Auditory midbrain implant: Histomorphologic effects of long-term implantation and electric stimulation of a new deep brain stimulation array

HYPOTHESIS: Chronic implantation and electric stimulation with a human prototype auditory midbrain implant (AMI) array within the inferior colliculus achieves minimal neuronal damage and does not cause any severe complications. BACKGROUND: An AMI array has been developed for patients with neural deafness and, based on animal studies, has shown to possess potential as an auditory prosthesis in humans. To investigate the safety of the AMI for clinical use, we characterized the histomorphologic effects of chronic implantation and stimulation within its target structure, the inferior colliculus. METHODS: Eight cats were chronically implanted for 3 months, and histologic sections were analyzed to assess long-term tissue effects. Four of the 8 cats were additionally stimulated for 60 days (4 h/d) starting 4 weeks after implantation to assess if clinically relevant stimuli further affected the tissue response. RESULTS: In general, both neurons and neuropil surrounding the implant track were apparently unaffected, whereas a fibrillary sheath (∼50 μm thick) developed around the array. There was a significant decrease in neuron density 50 to 100 μm away from the track with a significantly elevated number of glial cells out to approximately 250 to 350 μm. Chronic stimulation seemed to improve the tissue response and neuronal survival around the implant, although further studies are needed to confirm this finding. CONCLUSION: The histomorphologic effects and extent of neuronal damage observed for our AMI array are similar to those of other neural implants currently and safely used in humans. The minimal tissue damage surrounding the implanted array is encouraging with regard to the safety of the array for human use.