"The major advance of the subretinal approach is that the implant itself is light sensitive," says Robert MacLaren, a consultant vitreoretinal surgeon and professor of ophthalmology at University of Oxford's Merton College. MacLaren, who specializes in treating patients with AMD, retinitis pigmentosa, choroideremia and Stargardt disease, is the lead surgeon for Retina Implant's second clinical trial in the U.K. The trials will also be conducted in Germany, Hungary and Italy.
MacLaren likes the idea of placing the implant beneath the retina, where it can stimulate the retina's bipolar cells, which transmit signals from photoreceptors to ganglion cells. "Another advantage is that the implant is placed in the preferred location for stimulating the eye's photoreceptors," he says. "The fact that it's light sensitive simplifies the arrangement, although the actual surgery is still very complicated."
One of the difficulties designing a subretinal implant has been powering the device. Some researchers were hoping to tap light coming into the retina but they found the amount of energy inadequate, according to MacLaren. "This idea of a subretinal implant has been around since the 1970s," he adds. "But it hasn't been proved functional in a trial until Retina Implant did it."
Light, camera, action
Whereas the subretinal approach places the implant under the surface of the retina to stimulate bipolar cells, an epiretinal implant directly stimulates ganglia using signals sent from a camera and power sent from an external transmitter, both mounted on a pair of glasses. In the case of Second Sight technology, a receiver is implanted under the eyeball's clear mucus membrane, called the conjunctiva. A small camera on a pair of sunglasses captures an image and sends the information to a video processor, worn on the belt along with a wireless microprocessor and battery pack.* After the video processor converts the images to an electronic signal, a transmitter on the glasses sends that information wirelessly to the receiver, which in turn conveys the signals through a tiny cable to an electrode array, stimulating it to emit electrical pulses. The pulses induce responses in the retina that travel via the optic nerve to the brain, which perceives patterns of light and dark spots corresponding to the electrodes stimulated. Patients learn to interpret the visual patterns produced into meaningful images.
With the epiretinal approach, "you could potentially stimulate more of the retina than with a subretinal implant, and it would be easier to adjust for contrast and light," MacLaren says. A drawback to epiretinal implants is that they require a camera mounted on a pair of glasses, which is cumbersome and requires the patient to move his entire head (rather than simply the eyeball) to take in his surroundings, he adds.
Epiretinal implants have met with some success: For example, last year a 73-year-old man receiving a Second Sight Argus II implant at Moorfields Eye Hospital in London was able to see again for the first time in 30 years. All together, 30 people are testing Argus II implants and some of these devices have been in place for more than three years, according to the company, which anticipates a commercial launch of the Argus II in Europe later this year.
*Correction (6/16/10): This article originally stated that the video processor was mounted on the glasses.