Electronic Subretinal Implants Allow Patients to Read Letters and Form Words after Decades of Blindness
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(HealthNewsDigest.com) – This week the Royal Society London published a paper by Eberhart Zrenner and colleagues from Germany1 who showed that subretinal electronic implants allow patients blind from retinitis pigmentosa to localize and recognize objects again and even to read letters and form words after decades of blindness. This is a major breakthrough as it provides proof of concept that useful vision in daily life can be restored to a degree not shown previously. This development deserves a closer look.
About Retinitis Pigmentosa
Retinitis pigmentosa (RP) is one of the most common forms of inherited retinal degeneration affecting approximately 200,000 people in the U.S. and Europe.2 RP is a progressive condition with signs and symptoms often first appearing in childhood starting with night blindness, constricted visual fields, often ending in blindness; severe vision problems typically develop in early adulthood.3 There are currently no approved or commercially available treatment options that can restore vision, nor are there validated treatment options to retard the progression of the disease in most of RP-patients. However, researchers are making tremendous progress offering hope for the near future.
The main risk factor for RP is family history. For those affected, the retina which lines the back of the eye loses sensitivity because the light-responsive photoreceptor cells (rods and cones) carry the wrong genetic code. This adversely affects the production of proteins, leading to malfunction and eventual loss of the cells. It is a gradual process, which is why vision deteriorates over time (see below for diagrams that illustrate how the retina works).4
Initial signs and symptoms of RP are dependent on the nature of structural or functional changes induced by a certain mutation, e,g, whether rod or cone photoreceptor cells mutate first (both will eventually be compromised). In most forms of RP, rods are affected first. As rods are concentrated in the outer portions of the retina and are triggered by dim light, their degeneration affects peripheral and night vision. When the more centrally located cones responsible for color and sharp central vision become involved, the loss is in color perception and central vision (sometimes referred to seeing in tunnel vision). Most people with RP are visually seriously disabled by age 40, with a central visual field of less than 20 degrees in diameter5 and often even legally blind (visual field less than 5 degree or visual acuity less than 1/50, depending on country of residence).
Several clinical trials are in progress to investigate new treatments for RP. One of the most exciting and promising advances in the field for fully blind sufferers is the development of microchip, retinal implants that can restore vision to a low but useful functional level.
Rediscovering Vision with Retinal Implants
Retinal implants represent tremendous progress in the treatment of blindness caused by RP. Comprised of electronic chips, they are inserted in the eye of a blind person to generate artificial vision; this is a similar idea to the use of cochlear implants returning the gift of hearing to those impaired.
As with all research, there are different approaches to design and implantation of retinal implants. There are two main approaches scientists have taken in developing retinal implants that differ in several ways including placement of the chip, number of electrodes and image pixels produced by the chip and the mechanics involved in operating the chip.
There is emerging consensus among researchers that the subretinal (vs. epiretinal) approach is better positioned to produce positive results for patients. Subretinal implants involve implanting the chip below the retina, specifically below the macular region. The macular region is believed to be the ideal location because this is where light-sensitive photoreceptor cells are located which are responsible for producing clear images in normal-sighted people. It is also believed that the subretinal location provides more security in terms of keeping the implant from loosening and potentially causing damage to the retina. Moreover, as the image receiver under the retina moves exactly with the eye, localization by gaze and preventing fading of the picture is easily achieved.
In contrast, an epiretinal implant involves placing the chip right on the retina and requires several parts to work: a camera and transmitter mounted on the eyeglasses of the patient, an implanted receiver, electrodes secured to the retina with a tack to keep the device in place, and a image processing unit with a battery pack worn on the patient’s belt which powers the entire system. The camera captures images which are processed by the transmitter and receiver and turned into electrical pulses. The desired result is for the retina to respond to the pulses by perceiving them as patterns of light and dark spots which patients learn to interpret as meaningful images. Clinical trial results have yielded some success.
With the subretinal approach however, the only other tool required beyond the implant is an energy source: no camera outside the body is needed to interpret the image nor an external image processing unit. As such, the subretinal approach is a more direct replacement of vision than the epiretinal approach. Another significant difference is that the subretinal implant is designed to utilize substantially more electrodes: 1500 vs. 60. The increased number of electrodes allows for the light and dark images to appear sharper, making them easier to see.
Clinical Trial Results with Subretinal Implants
Retina Implant, the global leader in developing subretinal implants, began their first clinical trial on humans in November of 2005 in Tuebingen Germany; eleven people were implanted. The results were unprecedented and unlike patients who have received epiretinal implants, Retina Implant patients were able to see objects and shapes so clearly they could combine letters to form words, or essentially, read at a basic level as well as recognize foreign objects.
The clinical trial results are in line with research conducted by Perez et. al6 that found a minimum of several hundred electrodes are required to provide a level of vision that could enable a person to function independently in daily life. Retina Implant’s study is the first to implant patients with such a large number of electrodes and presents the first proof-of-concept that such devices can restore reasonable and useful vision in blind human subjects. It is important to note however, the ultimate goal of broad clinical application is not expected to be achieved in the near future, as a experience for longterm application has to be established in a larger study, as presently done by Retina Implant AG with a fully implanted device.
It is important to understand that the vision provided to implanted patients is quite different than the vision of a normal person. Lines and shapes of objects (table ware, mug, plate, shape of a tree or a banana) are typically all that can be expected to be seen initially by people with retinal implants, however scientists are finding that over time the human brain can retrain itself to interpret the lines into meaningful images (such as the clinical trial patients did when they could translate the lines they were seeing into letters and then into words). The ultimate goal is to restore vision to a point in which a person can perform normal, daily activities entirely independently.
Retina Implant began its second clinical trial this spring. The goal of this multi-site trial is to implant approximately 50 patients across Europe, as Prof. Zrenner, the coordinating principal investigator and Director of the Institute for Ophthalmic Research of the Tuebingen University has said. Of interest, scientists at MIT are also conducting clinical trials with their own version of a subretinal implant, however they do not anticipate implanting in humans until 2013.
Potential Future Applications of Retinal Implants
Retinal implants hold much promise for the blind community. In the future, patients who suffer from blindness due to other conditions like very advanced age-related macular degeneration (AMD) may be candidates for retinal implants.
Appendix: Images of the Retina, Cones, Receptors and Macula7
Light converges on the retina at the back of the eye.
Light shines through the vitreous humor and finally reaches the retina. The retina lines the inside of the back of the eye and records images in patterns of light and color. This layer of pink, mesh-like tissue is about the thickness of an onion skin and the size of a postage stamp.
The retina is covered with rods and cones, light-receptors that send electrical impulses through optic nerve fibers and to the brain.
The retina is made up of tiny, light-sensing structures called rods and cones. Chemical reactions in these ultra-sensitive cells transform light into electrical impulses. The impulses are processed in the inner retina and sent through the optic nerve to the brain to produce sight.
Visual acuity is strongest on the retina’s central spot, the macula.
The center of the retina, the area that lies directly behind the pupil, is called the macula. Visual acuity is strongest in the center of the macula, on a yellow-colored spot called the fovea. Light must be focused precisely on this area in both eyes to produce clear images. The major portion of the retina, the area surrounding the macula, provides peripheral, or side vision. Thus, central vision is sharpest and side vision provides less detail.
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