Artificial silicon retina microchip improves vision in some RP patients

Key Points

Decatur, GA-An artificial silicon retina microchip (ASR, Optobionics) improves visual function in some patients with retinitis pigmentosa (RP) for up to 24 months after implantation, said Ronald A. Schuchard, PhD, on behalf of the ASR Study Group. He is director, Rehabilitation Research and Development Center, Veterans Affairs (VA) Medical Center, Decatur, GA. Results came from a phase II clinical trial.

The microchip is designed to stimulate damaged retinal cells, allowing them to resume sending visual signals to the brain. The silicon chip is 2 mm in diameter and 25 μm thick and contains approximately 5,000 microphotodiodes, each with its own stimulating electrode. The microphotodiodes convert the light energy into electrical impulses that stimulate the remaining cells of the retina to provide a neurotrophic response.

In this trial, the microchip was randomly implanted in 10 right eyes and 10 left eyes of 20 subjects; the fellow eyes were used as controls. The subjects included 11 men and nine women aged 32 to 69 years. Baseline ETDRS visual acuity range was 20/80 to greater than 20/1600 in both the treated and fellow eyes. The trial was conducted at the Atlanta VA Rehabilitation Research and Development Center and Emory University in Atlanta, Johns Hopkins University in Baltimore, and Rush Medical Center in Chicago.

In addition, one case of retinoschisis, or macular hole, occurred in an implant eye. Events unrelated to implantation included an ovarian cyst surgery and one patient death.

Initial results

ETDRS and grating acuity testing were used to assess visual function. Contrast was evaluated using grating contrast sensitivity (not performed at baseline or before implantation), color contrast, and the Pelli-Robson chart at one site only. Visual field tests including scanning laser ophthalmoscopy, threshold testing with the 30-2 algorithm (Humphrey Field Analyzer, Carl Zeiss Meditec), and Goldmann V4e.

Functional vision tests included the Minnesota Low-Vision Reading Test projection with randomized letter size. "Unfortunately, with the fact that presentation of sentences with different letter sizes was randomized, we think that we introduced some variance into the testing, so it was not significant," Dr. Schuchard said.

Self-report questionnaires were used as well. The VA LV VFQ-54 identified two subjects with a positive response, 17 with no response (indicating no significant difference from baseline), and none with a negative response. The Turano Mobility Questionnaire revealed eight subjects with a positive response, 11 with no response, and zero with a negative response. Some of the items on these questionnaires were irrelevant to the everyday activities of the subjects, however, Dr. Schuchard said.

Mobility testing was conducted at several sites and included controlled lab testing and real environment testing. As with some of the other tests, mobility testing ultimately was not sensitive to the changes experienced by these patients, Dr. Schuchard said.

At the 1-year evaluation, two confounding factors were discovered: pupil dilation during vision tests and cataract progression. These factors increased glare and reduced contrast sensitivity, which may have affected visual function outcome measures in some participants.

"We propose that, potentially, this impacted all visual function measurements that we were doing in the phakic and pseudophakic patients," Dr. Schuchard said.

Increased glare and reduced contrast sensitivity also were found in patients who experienced cataract progression, which potentially affected all visual function measurements in the phakic patients and led to the possibility of IOL implantation.

Those findings, and the lack of a standard definition for a clinically significant treatment response for RP patients, led the investigators to devise their own meaning for the term. They concluded that >2 lines of vision improvement on the ETDRS chart constituted a clinically significant response, and >1 line of vision improvement was a statistically significant treatment response.

Subgroup analyzed

The potential negative effects the confounding factors had on glare and contrast sensitivity prompted investigators to separately analyze data from a subset of 10 subjects who had at most one confounding factor.