Pilot Evaluation of Sheep as In Vivo Model for Cochlear Implantation

The rise in the use of cochlear implants (CIs) has continued to fuel research aimed at improving surgical approaches and the preservation of residual hearing. Current in vivo models involve small animals not suitable for evaluating full-sized CIs nor are prohibitively expensive nonhuman primates. The objective of this study was to develop and evaluate an in vivo model of cochlear implantation in sheep.

Eight adult, female sheep were implanted with full-sized CIs from three manufacturers using a retrofacial approach to the round window. Partial electrode insertions were performed to a depth of 10 to 12 mm before closure. Round window electrocochleography (ECoG) and auditory brainstem responses (ABR) were conducted during and after surgery. Following a 30-day implantation, cochleae were explanted and imaged using both x-ray microscopy and histology.

The surgery was well tolerated although limited complications were observed in three of eight sheep. Electrode insertions were up to 12 mm before insertion resistance noted. ECoG and ABR responses were reduced postimplantation, reflecting changes in cochlear mechanics due to the presence of the implant, and/or insertion trauma. Histological and radiological image analysis showed the presence of intracochlear fibrosis as well as one instance of tip fold-over.

The use of sheep presents a feasible live-animal model to study cochlear implantations. Full-sized implants as well as surgical techniques can be evaluated on functional outcomes such as ABR and ECoG as well as histological markers for residual hearing including intracochlear fibrosis. Use of this model and surgical approach has potential to evaluate CIs and surgical techniques in both the acute and chronic setting.