A Pre-clinical Model For Chronic Electrical Stimulation Of The Retina Via Suprachoroidal Electrodes

Abstract

Purpose: : To develop a model for assessing the safety and efficacy of chronically stimulating the retina at perceptually relevant levels with a suprachoroidal electrode array.

Methods: : Normally sighted adult cats were implanted, suprachoroidally, with anatomically conformable, medical grade, one-piece electrode arrays using a clinically viable approach. The array contained 12 active platinum (Pt) disc electrodes (600µm diameter), 2 Pt return electrodes (2mm diameter) and an extraocular Pt ball electrode. Extradural recording electrodes were implanted over the contralateral visual cortex. The lead assembly was tunnelled to an exteriorised connector. Following recovery the cats were fitted with wearable multichannel stimulators delivering biphasic current pulses at rates up to 200Hz for 3 months. Monopolar and common ground electrode geometries were used for stimulation and threshold measurements. Maximum charge density for active electrodes was 86µC/cm2. Capacitive coupling and electrode shorting were used to ensure charge balance. Stimulus intensities were above physiological and perceptual thresholds but below aversive levels. Eye health was assessed using electroretinography, optical coherence tomography and fundus photography. Individual electrode impedances were monitored weekly. Cortical evoked potential thresholds were measured regularly during the study. Multi-unit responses were recorded using multichannel ‘Utah’ arrays to confirm thresholds and verify the origin of the responses was primary visual cortex. Pathohistological assessments were performed using the contralateral eyes as controls.

Results: : Implant surgeries including extraorbital lead exit routing, lead tunnelling and fixation of recording electrodes were successful, lasting ~5 hours. Post-surgical complications were minimal and clinical assessments revealed normal retinal structure. Electrodes and lead assemblies remained viable throughout the study period. Impedances were 15-25kΩ at 2-weeks post-implantation (3-6kΩ pre-implantation in saline). Cortical evoked potential thresholds ranged from 200-600µA for current pulses of 400µs per phase. Suprathreshold stimulation was possible for electrodes within comfort levels.

Conclusions: : An animal model for assessing the safety and efficacy of chronic electrical stimulation of the retina has been developed. A robust one-piece electrode array and helical lead assembly was shown to be a viable approach for chronic suprachoroidal stimulation. Results of this study, including surgical techniques, safe stimulation levels and changes in impedances over time, will inform development of clinical retinal prostheses.

Keywords: electrophysiology: non-clinical • retinitis • pathology: experimental