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.