Chronic Electrical Stimulation Of The Retina Via Suprachoroidal Electrodes Is Safe

Abstract

 

Purpose

 

To assess the safety and efficacy of chronic retinal stimulation with a suprachoroidal electrode array.

 

Methods

 

6 normally-sighted feline subjects were stimulated above physiological and perceptual, but below aversive, levels with a medical grade, conformable, suprachoroidal electrode array (containing: 12 Ø600µm Pt electrodes; and 3 large Pt return electrodes). Charge balanced, biphasic, current pulses (100-600µA; 400µs; <86µC/cm2; ≤200Hz) were used. Retinal structure and function was assessed monthly with ERG, OCT and fundus photography. Electrode impedances were measured weekly and electrically evoked cortical potentials (EEPs) measured monthly to confirm that stimuli were activating visual cortex. At the end of 3 months continuous stimulation, an acute experiment was conducted and thresholds re-measured. Eyes were enucleated and prepared for histopathology. Randomized, blinded assessments were performed by 2 independent pathologists to compare stimulated and non-stimulated H&E stained 5µm sections of eye-tissue using contralateral eyes as controls.

 

Results

 

All subjects accepted the surgical and stimulation procedure with no evidence of discomfort or unexpected adverse outcomes. Electrode impedances were stable over the total implantation period (up to 6 months), with typical impedances between 10-30kΩ (compared with 5-10kΩ, measured pre-implantation in saline). Clinical eye assessments were normal. EEPs were recorded throughout the chronic stimulation period. The changes in threshold between consecutive measurements (Fig. 1) did not significantly differ from each other (one-way ANOVA; p>0.05). In 70/73 electrode-adjacent tissue samples examined, there were no discernible histopathological differences in the minor scarring, fibroblastic response, or foreign body response between stimulated and unstimulated regions. There was no necrosis, acute/chronic inflammatory response or secondary retinal damage in any of the samples examined (Fig. 2). In the other 3 tissue samples there were minor focal fibroblastic and acute inflammatory responses for which there was no obvious cause.

 

Conclusions

 

Chronic suprathreshold electrical stimulation of the retina using Pt electrodes in a planar silicone carrier located in the suprachoroidal space evokes a minimal tissue response and no adverse histological findings. Moreover thresholds and electrode impedance remained stable over the duration of the study.

 

 

Fig.1

 

 

Fig.2

 

Keywords: 637 pathology: experimental • 508 electrophysiology: non-clinical • 688 retina