Retinal prosthesis, potential future approaches

Once a futuristic high-tech vision to cure the blind, retinal prosthesis has come to reality and has received Food and Drug Administration approval for patients with advanced retinitis pigmentosa and profound vision loss. It is fascinating to see that patients with visual acuity of bare light perception for many years, now with the help of retinal prosthesis, are able to recognize letters, detect motion and navigate. Those who have interacted with patients implanted with retinal prostheses know that how excited and grateful these patients are for a gain in visual acuity which may seem so trivial to most people. However, as more patients receive retinal prostheses, their expectations will increase, and they will demand a better visual gain from the device. Various research groups have developed many different types of retinal prostheses. Generally, all prostheses are composed of three major components, a video camera for capturing the images, an implanted microelectronic for converting the image data into a stimulus pattern and an electrode array for delivering the stimulus current to the retina. The major differences between various prostheses are in the packaging of the microelectronics and in the electrode array design. The electronic packages could be placed outside the orbit, around the eyeball or intraocular. The electrode array could be placed epiretinal, subretinal, suprachoroidal, intrascleral or episcleral. Each of these approaches has some advantages and limitations. From different components of vision such as visual acuity, visual field, motion detection, colour vision and contrast sensitivity, the first three are critical if we were to build a prosthesis that would satisfy essential needs of blind people. Higher visual acuity could presumably be achieved by a denser electrode array placed in contact with the retina or at its close proximity. This is challenging both in terms of microelectronic design and hermetic packaging, as well as placing the electrode array such that all the electrodes are equally at close proximity of the retina. Even thought it appears that epiretinal or subretinal electrode arrays have the highest chance of being close to the retina, because of some constraints that their discussion is beyond the scope of this paper, practically it is almost impossible that all the electrodes of an array be in contact with the retina. Moreover, the correlation of the number of electrodes of an array to the visual acuity has yet to be determined. For a blind person, improving mobility may be as important as acquiring the ability to read and write. In visually impaired people, mobility is directly related to the extent of visual field. Visual field, on the other hand, directly correlates to the size of the area of the retina that is being stimulated. Extrapolating from simulation studies on normal individuals, wide-field electrode arrays are expected to provide large visual field and better mobility. Although wide-field epiretinal electrode arrays have been developed, there are surgical and biological hurdles in implanting such arrays in the subretinal space. As the array becomes even larger, the same limitations also apply to epiretinal electrode arrays. An ideal retinal prosthesis should provide a highresolution vision and a large visual field, and should be tolerated in the eye without causing mechanical or chemical damage in long term. In addition, it should be implantable without requiring complex surgical techniques. For instance, a prosthesis that has 70% success rate in surgical implantation could not be an acceptable device. Likewise, if surgical implantation of a prosthesis is so complex that only few surgeons could safely perform, it would be far from an ideal device. The article by Saunders et al. in this issue of the journal is a nice effort in addressing this aspect of retinal prosthetic devices. Suprachoroidal, intrascleral or episcleral retinal prostheses may not theoretically be able to provide the highest possible visual acuity, but they are likely safer and surgically less challenging than subretinal or epiretinal prostheses. In addition, in these approaches it may be possible to provide a wide field of vision by simultaneously implanting multiple electrode arrays. As advances in technology will allow manufacturing more sophisticated retinal prostheses, the efforts would be towards providing a higher resolution vision and wider visual field to enable tasks such as