The Food and Drug Administration (FDA) of the United States has approved the first retinal implant for use in the United States. This approval is for Second Sight’s Argus II Retinal Prosthesis System, which provides limited sight to those patients blinded by a rare genetic eye condition called advanced retinitis pigmentosa. This condition damages the light-sensitive cells that line the outer layer of the retina and causes them to die. This severely reduces vision and eventually leads to blindness.
Second Sight has devoted more than 20 years of research and development to the development of the Argus II Retinal Prosthesis. It has succeeded in two clinical trials, and the funding for the development of this device – more than $200 million – came from the National Eye Institute, the Department of Energy and the National Science Foundation. The remaining money came from private investors. European regulators approved the Argus II for use in 2011 and it has been used in 30 patients in clinical-trial patients since 2007. The Ophthalmic Devices Advisory Panel of the FDA unanimously recommended approval for the Argus II in September 2012.
The Argus II includes a small video camera, a video processing unit and a 60-electrode implanted retinal prosthesis with a transmitter mounted on a pair of eyeglasses. This device replaces the function of degenerated cells in the retina. It must be stressed that the Argus II does not fully restore vision, but it can improve a patient’s ability to perceive images and movement. It uses the video processing unit to transform images from the video camera into electronic data that is wirelessly transmitted to the retinal prosthesis.
Retinitis pigmentosa affects about one in 4,000 people in the US and about 1.5 million people worldwide. It kills off the retina’s photoreceptors, which convert light into electrical signals that are transmitted by means of the optic nerve to the brain’s visual cortex for processing. Second Sight plans to adapt its technology to assist people afflicted with age-related macular degeneration, which is a similar but more common disease.
Second Sight has plans to make the Argus II available later this year in clinical centers throughout the US. They want to establish a network of surgeons who have the skills to implant the device and, eventually recruit hospitals to offer it.
The Argus II is not the only retinal implant under development. A medical start-up company called Retina Implant AG uses a different approach in its device. In this case, the prosthetic device, the Alpha IMS Implant, is inserted beneath a portion of the retina. The three- by three-millimeter microelectronic chip (0.1-millimeter thick) contains ~1,500 light-sensitive photodiodes, amplifiers and electrodes. The Alpha IMS Implant is surgically inserted beneath a portion of the retina known as the fovea (which contains a rich concentration of particular photoreceptors known as cone cells) in the retina’s macula region. The fovea enables the highest clarity of vision for people to read, watch TV and drive. This chip helps generate at least partial vision by stimulating intact nerve cells in the retina. The nerve impulses from these cells are then fed by means of the optic nerve to the visual cortex where they create impressions of sight. The power source for the chip is implanted under the skin behind the ear and connected by a thin cable to the chip. In May the company announced its first UK patients for its latest trial. To date surgeons have implanted the Alpha IMS Implant prosthetic in 36 patients through two clinical trials over six years.
Researchers from Stanford University researchers are developing self-powered retinal implants in which each pixel in the device is fitted with silicon photodiodes. These sensors detect light, and control the output of a pulsed electric current. Patients would be required to wear a set of goggles for these devices that emit near-infrared pulses that transmit power and data directly to the photodiodes. Inductive coils that must be surgically implanted in the patient’s head to power these other retinal prostheses. This design was reported in May 2012 issue of Nature Photonics, and in the article, they described in vitro electrical stimulation of healthy and degenerate rat retina by photodiodes powered by near-infrared light.
Other researchers are utilizing yet another design for retinal prosthesis design. Researchers from Weill Cornell Medical College in New York City have deciphered the neural codes that mouse and monkey retinas use to turn light patterns into patterns of electrical pulses that their brains translate into meaningful images. Next they programmed this information into an “encoder” chip that was combined with a mini-projector to create an implantable prosthetic. This chip converts images that come into the eye into a series of electrical impulses, and the mini-projector then converts the electrical impulses into light impulses that are sent to the brain. With this approach, instead of increasing the number of electrodes placed in an eye to capture more information and send signals to the brain, this approach increases the quality of the artificial signals themselves, which improves their ability to carry impulses to the brain.