Retinal implant

Retinal implants can to a certain extent take over the function of the photoreceptors destroyed by retinal degeneration in severely visually impaired or blind people, 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 work with their own camera.

What is the retinal implant?

The available retina 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 are transmitted by the ganglia, bipolar cells and nerves downstream of the retina instead of the signals from Photoreceptors can be further processed and sent to the visual centers of the brain.

The centers of vision ultimately generate the virtual image that we understand by “seeing”. As far as possible, the retina implants take over the function of the photoreceptors. Regardless of the technology used, retina implants are always useful if the ganglia, bipolar cells and nerve pathways to the brain and the visual pathways in the brain that are downstream of the photoreceptors are intact and can perceive their function. A basic distinction is made between subretinal and epiretinal implants.

Implants such as optic implants and others can ultimately be classified under the epiretinal or subretinal category, depending on the working principle. The subretinal implants use the natural eye to “obtain images” so that they do not need a separate camera. The epiretinal implants rely on an external camera that can be mounted on glasses.

Function, effect & goals

The most common area of ​​application for retina implants is in patients suffering from retinopathia pigmentosa (RP) or retinitis pigmentosa. It is a hereditary disease that is triggered by genetic defects and leads to retinal degeneration with breakdown of the photoreceptors. The almost same symptoms can also be caused by toxic substances or as undesirable side effects of drugs such as thioridazine or chloroquine (pseudoretinopathia pigmentosa).

In the case of RP disease, it is ensured that the downstream ganglia, bipolar cells and axons as well as the entire visual pathways are not affected, but rather retain their functionality. This is a prerequisite for the sustainable functionality of a retina implant. The use of retinal implants in age-related macular degeneration (AMD) is also being discussed among experts. The decision whether to use a subretinal or an epiretinal implant should be discussed in detail with the patient, considering all pros and cons. The most important distinction between a subretinal and an epiretinal implant is that the subretinal implant works without a separate camera.

The eye itself is used to generate electrical impulses on an implant area attached directly between the retina and choroid with the greatest possible number of photocells, depending on the incidence of light. The image resolution that can be achieved depends on how closely the photocells (diodes) are packed on the implant. According to the state of the art, around 1,500 diodes can be accommodated on the 3 mm x 3 mm implant. It can cover a field of view of around 10 degrees to 12 degrees. The electrical signals that are generated in the diodes, after being amplified by a microchip, stimulate the 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 glasses 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 any light impulses, but rather the image points that have already been converted into electrical impulses by the camera. Every single pixel is already amplified and located by a chip, so that the implanted stimulation electrodes receive individual electrical impulses which they pass on directly to “your” ganglion and “your” bipolar cell.

The transmission and further processing of the electrical nerve impulses to the virtual image, which the responsible visual centers in the brain generate, is analogous to healthy people. The aim of the implants is to give the best possible sight back to people who go blind because they suffer from retinal degeneration but have an intact nervous system and visual center. The retina implants used are constantly being technically developed in order to get closer to the goal of higher image resolution.

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Risks, side effects & dangers

The general risks such as infections and the risks of the necessary anesthesia when using a retina implant are comparable to those of other eye operations. Since the technology is a relatively new development, there is still no knowledge of whether specific complications such as B. Rejection of the material can occur by the immune system. No such complications have arisen in the operations performed so far.

The slight pain sensation on the day after the operation corresponds to the course of other interventions in the retina. A special feature and technical challenge with subretinal implants is the power supply. The power supply cable is led out from the side of the eyeball and runs further back in the area of ​​the temple 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 that no mechanical cable connection is necessary between the primary and secondary coil.

Subretinal implants have the advantage that they also use the natural eye movements, which cannot be the case with epiretinal implants with a separate camera. Both implant techniques have specific challenges to be worked on.