A second chance at sight

By Jeanne Erdmann

This article was originally published by IEC on April 11, 2007. It is re-published with permission. This article fits in to an occasional series on future technologies that may someday require IEC standards if there is a market need.

One feature of the retina in these degenerative diseases holds a treatment window open: a modest reserve of retinal cells can still carry light signals to the brain, even in those totally blind. These cells also carry the plans of an international array of scientists designing retinal implants aimed at people with retinitis pigmentosa and macular degeneration.

These retinal prostheses, also called biomimetics, represent a new type of microelectronic materials that mimic the biological function of the damaged parts of the nervous system they’re designed to replace. A variety of visual prostheses are currently under development in laboratories all over the world.

At the University of Southern California, ophthalmologist Mark Humayun and biomedical engineer James Weiland lead development of an intraocular retinal prosthesis at the Doheny Eye Institute in conjunction with Second Sight Medical Products in Sylmar, California. Their prosthesis began clinical trials in 2002 and remains the only device that has lasted in patients for five years. A similar design by the Intelligent Medical Implant (IMI) group in Bonn, Germany, is also in clinical trials.

"It’s a very exciting project to work on; it’s a difficult engineering problem. First of all, on an intellectual basis it’s challenging, and the humanitarian and altruistic nature of it are very appealing so it means a lot to me and all of the people who work on it," says Weiland.

Restoring sight
The incidence of retinitis pigmentosa is 1 out of 4 000; macular degeneration is the leading cause of blindness in Western countries. Both diseases cause blindness but in different ways. People with retinitis pigmentosa lose peripheral vision in their early twenties and are totally blind by their fifties. People with macular degeneration have blind spots in their front visual fields but peripheral vision remains.

Normally, light enters the eye through the cornea and lens, and focuses on the retina in the back of the eye, where specialized cells carry the signal along the optic nerve to the visual cortex in the brain. Visual prosthetic devices take advantage of retinal cells that can still carry light to the brain, even in the worst cases of macular degeneration and retinitis pigmentosa.

The prosthetic under design by the USC group and at IMI uses an external camera mounted on the frame of a pair of glasses. A wireless link transfers information from the camera to an implant that has been surgically attached to the retina. Wires lead from the electronic part of the implant to the retina. Each electrode, or pixel, creates one spot of light, which is then carried along the optic nerve, just as in a sighted person. A cable, reaching from the glasses to a processor worn at the patient’s waist helps sort out visual information.

In 2002, Humayun implanted the prosthesis in six people with the most severe case of retinitis pigmentosa (those with scant or no light perception). This first prototype – designed to see whether the implant is safe and effective – used an array of 16 electrodes, enough to sense motion and differentiate shapes albeit through rudimentary, gray scale images. "We were surprised at 16 pixels that after training the brain filled in more information than expected," says Humayun.

Patients could distinguish between a plate, a cup, and a knife, which means they can sit at the dinner table without knocking things over and can locate an exit sign. The second model, which is the first commercial device, ready in 2007 as a prototype, holds 60 pixels and should detect motion well enough to serve as a mobility aid for people with retinal degenerations.

In five or ten years, Humayun, Weiland, and their colleagues, expect release of a commercial device with 1000 pixels which should help people with 20/200 or worse vision and macular degeneration see well enough to recognize faces and read large print.

Artificial retinas, real challenges
Figuring out how to interface electronics with neurons has proven a decades-long effort. The prosthetic has to be small enough for the eye, remain stable while attached to the 0.3mm thick, wet retinal tissue, withstand eye movement, and remain leak-proof in the salty inner eye environment of the eye. Frames on the glasses need to be tough enough so the wires don’t get broken. Aesthetics count, too. Outwardly, the device looks like a pair of tinted sunglasses.

People with the implant have a learning curve, too. These patients may have been blind for more than 30 years and dependent on touch to get around; they have to learn to trust the implant. The brain needs to adjust to this type of stimulation, as well.

Andrew Moore CFO and head of business development at Bonn’s IMI says that people with degenerative retinal diseases have a useful database of memories because they were once sighted. IMI’s device may help shorten this learning curve because it can tune individual electrons, which takes into account the learning effects of each person as the information from the camera feeds back in and then gives values optimal for that person. IMI has completed two trials and is gearing up for a trial using the full system with 61 active electrodes. IMI also hope to have a commercial device on the market by 2010.

A vision for the future
The economic impact of blindness in the United States is estimated at nearly USD 50 billion annually, of which about 40% represents direct costs. Less than one-third of severely visually impaired persons are gainfully employed compared with a national average of 84%. People with severe visual impairment also earn less per year less than sighted people. Moore says returning independence to people can save money, especially taking into account the cost of several guide dogs over 10 years, as well as homecare workers. "But that doesn’t take into account the emotional costs to people who have been able to see before and who lost that sight."

Perhaps the best way to look at the future of retina implants is to look at the development of the cochlear implant and draw comparisons. The cochlear implant provides assistance for deaf people in a similar way (that is via electrical stimulation) to that planned for a retina implant for blind people. "As you may know, when the first cochlear implant was introduced it contained only two electrodes and was very rudimentary. As time passed and patient experience was gained and the technology made progress, more electrodes were added and a broader spectrum of patients could be helped," says Moore. Today, several major manufacturers produce cochlear implants and even infants born deaf can receive one.

"Although sight is much more complicated than hearing, it could well be that the future development of a retina implant, after the first product has been introduced to the market, could have parallels to the development of the cochlear implant market," adds Moore.

New technologies come into the IEC work programme via the national committees, so a company in this field could suggest to its IEC national committee that it propose a new work item for this technology. Ideas for new work could also come from the President’s Advisory Committee on Technology and from Sector Boards.

With Technical Committee 62, the IEC is very active in the field of electrical equipment in medical practice and standardizes, among others, pacemakers.

Source: International Electrotechnical Commission (IEC).