A 16-channel investigational epiretinal prosthesis chronically implanted in blind patients allows them to perceive discrete phosphenes and perform visual spatial tasks.

According to a presentation at the Cannes Retina Festival 24th Annual Meeting of the American Society of Retina Specialists (ASRS) & 6th Annual Meeting of the European Vitreoretinal Society (EVRS), held in Cannes, France, five patients implanted with the device (Second Sight Medical Products, Sylmar, Calif) continue to use it daily. “These patients are using the prosthesis to perform tasks such as finding doorways, following the action in sporting events, navigating, following individuals, locating objects across the room and eating,” said Robert J. Greenberg, MD, PhD, president and CEO of Second Sight.

The concept behind the prosthesis, Dr. Greenberg said, is that by electrically stimulating the residual nerve cells in the retina, you can create perceptions of light that lead to image perception. The first-generation prosthesis has 16 independently controlled electrodes, so there are 16 individual contacts that may be activated. The array can produce visual percepts at charge densities within the safety limits for platinum electrodes.

SURGICALY IMPLANTED

Mark S. Humayun, MD, from the Doheny Eye Institute at the University of South Carolina, implanted six patients monocularly (Figure 1); the first patient received the implantation in February 2002 and the sixth patient in June 2004. All patients had vision of bare light perception or worse and nonrecordable electroretinograms, Dr. Greenberg said. Following a vitrectomy, the 5-by-6–mm silicone-platinum electrode array was introduced into the eye and tacked on the epiretinal surface.

“The average length of implant is 3 years, and total duration is 16 patient years. So, we have quite a bit of experience with these first implants,” he said. “Every patient was able to perceive discrete phosphenes and also perform visual spatial tasks. There have been no significant adverse events such as infections or retinal detachments, and equally as important, there have been no device failures over the 4 years.”

Dr. Greenberg added that the group had one partial explant in a heavy smoker.

The investigators have looked at long-term stimulation thresholds or how much current it takes to actually activate the device. “We saw the level increase in the first couple of [patients], which concerned us that there might be long-term stability problems. But, in our last three [patients], out to 1,000 days, those thresholds have been stable,” he said.

CAN IDENTIFY LIGHT

Patients with the implant (Figures 2 and 3) were asked to identify the location of a spot of light they saw when individual electrodes were activated. “We ask the patient to hold a ball in the 3-D space where they see the light in order to localize those individual spots and determine where in visual space they are seeing each individual electrode,” he said.

Dr. Greenberg’s group has also conducted performance tests with the patients. These are things such as finding objects, counting objects and recognizing the difference of objects such as a plate, knife and cup. “The patients performed well above what could be expected by chance. This is significant because these patients have no or barely any light perception, and for the first time they can perform significant visual tasks.”

A second-generation device is in development after several years of testing. This implant will have between 50 and 100 electrodes, and it is significantly smaller than the first-generation device, Dr. Greenberg said. “Similar to the first generation device it’s designed to last a lifetime. We are completing testing of that device now and are expecting to begin human trials later this year.”

INTERNATIONAL TRIAL OF NEW DEVICE

The new device will be studied in a multicenter worldwide trial with five sites in North and South America and five sites in Europe. The follow-up will be roughly 3 years. “We have begun signing up centers, but if anyone is interested in participating, we are interviewing additional centers.”

Second Sight has partnerships with institutions, universities and nonprofit organizations, including a number of US National Laboratories, The Doheny Eye Institute, The Salk Institute, UC Santa Cruz and The Alfred E. Mann Foundation.

The epiretinal prosthesis program is supported by the National Eye Institute and the Department of Energy.

Robert J. Greenberg, MD, PhD, is president and CEO of Second Sight, in Sylmar, Calif. He may be reached at bob@2-sight.com or 818-833-5050.

Greenberg RJ, Humayun MS, Hopkins J, et al. Results from a chronically implanted 16-channel epiretinal prosthesis in blind subjects. Presented at the Cannes Retina Festival 24th Annual Meeting of the ASRS & 6th Annual Meeting of the EVRS. Sept. 9-13, 2006. Cannes, France.