What is a retinal prosthesis?
A retinal prosthesis is a non-living, electronic substitute for the retina. Popular and brand names for retinal prostheses are the Bionic Eye and the Argus II. The aim is to restore vision to someone blinded by retinal eye disease. A retinal prosthesis is different from an implanted lens or a low-vision device, which works to maximize a person’s existing vision.
What does it do?
In people with advanced retinal disease, the light-capturing cells of the retina, called photoreceptors, have been lost, but the network of nerves that sends visual information to the brain often is intact. A retinal prosthesis bypasses the photoreceptors and sends visual signals to the brain.
Will it restore my vision?
The prostheses that have been tested so far do not provide natural sight. For example, people who are using them are able to recognize a doorway or the shape of a person, or in some cases can make finer distinctions, such as the difference between a fork and a spoon. These retinal prostheses provide a “simulation of sight” which means that the users have to re-learn how to see. Their brains need to learn how to interpret this new kind of information.
Who could use it?
Retinal prostheses are intended for people who are blind or have only minimal light perception, but who once had sight. With prostheses, the brain must interpret the device’s signals. Someone blind from birth never developed this capacity, and therefore it might not benefit them.
Are any approved in Canada?
Yes. The Argus II Retinal Prosthesis is approved by Health Canada. It is also approved in Europe and the USA. The Foundation Fighting Blindness played a key role in bringing the Argus II (sometimes called the Bionic Eye) to Canada by helping to fund an observational clinical trial of the device at the Toronto Western Hospital led by Dr. Robert Devenyi.
What will it cost?
The Argus II Retinal Prosthesis is now being marketed in Europe for about $100,000 USD, plus the cost of the surgery to implant it. Second Sight (the manufacturer) is actively seeking coverage of the device through public insurance or government subsidies. The costs of other retinal prostheses are not yet known.
How does it work?
Just as there are multiple kinds of smart phones, there are different approaches to this technology.
Camera + Epi-Retinal Chip
The Argus II by Second Sight is the leader in this category. It captures images with a mini-camera embedded in glasses that also carry a battery pack. A 2D array of many tiny electrodes is implanted surgically on the front surface of the retina (epi-retinal). Images from the camera are converted into electrical pulses sent wirelessly to the implant. The pulses stimulate the retina’s remaining cells to send patterns of nerve impulses, representing the images, along the optic nerve to the brain. Patients can learn to interpret the patterns and regain some functional vision. Most of the people who have received an Argus II implant have had some visual perception restored, allowing them to better orient themselves in a room or negotiate daily tasks. There appear to be significant variations in results between users.
The Intelligent Retinal Implant System is another camera/chip combo, similar to the Argus II. It is in clinical trials in Germany and the UK. Bionic Vision Australia is also working on a similar product.
Retinal Implant AG has created a sub-retinal implant, which sits behind the retina instead of in front of it. This electronic chip contains tiny photocells to capture light, amplifiers to boost their signal, and electrodes to stimulate retinal nerve cells. Since photocells are part of the chip, the device does not need an external camera, and the sub-retinal placement should be more secure and stable than the epi-retinal option; but more complicated surgery is required to implant it. Clinical trials of this device are ongoing in Germany, Italy and the UK, and in the USA.
Other groups developing chips include Artificial Silicon Retina Microchip, the Boston Retinal Implant Project, and Nano Vision although the later two are not yet at the human trial stage.
Sub-retinal chips may allow somewhat higher resolution images than epi-retinal chips. However, since even the tiniest electrodes in these prostheses are bound to stimulate more than one retinal cell, so the wearer’s visual acuity may never approach normal sight. This limitation has led to hybrid strategies, in which remaining retinal nerve cells are made light-sensitive and then stimulated by patterns of light instead of electricity.
Encoding Neural Signals
Dr. Sheila Nirenberg of Cornell University is one of several researchers, who are developing this new hybrid approach to prosthetics. In Dr. Nirenberg’s studies, a camera sends images to a computer, which measures local differences in intensity across the image and encodes this information in pulses of light that mimic the natural ‘language’ of the central nervous system. The size of these pulses of light can be smaller than the smallest retinal nerve cells; they can be projected through the pupil onto individual retinal cells. Using a new approach called optogenetics, a form of gene therapy endows these nerve cells with the ability to respond directly to light, so that the computer-generated light pulses stimulate them to send high-resolution, ‘realistic’ image representations to the brain. This approach is being tested in animals. If it proves to be effective, it should provide much higher-quality images and a more natural visual experience. Dr. Gautam Awatramani at the University of Victoria is one scientist funded by the Foundation Fighting Blindness donors to study similar therapies.
Direct to Brain Prothesis
Scientists at the Monash Vision Group in Australia have developed a different type of vision prosthesis. It avoids the retina altogether. This device uses a video camera to capture images and send its electronic signals directly to the visual cortex of the brain.
While brain surgery sounds like a more difficult, and risky option, the surgery required is relatively straightforward. More importantly, if it is successful, the device could have some important advantages. For example, it could help people with retinal degenerative disease, but it might also help people whose optic nerve has been damaged due to glaucoma or injury
As well, this prosthesis would not be implanted into the retina and thus would not block or damage retinal tissue. So the prosthetic could be used to augment vision for people with some remaining sight, and would not impair their remaining vision.
The Monash Vision Group and its partners have committed to having their “direct to brain bionic eye” ready for first patient tests very soon.
Updated May 18, 2016: Dr. Mary Sunderland, Director of Research & Education, Foundation Fighting Blindness. Initially reviewed by Dr. Bill Stell, Professor of Cell Biology and Anatomy, at the University of Calgary and Dr. Gautam Awatramani, University of Victoria.