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Many patients with vision loss resulting from retinitis pigmentosa and other diseases that damage the outer cellular layer of the retina may someday be helped by the Artificial Silicon Retina retinal prosthesis (ASR, Optobionics Corp., Wheaton, Ill.), a tiny device now being studied. Preliminary findings of a 2-year safety and feasibility study show significant gains for some patients and more modest improvements for others.
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| Vincent Chow and Alan Y. Chow, MD, cofounders of Optobionics |
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The ASR is implanted in the subretinal space; it is the brainchild of brothers Alan Y. Chow, MD, and Vincent Chow, who cofounded Optobionics. Alan Chow conceived the idea and his brother created the silicon chip, which is about 2 mm in diameter, 25 µm thick, with approximately 5,000 microscopic solar cells on its surface.
Incoming light powers the ASR. Each photocell has an electrode that is fabricated on its surface, which produces tiny electric currents in the eye when light strikes the chip. The electrodes transfer these currents onto retinal cells.
"The purpose of the large number of microphotodiodes is to create a pixilized image, similar to that on a computer screen, focused on the implant," said Alan Chow, MD, Optobionics president and chief executive officer.
To date, six RP patients have been implanted with the ASR in a Food and Drug Administration-authorized safety study, slated to include up to 10 patients. The intent of the study was to see how well the silicon retinal implant would be tolerated, said Gholam A. Peyman, MD, who implanted several of the devices. "They wanted to know if the device is tolerated inside the eye, if it causes an inflammatory response, if it causes any cloudiness of the retina, if it is going to be rejected, or if it is going to be encapsulated," said Peyman, a professor of ophthalmology and codirector of vitreoretinal services at Tulane University Health Sciences Center, New Orleans.
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Dr. Alan Chow prepares the artificial silicone retina during surgery.
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"Therefore, these implants were not placed in the center part of the retina." Patients have been closely monitored during the 1- to 2-year follow-up and have shown no signs of rejection. "The implant is well-tolerated, because it did not cause any scarring of the retina or initiate any inflammatory response, either in that area or in the vitreous cavity," Peyman said. "In addition, it stayed in place and did not move."
So far, the six patients have had varying degrees of visual success with the device. Initially, some were able to see some light in the area where the device was implanted - this however, was just a transient response occurring at the beginning of a light pulse stimulus, Alan Chow found. "What was more exciting is it seemed as if the implant stimulated retinal cells in the surrounding area, as well improved their inherent function," he said. "Even though the implant was placed about 20° superior and temporal to the macula, some patients developed improved macular and peripheral retinal function." For example, some patients who had just minimal light perception to bright lights in a small sector of their visual field before surgery noticed brightening of their entire visual field in standard daylight conditions after the device was implanted, Chow said. He thinks the chip may be inducing a neurotrophic-type response in the retina, which may be waking up barely functioning retinal cells at a distance from the ASR chip.
Several patients have had significant visual gains. "One patient went from essentially no light perception to being able to see light in the inferior nasal sector of her visual field, which is where the implant projects, which then expanded to the left and central visual field," Alan Chow said. "She indicates that there are times when she can see shapes of people in front of her and can at times even tell if the person is heavyset or slim.
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| A clinical view of the artificial silicone retina in the subretinal space |
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Even though that is the extent of her vision, she's pretty happy." The patient who made the most significant gains was a man who was able to see his face in the mirror for the first time in 20 years. This patient was able to see two or three times as many letters on the ETDRS chart after receiving the ASR device. Others, however, have had much more modest improvements, although, percentage-wise, it is still great. "It is puzzling why some patients can see better and some have only slight improvement in light perception," Peyman said.
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| The whole artificial retina surgical team |
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Alan Clow suggests that it may be related to the level of their initial vision
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| A case of retinitis pigmentosa |
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"The study was not really intended to evaluate the efficacy of this implant; it was intended to look at its safety and, therefore, a lot more work needs to be done," Peyman said. If all goes well, the chip may be available in the next 5 to 10 years, he said. While early results are encouraging and progress is being made, it is important for investigators to proceed with meticulous care, under Food and Drug Administration protocol, he said.
Contact Information
Alan Chow: 877-277-2447, fax 630-690-3493
Peyman: 504-584-2460, fax 504-988-1390, gpeyman@tulane.edu
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