Macular degeneration is an irreversible condition and the leading cause of blindness in people over the age of 60. In America alone, more than 15 million people are affected by this condition. When the nerve cells in the macula degenerate, it interrupts the pathway to the optic nerve and as a result, the signals that convert to visual images can no longer be received by the brain. The disease Retinitis Pigmentosa can also cause retinal degeneration in children and adults resulting in blindness. An estimated 4,000 Americans are affected by Retinitis Pigmentosa.
What is a Retinal Prosthesis?
Natural vision relies on millions of photoreceptor cells located in photoreceptors located within the outer layers of the retina. When macular degeneration occurs these layers are gradually lost, resulting in progressive visual loss. However, research has found that inner retinal layers such as RGCs are partially spared. In theory, therefore, by creating devices such as retinal prostheses, partial vision may be restored.
A retinal prosthesis is an implantable device that sends electronic signals to the optic nerve. These devices are designed to bypass the non-functioning retina in individuals with macular degeneration. When incoming light is received the information is processed in the form of electrical impulses and transmitted to the remaining inner layers of the retina, restoring enough visual function to perform the most common daily tasks. The retina is considered to be the best target for treating outer retinal diseases. The primary goal of artificial retinal prostheses is to provide patients with enough functional vision to perform normal daily activities.
How Does it Work?
A device is either placed epiretinally (on the surface of the retina) or subretinally (outside the retina). Epiretinal devices are easier to implant and less risky, however, subretinal devices may benefit from intact middle retinal layer processing pathways. Electrical impulses are delivered by micro-photodiode arrays or microelectrode arrays, each with distinct advantages.
Due to a substantial increase in chronic diseases such as diabetes in recent years as well as age-related causes, the number of visually impaired and blind patient cases are also increasing. Blindness related to systemic and ocular diseases as well as ageing-related macular degeneration will continue to increase unless innovative technologies and major research efforts are developed to overcome them. According to WHO, an estimated 161 million people worldwide are visually impaired. Of these 124 million have low vision and 37 million are completely blind. Research to restore functional vision in people affected by AMD and RP has been ongoing for the past 30 years but in recent years artificial retinal prostheses have reached major goals in this area. Retinal prosthesis devices convert visual information about the world into electrical signals to restore lost vision. This technique is considered a safe and viable option for those who suffer from visual impairment due to substantial loss of photoreceptors.
Retinal prostheses are considered to be the most advanced of all forms of visual prostheses and account for the most ongoing and completed clinical trials. As a sub-retinal approach, the active MPDA (Micro Photodiode Array) has been implanted in more human subjects to date than any other sub-retinal devices.
With a number of retinal prosthesis systems currently undergoing or nearing human clinical trials for retinal degenerative diseases, the future of retinal prosthesis devices holds great promise. Even though outcomes so far have been modest and limited to localization, high-contrast object recognition, basic navigation, and reading, the prospect of providing patients suffering from outer retinal diseases with enough visual function in these basic areas while looking forward to more dramatic outcomes in the future is worth investing in.