Visual prostheses: progress and new challenges

Research and development of visual prostheses have progressed substantially over the past two decades, with fourcompanies having implanted devices successfully in patients and accumulated substantial biocompatibility data.Despite the many challenges overcome to reach this stage, more remain, said Joseph F. Rizzo III, MD, Departmentof Ophthalmology, Massachusetts Eye & Ear Infirmary, Boston. He also is associate professor of ophthalmology at Harvard Medical School and co-director of the Boston RetinalImplant Project.

While much of the work in the first 20 years involved engineering the devices, one of the difficultiesresearchers now face is how to measure vision reliably in individuals who are severely blind and demonstrate anydifference between pre-implantation and post-implantation levels of vision, Dr. Rizzo said. A cloak of secrecynecessitated by commercialization of new implants also is frustrating to scientists who are unable to gainaccess to information, and many other issues also await resolution, he said.

"Ultimately, the real challenge is the demonstration that we can improve the quality of life for blind peoplewith a visual prosthetic device," Dr. Rizzo said. "Although I would say that we haven't yet definitively shownthat, there is good reason for optimism that in the next 5 or 10 years, we'll find that some patients chose touse visual prosthetics at the very least to allow them to walk safely in unfamiliar environments."

Researchers have studied several approaches to prostheses, including those based on the retina, optics, and thelateral geniculate. Retinal approaches have the potential to restore vision in instances where there isselective degeneration of the outer retina, such as age-related macular degeneration or retinitis pigmentosa.Research into this approach began in the 1980s, and now more than 20 groups worldwide are conducting research onretinal prostheses.

Engineering expertise has contributed substantially to the development of various experimental visualprostheses, Dr. Rizzo said, explaining that engineers' efforts in the face of many obstacles have made itpossible for several companies to achieve their goals of developing and implanting devices that electronicallystimulate the retina. Engineers have tackled issues such as designing implantable devices that will not damagethe retina, minimizing the amount of power sent into the eye, and creating or adapting materials that cansurvive in the optical environment.

To date, four companies have developed and implanted visual prostheses, and evidence indicates that eyes seem totolerate the placement of these devices very well and also tolerate electric stimulation for periods of monthsor years.

"There's no doubt that this approach in general can provide some level of vision to people who have beenseverely blind for decades," Dr. Rizzo said. "The question remains what the ultimate level of vision can be, andthat's probably an evolving story."

Other approaches to the treatment of neural forms of blindness are being investigated as well. They includetraditional means such as low vision rehabilitation, sensory substitution, molecular, transplantation, andtrophic factors. All are worth pursuing because it is impossible to know which ones are most likely to besuccessful, and the likelihood is that certain approaches will be better for certain diseases. Ultimately, thecollective body of research should be helpful to a wide range of patients, Dr. Rizzo said.

He also cautioned clinicians not to overlook traditional approaches to low vision such as rehabilitation, notingthat substantial progress with prostheses and other novel approaches may require decades and do not offerimmediate hope to the majority of patients.