Improving vision with electronic retinal implants

Electronic retinal implants can restore enough vision for some patients to pursue tasks of daily living, according to RE MacLaren, part of a team testing the devices.

“The electronic retinal implant is a complex device that requires complex surgery, but the results in improving vision in patients with end-stage RP have shown proof of concept and in some cases have been spectacular,” wrote MacLaren in Nature Eye.

In a multicentre trial of the Alpha IMS (Retina Implant AG), completed in 2012, 21 out of 29 participants reported significant improvement in activities of daily living.

Age-related macular degeneration, retinitis pigmentosa and other forms of retinal degeneration have become the most common causes of untreatable blindness in the developed world.

Despite relatively good preservation of the inner retina, these conditions may cause loss of cells in the outer retina such as the retinal epithelium (RPE) and photoreceptors.

Many neuronal synapses separate photoreceptors from retinal ganglion cells, which may survive under conditions that damage the photoreceptors. In end-stage retinitis pigmentosa, for example, vasoconstriction affects vessels of the optic nerve head. The normal pink appearance becomes waxy because the ganglion cell axons are still present, even as the retinal capillary bed undergoes vasoconstriction. Enough ganglion cells may survive to transmit a visual signal.

To take advantage of this possibility, Eberhart Zrenner et al. in Tübingen, Germany, have been working on a retinal implant for almost 20 years.

Their device differs from the Argus II epiretinal implant because it is placed in the subretinal space, the correct anatomical location for a pixilated image, MacLaren wrote.

It contains light-sensitive components in addition to electrodes, so it functions similarly to a photoreceptor array; signals are transmitted to the residual biopolar and horizontal cells of the inner retina.

This makes it more complicated than the Argus II, and explains why it has taken so long to develop, according to MacLaren.

Its power supply originates in an induction coil behind the ear, so it requires additional time to implant. On the other hand, it doesn’t require an external camera visible around the face.

Instead, the implant relies on the optics of the eye to provide a focussed image on the retina and this also means that the image is scanned with the ocular saccades. This could result in a more natural interpretation of the visual scene, according to MacLaren.

A complementary metal-oxide-semiconductor (CMOS) camera-like chip with 1500 pixels about 3 x 3 mm in diameter makes up the light-capturing component of the device.

Each pixel is 70 x 70 µm in diameter and comprises a light-sensitive photodiode and an electrode that stimulates the overlying retina. The photodiode acts like the outer segment of the photoreceptor, while the electrode acts like the photoreceptor synapse, transmitting signals to the inner retina.

This incorporation of the sensory and stimulatory functions in a single chip also sets the device apart from the Argus II, which consists of a stimulatory electrode inside the eye picking up the signal from an external camera.

The Alpha-IMS retinal implant spans a visual field of about 12 degrees, depending on the size of the eye. Its pixel density could yield a theoretical best-corrected visual acuity of 6/75. This would assume perfect contact of the individual 70 µm electrodes with overlying functional bipolar/horizontal cells. In the trial, 2 patients came close to this theoretical maximum resolution.

In the absence of photoreceptors to amplify energy from photons entering the eye, the device relies on an induction loop connection through the skin behind the ear. This uses a magnetic interface resembling that of a power cochlear implant.

A cochlear implant surgeon, and an anaesthetist who is comfortable with relatively long periods of general anaesthesia, must both participate in this surgery.

In 2011, MacLaren and colleagues implanted the first generation of Alpha-IMS retinal implant as part of a multicentre clinical trial to assess the retinal implant in clinical practice. The trial made up part of a clinical investigation led by Retina Implant AG, that led to a CE Mark of approval.

In the multicenter trial, patients reported improvements in routine visually guided tasks, recognition tasks and mobility. Twenty-five of the 29 participants regained some visual acuity, light perception or object recognition.

After about 12 months, however, the implants started failing. After explantation, the researchers traced the problems to cable breakage and other deterioration related to “using a complex electronic device in a saline environment,” MacLaren wrote.

The researchers are now testing a new version of the device, the Alpha AMS, in which they have attempted to address these weaknesses. The new trial, funded by the National Institute for Health Research Invention for Innovation award, began in 2015. The first patient was still using the device after 16 months as of January 6, 2017.