Retinal prosthesis offers hope to blind patients

An artificial retinal prosthesis allows patient with retinitis pigmentosa (RP) to locate objects, detect movement, and improve orientation and mobility skills.

The Argus II Retinal Prosthesis System (Second Sight Medical Products), which was approved by the U.S. Food and Drug Administration in 2013, is limited to RP patients who are currently no light perception (NLP) or bare LP.

How the device works

The device consists of:

• A 60-electrode array surgically implanted over the macula (Figure 1) driven by a neurostimulator attached to the outside of the sclera

• An inductive coil link used to transmit power and video data to the implant

• An external belt-worn video processing unit (VPU)

• A miniature video camera mounted on a pair of glasses

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The video camera captures visual information and relays it to the VPU. The VPU in turn digitizes the signal in real time and creates a series of stimulus pulses. These pulses are then transmitted to the microelectrode array on the retina where they stimulate the remaining cells in the retina. The nerve cells in the retina transmit the visual information along the optic nerve to the brain, creating the perception of patterns of light. The patient then must learn to interpret these visual patterns, regaining some functional vision.

The prosthesis itself is implanted surgically, typically by an experienced vitreoretinal surgeon, using existing surgical techniques. Median surgery time is approximately three hours. After the implantation, most patient need to work with occupational or low-vision rehabilitation specialists for up to three months to understand the device and how to use it to achieve their goals.

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Looking at results

A three-year study evaluated the safety and visual function in 30 subjects with bare LP vision only in 10 centers in Europe and the United States. The study included 29 patients with RP and one with choroideremia. The Argus II system was implanted in a single eye (typically the worse-seeing eye). Subjects served as their own controls comparing the implanted eye vs. fellow eye as well as their functioning with the system on vs. off.

Of the 30 subjects, 29 had functioning Argus II systems three years after implantation. Some 63.3 percent of the subjects (19/30) had experienced no device- or surgery-related serious adverse events. After three years, a total of 23 serious adverse events occurred in 11 subjects, all of which were treated with standard ophthalmic approaches. One subject had the device removed due to recurrent conjunctival erosion.

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Visual function results indicated that 89 percent of the subjects performed significantly better with the system on than off on an object localization task. The mean acuity values for those patients that showed improvement was 2.5 logMAR, which represented an average gain of four letters on the Early Treatment Diabetic Retinopathy Study (ETDRS) chart for these patients.

Further, orientation and mobility testing demonstrated that patients were able to perform practical tasks with more success with the system on than off three years after implantation. In addition, an assessment of function and well-being found that 65 percent of the subjects received benefit from the system at three years, with none affected negatively.

Advances in helping patients see

Argus II represents a significant step forward in returning some function to eyes with devastating visual loss from RP. While implanted in only a relatively small number of patients, the safety profile seems acceptable with the vast majority of patients benefiting from the system. Ongoing research and technology will no doubt create more options for eyecare practitioners to help even more patients in the future.

Reference

1. Ho AC, Humayun MS, Dorn JD, da Cruz L, Dagnelie G, Handa J, Barale PO, Sahel JA, Stanga PE, Hafezi F, Safran AB, Salzmann J, Santos A, Birch D, Spencer R, Cideciyan AV, de Juan E, Duncan JL, Eliott D, Fawzi A, Olmos de Koo LC, Brown GC, Haller JA, Regillo CD, Del Priore LV, Arditi A, Geruschat DR, Greenberg RJ; Argus II Study Group. Long-Term Results from an Epiretinal Prosthesis to Restore Sight to the Blind. Ophthalmology. 2015 Aug;122(8):1547-54.

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