‘Bionic’ Retinal implants
Retinitis pigmentosa (RP) affects approximately 1 in 4,000 worldwide, making it the most common inherited retinal degenerative disease. Light-detecting cells in the retina deteriorate, eventually leading to severe visual impairment, although individual progression rate differs.
Exciting research into electronic retinal implants have yielded encouraging results for RP patients.
Dr Thomas Edwards (Pictured) from the Nuffield Laboratory of Ophthalmology, John Radcliffe Hospital at the University of Oxford discusses recent innovations and his upcoming trial with this developing technology.
Research on electronic retinal implants has been ongoing for a decade at several centres around the world. Dr Edwards explains that the device he is testing is designed to replace lost retinal photoreceptor cells - the outermost light-sensitive layer of the retina. One inclusion criteria for patients is that the inner retinal layers must be intact in order to pass on the electrical signals from the implant to the brain.
IT'S A BIT LIKE ASKING A MOBILE PHONE TO WORK UNDER WATER.Dr. Thomas Edwards
A small microchip with 1,500 miniature electronic photodiodes is implanted below the retina. Microelectrodes then stimulate the overlaying retinal layers in response to incident light. The microchip is connected to an inductive coil, which is implanted beneath the skin behind the ear, much like a cochlear implant. The inductive coil is charged wirelessly by a second inductive coil placed on the overlying skin, using the same principle as an electric toothbrush charger.
Dr Edwards notes that one of the greatest challenges to the device is functional longevity. “It has to survive in a very hostile environment. It&rsquot;s a bit like asking a mobile phone to work under water,” he said. For functionality, the microchip must be exposed to light and be in contact with the overlying retina. It cannot be placed in a protective case. However, device survival rates have been promising with some patients reporting positive results as far out as a year post-implantation.
RP is a very individual disease with a wide spectrum of progression. The sub-retinal implant has been used in patients with no light perception vision, or only vague light perception vision, as this is the group of RP patients most likely to benefit from the device. Results with the first-generation subretinal device have been encouraging, with some patients able to discern shapes, while others have been able to read letters on a screen. Many of the implant recipients have had extremely low vision for many years, so it has taken some adjusting to get used to the visual input stimulated by the sub-retinal implant, with some improvement expected as patients learn to use the electronic chip. The current technology facilitates a fairly narrow field of vision, but Dr Edwards highlights the potential for multichip or larger chip devices in the future. “Research and technological development must be done in a step-wise fashion,” he cautions.
He also stresses the need for research participants to have high levels of motivation and an understanding that the device is being used for research in the UK, and not yet as a treatment. “It is the longest ophthalmology operation around, most of eight hours,” he said. A study of the latest sub-retinal chip will be getting underway later this year in Oxford. Potential participants should have no light perception or no ‘functional’ light perception vision in at least one eye (i.e. advanced RP) and must have good general fitness. Results from the first-generation implant were encouraging, prompting this planned new trial of the latest device. The trial will be open to UK residents only.
In addition to the subretinal implant Dr Edwards is researching, other approaches are being explored with a device that sits on top of the retina. He also notes that other strategies for RP include gene and stem cell therapy, which may lead to promising treatments.
The retinal implant is not yet designed to aid individuals with significant retinal tissue loss or scarring, including age-related macular degeneration (AMD). The device also is not suitable for diseases that affect the optic nerve, such as glaucoma. However, for the 1.5 million affected by RP worldwide, developments in electronic retinal implant research could be life-changing.
The Vision Institute - Retinitis Pigmentosa, Available here: http://www.institut-vision.org/index.php?option=com_content&id=204&Itemid=66&lang=en&limitstart=1
Will retinal implants restore vision? (Science) Available here: http://www.stat.ucla.edu/~yuille/courses/FIAT_Stat19/handout4b.pdf
Outer retinal degeneration: an electronic retinal prosthesis as a treatment strategy (JAMA) Available here: http://archopht.jamanetwork.com/article.aspx?articleid=416252
Visual perception in a blind subject with a chronic microelectronic retinal prosthesis (Vision Research). Available here: http://www.sciencedirect.com/science/article/pii/S0042698903004577