Some blind patients could regain simple sight with a new retinal implant, writes Eoin Burke-Kennedy
ITS INTRICATE architecture used to be cited as proof of a divine creator.
Even Darwin acknowledged that it would be difficult to believe such a structure derived from the process of natural selection.
Yet despite its complexity, the human eye has become the latest body part to be artificially constructed for use as a prosthetic.
The retinal implant, dubbed the bionic eye, is an electronic chip inserted into the eye to mimic, albeit crudely, the function of the retina - the light-sensitive tissue lining the back of the eye.
Although the technology is embryonic, trials indicate the implant can restore rudimentary sight to people blinded by retinitis pigmentosa, a range of hereditary eye disorders.
The implant works through a light-sensing camera, fixed to a pair of glasses. Light from the camera is processed and sent as a wireless signal to a panel of electrodes in the eye - essentially switching them on and off.
The electrodes stimulate the remaining nerve cells in the retina which send signals down the optic nerve to the visual part of the brain giving the sensation of vision.
In effect scientists are hot wiring the function of the retina but the device only works if the visual centres in the brain are intact.
In other words, patients who have suffered a total retinal detachment or who have lost vision from diseases such as glaucoma where the optic nerve is damaged would not benefit from such an approach.
The technology hit the headlines in Europe last month when two patients at London's Moorfield Eye Hospital were fitted with implants as part of a trial sponsored by US medical firm Second Sight.
Experts say that the real breakthrough, however, is not in the surgery - which has been done before - but in the technology, particularly the resolution of the chip.
Irish charity Fighting Blindness is sponsoring research in the Tyndall Institute at University College Cork which is examining ways of increasing the efficiency and density of the chip used in retinal implants.
Head of the Cork team Dr John Alderman explains that to get the device into the eye it needs to be extremely small and there are physical limits to the numbers of light-sensing electrodes that can be placed on the chip.
The current trial uses a chip with an array of only 60 electrodes whereas a healthy retina contains hundreds of millions of photoreceptor cells.
"So you're not putting back the incredibly complex vision that you get from the natural eye," Alderman says.
But implant patients in US trials, some of whom were completely blind for several decades, have had a limited field of vision restored.
According to reports, this ranged from an ability to distinguish between light and dark to being able to tell where a window or a door bell is positioned. Some subjects were able to identify simple objects on a table.
"But it is not what anybody would understand as normal vision," Alderman says.
"Nevertheless this limited restoration can have a significant impact on people's lives in terms of orientation, movement and independence."
Retinitis pigmentosa (RP)is the most common vision handicap in people of a working age, affecting about one person in every 3,000.
The inherited condition causes a degeneration of the photoreceptor cells in the retina.
It generally shows itself in the late teens or early 20s, after which vision progressively deteriorates.
"It is not inevitable that everybody with RP loses all sight but a significant minority of patients do and a retinal chip would be particularly suitable for people in that category," says Dr Paul Kenna, clinical ophthalmologist and surgeon at the Royal Victoria Eye and Ear Hospital, Dublin.
Significantly, the technology has the potential to treat a patient population that gene therapy would not be able to do anything with, he explains.
"For people who have lost all of their photoreceptor cells, gene therapy is not going to work as it is too late. So the implant offers hope."
But Kenna warns that getting the chip under the retina is an extremely delicate procedure "so the potential for doing damage during the surgery itself is not insignificant".
"The eye is not an ideal environment for a piece of electronics and it may prove corrosive, but only time will tell how long these chips are tolerated by the body," he says.
A variant of the technology, currently being tested in Germany, uses an implant which contains the camera inside the chip.
A major challenge, as the devices get more sophisticated, is to limit the amount of heat generated by the chip and to supply it with enough battery power to run for an extended time.
The original developer of the US model, Mark Humayun, professor of ophthalmology at the Doheny Eye Institute at the University of Southern California, says it is a fine balance between over-engineering the device and having it robust enough to perform for long periods in the body.
Humayun explains that, although the intention is to increase the amount of pixels or electrodes of light in the implant, "this must be balanced with how much heat is dissipated and how fragile the electronics device can become".
He recalls that when his team originally tested a chip with only 16 electrodes in 2002 everybody said it would do nothing to improve vision as the field was too small and that it would only prove useful in providing safety information on how the eye and body tolerated the implant.
This proved not to be the case and patients did recover limited sight over time.
"Even though the image is very crude compared to what we normally see, what we have discovered is that the brain is able to fill in a lot of the missing information for these patients with these devices," Humayun says.
"We may not need to go to a mega pixel of say 10,000, we may only need to get to the high hundreds, or a thousand, before patients will be able to read and recognise faces," he adds.