In the eyes of people with Usher syndrome, the photoreceptors, the rods and cones, slowly die.
Various types of implants are under development for recovering the eyesight in an artificial way. The various implants are found at another place in the visual system. This visual system runs from the eye to the brain. Studies with test persons are globally conducted in various clinics. The target group for this application generally consists of patients suffering from retinitis pigmentosa in an advanced phase.
DIRECTLY ON THE BRAIN (CORTICAL)
Various groups around the world are trying to make implants that can be directly connected with the area in the brain that is responsible for seeing (visual cortex). These in fact skip the eye and the optic nerve. The first cortical implants were implanted in completely blind people already in 1967 (Brindley and Lewin) and in 1976 (Dobelle), but these were successful only for a short time. The last few years, research has been done actively into the possibility of implants in the brain. However, at this moment these only include studies in the lab and with test animals. In the Netherlands the NESTOR group is working on this.
ON THE RETINA (EPIRETINAL)
The epiretinal implants work by means of glasses with a camera passing on images to the optic nerve which is connected with the brain. Just like with a cochlear implant, an electrode is first implanted in the retina of the eye. The glasses are connected with the electrode. Examples are SSMP’s Argus II and Pixium Vision’s Iris.
IN OR BELOW THE RETINA (SUBRETINAL) I
In case of subretinal implants a chip is placed in the retina. This chip is powered by means of a box behind the ear. Subretinal implants do not always require special glasses. The chip converts images coming in through the eye into electrical signals. These signals go through the optic nerve to the brain. An example of this is the Retina Implant Alpha AMS/IMS. However, the company Retina Implant is bankrupt, as a result of which this implant is not available for the time being.
Similar implants are being developed by the University of California in San Diego, USA. They make use of a grid of nanowires. However, this is still in the animal model research phase.
ARTIFICIAL RETINA (OPTOGENETICS)
Another therapeutic approach towards recovering the eyesight is the optogenetic retina. This therapy is intended for patients who have lost all of their photoreceptors.
The idea of optogenetics is not to recover the photoreceptors, but to transform other cells in the eye that have not been affected by the disease and to make them photosensitive. These include other cells (so no photoreceptors) that have been connected with the optical nerve. These cells are injected one-off with gene therapy, making them sensitive to infrared light. Transformed cells are not activated by natural light.
With special glasses camera images are converted into infrared images. This infrared light is sent to the transformed (optogenetic) cells. These cells then send a biological signal to the optical nerve.
A visual implant as described above converts images into pixels, which are sent by means of electronic signals. Optogenetics is a development that is like a retina implant, but as the intervention takes place at cellular level, this will hopefully produce a more refined visual image.
At this moment this study it still in an early innovative phase with animal models.
Additionally, mathematical researchers try to further develop the specific cameras for optogenetic retina for even more precise coding of spacial vision and creating an image with more contrast.