Neil Osterweil

May 09, 2011

May 9, 2011 (Fort Lauderdale, Florida) — It's a far cry from replacing the human eye, but an implantable retinal prosthesis (Argus II, Second Sight Medical, Sylmar, California) has allowed 30 legally blind patients to see light as phosphenes, and has helped some improve their orientation and mobility in daily tasks, investigators reported here at the Association for Research in Vision and Ophthalmology 2011 Annual Meeting.

"The experience with Argus II subjects implanted is up to 73.4 years cumulatively, and the tests demonstrate that the Argus II, like the Argus I, continues to withstand long-term implants that allow us to study this interface," said Mark Humayun, MD, from the Doheny Eye Institute at the University of Southern California in Los Angeles.

"Our subjects are able to detect light and do well with certain performance parameters. We're continuing to study this in their daily lives and develop it into a rehabilitation program," he added.

The Argus II retinal prosthesis system couples a 60-electrode grid implanted in the retina with a spectacle-worn video camera and a visual processing unit that users can wear around the neck or waist. The visual information is sent wirelessly from the camera to the processing unit, which converts the information into pattern stimulation.

As of February 2011, 30 patients had received the device (9 women and 21 men; median age at time of implant, 58 ± 10 years [range, 28 to 77]). Of the 30, 29 have retinitis pigmentosa; the remaining subject has choroideremia. The average time patients have lived with the device is 2.4 years.

One of the devices required explantation at 14 months for recurrent conjunctival erosions; 1 had a communication failure at 10 months but is still in place.

"One of the things that concerned us very early on — and that we have addressed now — was the rate of endophthalmitis," Dr. Humayun said. "This was related to how you managed this device intraoperatively, as well as sterile practice techniques."

Visual Acuity Improved

With the 16-electrode Argus I device, the best visual acuity was about 20/4000. With the second-generation device, 27% of participants achieved a best visual acuity of 20/1260 with the system on (none achieved it with the system off).

In a grating visual acuity test in which patients are asked to touch a screen displaying an image of a square grating superimposed on black and white vertical bars, 8 subjects showed an improvement with the system switched on; with the switched system off, none did.

When asked to identify letters or numbers in 4 groups of increasing difficulty (L,T,E,J,F,H,I,U), 95.2% of subjects correctly identified the displayed character (shown 9.8 inches high on a screen) more often than chance with the system on in the first group; with the system off, 9.5% did. On/off performance with the other 3 groups of characters was, respectively, 78.9% vs 5.3%, 80.0% vs 10.0%, and 87.5% vs 0.0%.

In a test of orientation in which patients were told to follow a line on the floor by sight, all test runs were significantly better with the system switched on than when it was off. The line was painted on rather than taped to reduce the possibility that patients could feel their way along the course.

Users of the device have reported being able to find utensils on a table, locate parked and moving cars, find doors, avoid bumping into door frames, and navigate around new places in good lighting.

Dr. Humayun showed videos in which some patients were able to cross a street and negotiate a curb, and perceive colors on a screen. The videos included test runs in a variety of lighting and contrast situations.

Devices Evolving

A researcher working on a different device, the Boston Retinal Implant, told Medscape Medical News that although retinal prostheses are rapidly improving, they are still not ready for prime time.

"The field has moved forward in a very positive way, I think that each of these stylized testing methods demonstrate individual functional capacities, but for no group has it really yet been translated into practical day-to-day capabilities. The field just hasn't gotten there yet, but I think it's reasonably likely that it will," said Joseph Rizzo, MD, from the Massachusetts Eye and Ear Infirmary and Harvard Medical School in Boston.

The Argus II device has received approval for marketing in Europe; an application is pending in the United States.

The study is supported by Second Sight Medical Products. Dr. Humayun reports receiving research funding, holding a patent, and having a personal financial interest in the Argus II device. His coauthors report receiving either financial support or honoraria from the company. Dr. Rizzo has disclosed no relevant financial relationships.

Association for Research in Vision and Ophthalmology (ARVO) 2011 Annual Meeting. Abstract 2594. Presented May 3, 2011.