Comparison of electrically evoked cortical potential thresholds generated with subretinal or suprachoroidal placement of a microelectrode array in the rabbit

The aim of the study was to directly compare the threshold electrical charge density of the retina (retinal threshold) in rabbits for the generation of electrical evoked potentials (EEP) by delivering electrical stimulation with a custom-made microelectrode array (MEA) implanted into either the subretinal or suprachoroidal space. Nine eyes of seven Dutch-belted rabbits were studied. The electroretinogram (ERG), visual evoked potentials (VEP) and EEP were recorded. Electrodes for the VEP and EEP were placed on the dura mater overlying the visual cortex. The EEP was recorded following electrical stimulation of the MEA placed either subretinally beneath the visual streak of the retina or in the suprachoroidal space in the rabbit eye. An ab externo approach was used for placement of the MEA. Liquid perfluorodecaline (PFCL; 0.4 ml) was placed within the vitreous cavity to flatten the neurosensory retina on the MEA after subretinal implantation. The retinal threshold for generation of an EEP was determined for each MEA placement by three consecutive measurements consisting of 100 computer-averaged recordings. Animals were sacrificed at the conclusion of the experiment and the eyes were enucleated for histological examination. The retinal threshold to generate an EEP was 9 +/- 7 nC (0.023 +/- 0.016 mC cm(-2)) within the subretinal space and 150 +/- 122 nC (0.375 +/- 0.306 mC cm(-2)) within the suprachoroidal space. Histology showed disruption of the outer retina with subretinal but not suprachoroidal placement. The retinal threshold to elicit an EEP is significantly lower with subretinal placement of the MEA compared to suprachoroidal placement (P < 0.05). The retinal threshold charge density with a subretinal MEA is well below the published charge limit of 1 mC cm(-2), which is the level below which chronic stimulation of the retina is considered necessary to avoid tissue damage (Shannon 1992 IEEE Trans. Biomed. Eng. 39 424-6).