THE RETINAL PROSTHESIS – THE STIMULATION THRESHOLD IS LOWER WITH A SUBRETINAL MICROELECTRODE ARRAY

Abstract

Abstract: : Purpose: To compare the visual cortex stimulation threshold of a microelectrode array (MEA) placed in either the suprachoroidal or subretinal space of the rabbit.Methods: Eighteen eyes of 17 rabbits were studied. Electrical evoked potential (EEP) electrodes were placed onto the dura mater above the visual cortex and forehead. The EEP was recorded following electrical stimulation of the MEA placed in either the suprachoroidal (9 eyes) or subretinal space (9 eyes) using an ab externo approach. 0.4 ml of liquid perfluordecaline was placed above the MEA to flatten the neurosensory retina after implantation in the subretinal space. The minimal electrical threshold to generate an EEP was determined for each eye in three consecutive trials with each trial containing over 100 averaged measurements. In 3 eyes the maximal EEP amplitude was determined by a successive increase in electrical stimulation energy. The enucleated eyes were processed histologically and examined with light microscopy.Results: The mean electrical threshold to generate an EEP after suprachoroidal placement was 469±217nC; after subretinal placement, 78±113nC (p<0.0005). In 6 of 9 eyes with subretinal placement, the MEA was placed beneath the myelinated nerve fiber layer and close to the optic nerve. In these eyes, the electrical threshold was 8±3nC – a log unit less than in the other eyes. The subretinal EEP amplitude reached a plateau around 10,000nC. Light microscopy after subretinal placement showed destruction of the outer segment layer presumably from operative damage.Conclusions: The electrical threshold required to generate an EEP in the rabbit is significantly less after subretinal placement compared to suprachoroidal placement. The finding of the lower thresholds in the subretinal approach is not unexpected given the normally high resistivity imposed by the retinal pigment epithelium. Thus, the subretinal approach is preferable because of the electrical benefits, even though the surgical approach to the suprachoroidal space is easier and less risky.

Keywords: electrophysiology: non–clinical • retina • visual cortex