Retinal implants can take over the function of photoreceptors destroyed by retinal degeneration in severely visually impaired or blind people to a certain extent, provided that the optic nerves and visual pathways of the brain are functional. Depending on the degree of destruction of the retina, different techniques are used, some of which use their own cameras.
What is the retinal implant?
Retinal implants are generally useful whenever the ganglia, bipolar cells, and nerve pathways to the brain downstream of the photoreceptors and the visual pathways in the brain are intact and able to perform their function. The available retinal implants, also known as visual prostheses, use different techniques but always aim to convert images of the central visual field into electrical impulses in such a way that they can be further processed by the ganglia, bipolar cells and nerves downstream of the retina instead of the signals from the photoreceptors and can be transmitted to the visual centers of the brain. The visual centers ultimately create the virtual image that we understand by “vision.” The retinal implants take over – as far as this is possible – the function of the photoreceptors. Regardless of the technique used, retinal implants are always useful if the ganglia, bipolar cells and nerve pathways to the brain downstream of the photoreceptors and the visual pathways in the brain are intact and can perform their function. In principle, a distinction is made between subretinal and epiretinal implants. Implants such as optic implants and others can also ultimately be categorized as epiretinal or subretinal, depending on the principle of operation. Subretinal implants use the natural eye for “image acquisition,” so they do not require a separate camera. Epiretinal implants rely on an external camera, which may be mounted on eyeglasses.
Function, effect, and goals
The most common application for retinal implants is in patients who have retinopathia pigmentosa (RP) or retinitis pigmentosa. This is a hereditary disease caused by genetic defects and leads to retinal degeneration with degradation of the photoreceptors. The approximately same symptoms can also be caused by toxic substances or as undesirable side effects of drugs such as thioridazine or chloroquine (pseudoretinopathia pigmentosa). RP ensures that the downstream ganglia, bipolar cells and axons as well as the entire visual pathways are not affected but retain their functionality. This is a prerequisite for the sustainable functionality of a retinal implant. The use of retinal implants for age-related macular degeneration (AMD) is also discussed among experts. The decision whether to use a subretinal or epiretinal implant should be discussed in detail with the patient, weighing all pros and cons. The most important distinction between a subretinal and an epiretinal implant is that the subretinal implant does not require a separate camera. The eye itself is used to generate electrical impulses on an implant area placed directly between the retina and the choroid with the greatest possible number of photocells, depending on the incidence of light. The image resolution that can be achieved depends on how densely the photocells (diodes) are packed on the implant. According to the state of the art, about 1,500 diodes can be accommodated on the 3 mm x 3 mm implant. A field of view of about 10 degrees to 12 degrees can thus be covered. The electrical signals generated in the diodes, after amplification by a microchip, stimulate the respective responsible bipolar cells by means of stimulation electrodes. The epiretinal implant cannot use the eye as an image source, but relies on a separate camera that can be attached to a spectacle frame. The actual implant is equipped with the largest possible number of stimulation electrodes and is attached directly to the retina. Unlike the subretinal implant, the epiretinal implant does not receive light pulses, but the pixels already converted into electrical pulses by the camera. Each individual pixel is already amplified and located by a chip, so that the implanted stimulation electrodes receive individual electrical impulses, which they pass directly to “their” ganglion and to “their” bipolar cell.The transmission and further processing of the electrical nerve impulses to the virtual image generated by the responsible visual centers in the brain proceeds analogously to healthy persons. The aim of the implants is to restore as good as possible the vision of people who go blind because they suffer from degeneration of the retina, but who have an intact nervous system and intact visual center. The retinal implants used are constantly undergoing technical development to get closer to the goal of higher image resolution.
Risks, side effects, and dangers
The general risks, such as infection and the risks of the anesthesia required, are comparable to those of other eye surgeries when using a retinal implant. Because the technology is a relatively new development, no evidence is yet available on whether specific complications, such as rejection of the material by the immune system, may occur. No such complications have occurred in the procedures performed to date. The slight sensation of pain on the day after surgery corresponds to the course of other procedures in the retinal area. A special feature and technical challenge of subretinal implants is the power supply. The power supply cable is led laterally out of the eyeball and runs in the area of the temple further to the back where the secondary coil is attached to the skull bone. The secondary coil receives the necessary current from the externally attached primary coil via induction, so no mechanical cable connection between the primary and secondary coils is necessary. Subretinal implants offer the advantage of also using natural eye movements, which may not be the case with epiretinal implants with a separate camera. Both implant techniques involve specific challenges that are being worked on.