Steps toward the development of a chronic retinal implant

Summary form only given. This talk describes the efforts at MIT and the Massachusetts Eye and Ear Infirmary over the past 15 years to develop a chronically implantable retinal prosthesis. The goal is to restore some useful level of vision to patients suffering from outer retinal diseases, primarily retinitis pigmentosa and macular degeneration. We initially planned to build an intraocular implant, wirelessly supplied with signal and power, to stimulate the surviving cells of the retina. In this design electrical stimulation is applied through an epiretinal microelectrode array attached to the inner (front) surface of the retina. We have carried out a series of six acute surgical trials on human volunteers (five of whom were blind with retinitis pigmentosa and one with normal vision and cancer of the orbit) to assess electrical thresholds and the perceptions resulting from epiretinal retinal stimulation. The reported perceptions often corresponded poorly to the spatial pattern of the stimulated electrodes. In particular, no patient correctly recognized a letter. We hope that chronically implanted patients will adapt over time to better interpret the abnormal stimuli supplied by such a prosthesis. Experiences with both animals and humans exposed surgical, biocompatibility, thermal and packaging difficulties with this epiretinal approach. Two years ago we altered our approach to a subretinal design which will, we believe, reduce these difficulties. Our current design places essentially the entire bulk of the implant on the temporal outer wall of the eye, with only a tiny sliver of the 10 micron thick microelectrode array inserted through a scleral flap beneath the retina. In this design the entire implant lies in a sterile area behind the conjunctiva. We plan to have a wireless prototype version of this design ready for chronic animal implantation this Spring