Long-term histological and electrophysiological results of an inactive epiretinal electrode array implantation in dogs

PURPOSE. Short-term pattern electrical stimulation of the retina via multie lectrode arrays in humans blind from photoreceptor loss has shown that ambu lator; vision and limited character recognition is possible. To develop an implantable retinal prosthesis that would provide useful vision, these resu lts need to be sustained over a prolonged period of retinal electrical stim ulation. As a first step toward this goal, the biocompatibility and the fea sibility of surgically implanting an electrically inactive electrode array onto the retinal surface was tested. METHODS. A 5 x 5 electrode array (25 platinum disc-shaped electrodes in a s ilicone matrix) was implanted onto the retinal surface using retinal tacks in each of the 4 mixed-breed sighted dogs. Color fundus photography, fluore scein angiography, electroretinography, and visual evoked potentials were o btained preoperatively, at 1-week intervals for 2 weeks postoperatively, th en at 2-week intervals up to 2 months postoperatively, and thereafter at 1- month intervals. One dog was killed at 2 months after implantation and a se cond dog after 3 months of implantation. Histologic evaluation of the retin as was performed. The remaining two dogs continue to be followed beyond 6 m onths after the implantation surgery. RESULTS. No retinal detachment, infection, or uncontrolled intraocular blee ding occurred in any of the animals. Retinal tacks and the retinal array re mained firmly affixed to the retina throughout the follow-up period. Hyperp igmentation of the retinal pigment epithelium was observed only around the site of retinal tack insertion. No fibrous encapsulation of the implant or intraocular inflammation was visible. A- and b-wave amplitudes of the elect roretinogram were depressed at the first postoperative week testing but rec overed over the ensuing 1 week and were not statistically different from th e normal unoperated fellow eye throughout the postoperative period. N-1 and P-1 wave amplitudes of the visual evoked potentials were not significantly different from the normal fellow eyes at any of the postoperative test int ervals. Fluorescein angiography showed that the entire retina including the area under the electrode array remained well perfused. Similarly, histolog ic evaluation revealed near total preservation of the retina underlying the electrode array. CONCLUSIONS. Implantation of an electrode array on the epiretinal side (i.e ., side closest to the ganglion cell layer) is surgically feasible, with in significant damage to the underlying retina. The platinum and silicone arra ys as well as the metal tacks are biocompatible. With the success of implan ting an electrically inactive device onto the retinal surface for prolonged periods, the effects of long-term retinal electrical stimulation are now r eady to be tested as the next step toward developing a prototype retinal pr osthesis for human use.