fibrosis area. After 10 weeks of stimulation, 67% of implanted, stimulated animals had more than 10 dB of additional threshold shift at 1 kHz, compared to 17% of implanted, non-stimulated animals and 0% of non-implanted animals. This 1-kHz hearing loss was not associated with changes in any of the cochlear measures quantified in this study. The variation in hearing loss after surgery and electrical stimulation in this animal model is consistent with the variation in human patients. Further, these findings illustrate an advantage of a normal-hearing animal model for quantification of hearing loss and damage to cochlear structures without the confounding effects of chemical- or noise-induced hearing loss. Finally, this study is the first to suggest a role of the stria vascularis and damage to the lateral wall in implantation-induced hearing loss. Further work is needed to determine the mechanisms of implantation- and electrical-stimulation-induced hearing loss.
authors.affiliation: Oregon Hearing Research Center, Department of Otolaryngology, Mail Code: NRC04, Oregon Health and Science University, 3181 SW Sam Jackson Park Road, Portland, OR 97239, USA. Electronic address: tanakach@hawaii.edu.
authors.forename: Chiemi
authors.initials: C
authors.lastname: Tanaka
authors.affiliation: Oregon Hearing Research Center, Department of Otolaryngology, Mail Code: NRC04, Oregon Health and Science University, 3181 SW Sam Jackson Park Road, Portland, OR 97239, USA. Electronic address: nguyanh@ohsu.edu.
authors.forename: Anh
authors.initials: A
authors.lastname: Nguyen-Huynh
authors.affiliation: Oregon Hearing Research Center, Department of Otolaryngology, Mail Code: NRC04, Oregon Health and Science University, 3181 SW Sam Jackson Park Road, Portland, OR 97239, USA. Electronic address: loerakatherine@gmail.com.
authors.forename: Katherine
authors.initials: K
authors.lastname: Loera
authors.affiliation: Oregon Hearing Research Center, Department of Otolaryngology, Mail Code: NRC04, Oregon Health and Science University, 3181 SW Sam Jackson Park Road, Portland, OR 97239, USA. Electronic address: starkg@ohsu.edu.
authors.forename: Gemaine
authors.initials: G
authors.lastname: Stark
authors.affiliation: Oregon Hearing Research Center, Department of Otolaryngology, Mail Code: NRC04, Oregon Health and Science University, 3181 SW Sam Jackson Park Road, Portland, OR 97239, USA. Electronic address: reiss@ohsu.edu.
authors.forename: Lina
authors.initials: L
authors.lastname: Reiss
mesh_terms: D000161:Acoustic Stimulation; D000162:Acoustics; D000818:Animals; D003051:Cochlea / Q000293:injuries; D019929:Cochlear Implantation; D003054:Cochlear Implants; D004195:Disease Models, Animal; D004558:Electric Stimulation; D016057:Evoked Potentials, Auditory, Brain Stem / Q000502:physiology; D006168:Guinea Pigs; D006198:Hair Cells, Auditory / Q000502:physiology*; D006309:Hearing / Q000502:physiology*; D034381:Hearing Loss / Q000175:diagnosis / Q000503:physiopathology* / Q000601:surgery; D006317:Hearing Loss, Noise-Induced; D008297:Male; D013136:Spiral Ganglion / Q000502:physiology*; D013997:Time Factors