In his article “Will Retinal Implants Restore Vision?” (Science; 02/08/2002 Vol. 295 Issue 5557 p. 1022-1032), Eberhart Zrenner delves into the questions surrounding the possibilities of using technology to cure blindness. Zrenner, a member of the distinguished Department of Pathophysiology of Vision and Neuro-Ophthamology at the University Eye Hospital in Tubingen, Germany, explores the evolving concepts of Subretinal and Epiretinal Implants use in repairing degenerative or hereditary vision impairments plaguing millions the world over. Vision, regarded as the most valuable sense animals possess, is also, according to Zrenner, the most difficult to fix. The author methodically delivers the strengths and weaknesses of two alternative technologies being developed to stifle the loss of sight in today’s world of ever-rising medical breakthroughs. While delivering an unbiased informative article, Zrenner never loses his way conveying the promising, but still yet flawed, research into Subretinal and Epiretinal Implants that may one day be perfected to restore sight.
Zrenner begins his article by stating “Vision is an enormously complex form of information processing that depends on a remarkable neuroprocessor at the back of the eye called the retina (1022). This helps to give the reader the sense of the uphill battle involved in vision restoration. He goes on to inform us that “Blindness can result when any step of the optical pathway-the optics, the retina, the optic nerve, visual cortex, or other cortical areas involved in the processing of vision-sustains damage.” (Zrenner 1023). There are two methods filtering to the top of the list of treatments in the field of research: Subretinal and Epiretinal implantations.
The Subretinal implant “is implanted between the pigment epithelial layer (pupil to layman’s) and the outer layer of the retina, which contains the photoreceptor cells.” (Zrenner 1024). Light is collected and transferred to neurons in the retina that are undamaged by way of microphotodiodes. The Microphotodiodes are tiny camera-like sensors which take images of formed light and help the damaged retinal cells arrange the data into images the brain the can read. According to the author, the are many advantages to Subretinal implants, such as “the microphotodiodes directly replace damaged photoreceptor cells; the retina’s remaining intact neural network is still capable of processing electrical signals; positioning and fixing of the microphotodiodes in the subretinal space is relatively easy; and no external camera or image processing is required.” (Zrenner 1024). These implants allowed animal test subjects to form cognitive relationships between the differences of dark and light.
Unfortunately, Subretinal implants also have their limitations. Zrenner states “In vivo experiments also reveal weaknesses in subretinal implant prototypes. For example, the current generated by a single microphotodiode with it’s small light-sensitive area is not sufficient to stimulate adjacent neurons with the ambient light available from the environment” (1025), meaning, simply stated, that there is just not enough power contained within the implant itself to stimulate the damaged sectors of neurons.
The Epiretinal implant differs in most ways from its experimental counterpart. “The epiretinal implant has no light sensitive elements. A very tiny field sensor, like a camera, is positioned either outside the eye or within an intraocular plastic lens that replaces the natural lens of the eye. Foil-bound wires connect the field sensor at the anterior of the eye with an electrode army implanted on top of the inner retina.” (Zrenner 1025) Also, Zrenner states that “Unlike the subretinal implant, the epiretinal implant does not use the remaining network of the retina for information processing.” (1026)
While the prototypes have yielded strong results in animal testing as well, epiretinal implants are not without their own shortcomings. “Whereas the subretinal implant uses the remaining neural network of the retina, the epiretinal implant does not and thus must provide additional processing to prepare the visual information. On the other hand, the information-transfer characteristics of the epiretinal implant are more amenable to external control. Fixing the epiretinal implant is very difficult and carries the risk of stimulating cellular proliferation.” (1026)
Will retinal implants work on human patients? According to Zrenner, that remains to be seen. “Both test groups reported a sensation of light patterns by the patients, but perception of geometric patterns was reported in only a few instances, and did not often meet expectations in relation to the pattern of stimulation.” (1027) It is obvious that the scientific community, and Zrenner himself, have doubts about either implant as it stands being a cure for vision impairment, but the author remains optimistic. “It is indeed feasible to elicit action potentials in the visual cortex using electrical impulses generated by subretinal or epiretinal devices, but a number of obstacles remain to overcome.” (Zrenner 1027)
In the article “Will Retinal Implants Restore Vision?” Eberhart Zrenner fluently and comprehensively lists the strengths and weaknesses of Subretinal and Epiretinal implants, and the trials faced by those who pioneer this field of vision restoration. The information presented is informative and unbiased, but optimistic in conveyance of the facts at hand. It is clear through Zrenner’s tone that he feels that total vision repair is a vast hurdle for science to leap, but the distance becomes shorter as more data is collected and analyzed. Zrenner has constructed an article meant to inform, not entertain or express his own viewpoints, and in that respect, he is very successful.