Decrease in Electrode-Retina Distance over Time and its Effect on Electrical Impedances in a Suprachoroidal Retinal Prosthesis

Abstract

Purpose: The initial patient tests of a suprachoroidal retinal prosthesis prototype have shown success in both the surgical procedure and initial psychophysics testing. In this study, we investigated the relationship between electrode-retina separation and electrical impedances in a patient with an implanted suprachoroidal device.

Methods: Three patients with end stage retinitis pigmentosa (light perception vision) were implanted with a suprachoroidal retinal prosthesis prototype, which consisted of 24 electrodes. Optical coherence tomography scans were taken weekly, and high quality images were obtained from one patient (52yo female) with steady fixation. From these scans the array position, distance between electrode and retina, and retinal thickness above the electrodes were measured. Scans were taken both horizontally and vertically through all rows and columns of electrodes. Electrical impedances were measured weekly using a 500µs phase width and 20µs interphase gap biphasic pulse. The results of 50 pulses delivered at 500Hz were averaged.

Results: The average electrode-retina separation decreased gradually with time from an average of 422µm at the initial stimulation (55 days postop) to 362µm at 6 months postop (p=0.02). Average retinal thickness did not change significantly over this time (186µm to 184µm, p=0.71). The electrical impedances increased over this same time period, from an initial average impedance of 17.75kΩ to 20.08kΩ at 6 months postop (p<0.005). The decrease in electrode-retina distance was significantly correlated with the increased impedances (r=-0.59, p<0.005).

Conclusions: Following suprachoroidal retinal prosthesis implantation, the electrode-retina distance gradually decreases with time. The improved proximity of electrode to retinal tissue correlated with increased electrical impedances in this patient.

Keywords: 550 imaging/image analysis: clinical