Retinal replacement--the development of microelectronic retinal prostheses--experience with subretinal implants and new aspects.

BACKGROUND: Progress in the field of microelectronics has led to the development of visual prostheses for the treatment of blinding diseases. Different concepts of retinal replacement are currently under investigation. The aim of the retinal prostheses is to replace the function of lost photoreceptors in degenerative diseases, such as retinitis pigmentosa.
METHODS: Within the field of visual prosthetic developments mainly two retinal based concepts are under investigation. One of the concepts is the epiretinal implant which acquires images of an external camera and after preprocessing by a computer reading this visual information into the human visual system. In the subretinal prosthesis design concept an array of stimulation electrodes is placed in the subretinal space. True to the concept the image falling on the retina and its light impulses are converted into electrical currents by microphotodiodes and the retina is stimulated with these locally. To test the feasibility of the concepts the biocompatibility and to determine basic stimulation parameters a lot of animal experiments and first human experiments were carried out.
RESULTS: Currently the research conducted by teams in Germany, the USA and Japan into epiretinal and subretinal implants has reached the stage where clinical trials can now be performed. Individual pilot studies were carried out for both the epiretinal and the subretinal implant by different research groups. DISCUSSION: The results achieved by the researchers indicate that cortical action potentials can be triggered by electric retinal stimulation with both concepts. The experimental work has highlighted a whole range of obstacles, not all of which have yet been fully resolved. These findings offer hope that coarse restoration of vision may be feasible by electrical stimulation.