Photoreceptor Damage:

Causes and Possibilities

by John Angell

Over 10,000,000 people around the world suffer from some sort ofblindness or handicap due to photoreceptor damage. These effects canbe caused by a number of afflictions, including retinitis pigmentosa,macular degeneration, and tumors. These illnesses vary in severityfrom being a mere hindrance to completely blinding the individual.Until recently, those affected were left without hope of a cure oreven a treatment that would somewhat improve their vision. But overthe last few years, several groups of scientists have been working ona partial cure in the form of neuroprostheses, artificial deviceswhich are inserted in the eye behind or on top of the damaged retinalarea. These photoreceptive chips, in theory, should provideinformation too the healthy neurons residing in the retina,substituting for the damaged photoreceptors.

When we open our eyes, millions of tiny events occur that allowus to see. Our pupils automatically constrict in accordance to thelight level, the variable lens bends and adjusts to fit the distanceof what we are looking, and our photoreceptors receive information inaccordance to the previous factors. (This is extremely simplified,but it will suffice for now.) Photoreceptors are tiny, specializedneurons located in the retina at the back of the eye. There are twotypes of photoreceptors, rods and cones. Each follow the sameprinciples: when light hits them they respond with a chemicalreaction using a substance known as rhodopsin. Once this reactionoccurs a chain of events sends this message down a number ofsophisticated and specialized neurons, eventually reaching the brainand resulting in what we call sight.

Rods (numbering one hundred million or so in each eye) areprimarily in the periphery of our visual field. They are extremelysensitive to light and are often ÒtiedÓ together on a lower level toallow for greater sensitivity. Rods do not see in with goodresolution and cannot differentiate colors.

Cones (only five million or so exist) are mostly found on thecenter of the visual field, a place called the fovea. The words youare reading now are being processed by cones in the fovea. Theyoperate in brighter light than rods and detect color (there are threetypes, each responding to a particular range of wavelengths). Conesdo not pool their output and exist for resolution, not meredetection. The only drawback with the cone system is the amount oflight saturation necessary to stimulate them and send their signal tothe brain. Cones require bright light and do not function in dimlight, which explains why we only see in black and white while in thedark.

Working together, rods and cones create our visual field,resulting in astounding resolution and sensitivity when compared tomost other animals. Unfortunately, nothing in nature is eitherperfect or infallible. Many different factors may affect andpermanently damage these photoreceptors.

Perhaps the most common cause of photoreceptor failure isretinitis pigmentosa, which affects over 100,000 people in the UnitedStates alone. Very little is know about the cause of the disease, buta problem in the pigment epithelium, a layer of blood and nutrientsupplying vessels beneath the photoreceptors, has been theorized.Retinitis pigmentosa begins in early adulthood, around the age oftwenty, first damaging the rods on the periphery of the visual field,causing night tunnel vision.As the individual ages and the diseaseprogresses, the visual field gradually decreases until the diseaseattacks the cones in the fovea, leaving many subjects completelyblind. Retinitis pigmentosa is genetically inherited, but does notaffect everyone with the retinitis gene.

Second on the list of crippling conditions which causes receptordamage is macular degeneration, the most common form known asage-related macular degeneration. The macula is an area of tissuemeasuring roughly 5mm in diameter surrounding and including thefovea. In early macular degeneration, the disease causes the conereceptors to thin slightly and small yellow lumps begin to form onthe retina. This progresses slowly and may not cause any realproblems at all. In up to 20 percent of the cases, however, themacula also experiences the formation of small new blood vessels(strikingly similar to diabetic retinopathy) which eventually breakand bleed fluid into the vitreous humor (the jelly-like substanceinside the eye). It only takes a few months for this leakage todestroy the cones. Macular degeneration is fairly common in olderpeople and there has been some success in slowing its cripplingeffects using a laser to photocoagulate these new blood vessels andreduce their leakage. The perceptual effects of macular degenerationare the opposite of retinitis pigmentosa: peripheral and night visionremain remarkably intact but central and color vision are completelydestroyed. In other words, the victim can see everything around whathe or her is looking at but cannot see straight ahead at all. Peoplesuffering from this cannot read, drive a car, or even walk normally.Also they cannot see properly in bright light because thehypersensitive rods become oversaturated.

Diabetic retinopathy is a result of the invention on insulin.Until insulin was introduced, a diabetic could not expect to reach25. But with the prolonging of life due to this miracle of medicine,new problems emerge. In the United States alone, over 4 millionpeople suffer from diabetic retinopathy. In early stages, thecapillaries in the retina begin to swell, squeezing thephotoreceptors and causing visual impairment. Sometimes the diseasestops here and only slightly hinders the sight of the victim. Othertimes, however, the disease progresses into what is known asneovascularization. As in macular degeneration, new and abnormallyshaped blood cells form but actually cut off vital nutrients andoxygen to the retinal neurons, literally starving them to death.Neovascularization can also cause retinal scarring and detachment.

Less common forms of photoreceptor damage include opticneuropathy, vascular disturbance, tumors, extreme light damage.Tumors may slowly grow beneath the retina squeeze the photoreceptorsuntil they eventually die. This results in patchy vision in whateverarea is affected. Extreme light intensity is not common and is easilyavoidable. Galileo suffered from damage to his photoreceptors when hetrained his telescope on the sun in attempt to view its surface.Also, residents of Nagasaki and Hiroshima were blinded when they sawthe flash of the first atomic bombs. The photoreceptors in their eyeswere, quite simple, burned out.

As stated before, over 10,000,000 people worldwide suffer fromdamage to the retina and/or photoreceptors. That number can only beexpected to grow in the future. Over the last few years, however, newpossibilities have become available (though they seen extremelyscience fiction). Retinal implants, or neuroprostheses, which arewafer sized chips measuring only 3mm in diameter and 50 micronsthick, have given hope to millions of people. They are similar intheory to the first sensory organ replacement, the Cochlea Implant,which dramatically improves the hearing to those suffering fromcertain ear conditions. Surgery for retinal implants is more invasiveand more dangerous than the Cochleal Implant, but it is certain thatmany people are willing to take the risk.

The first of these neuroprostheses is being studied by a group ofdistinguished German scientists led by Professor Ebert Zrenner. Thesubretinal implant, or subret, is a tiny disk implanted within thedamaged photoreceptors at the back of the eye. Once implanted, tinymicroelectrodes connect to horizontal and bipolar cells which wouldnormally be connected to the photoreceptors. On this disk are 7000microphotodiodes arranged in a checkerboard pattern, each measuringonly 20X20 mm (about the size of the photoreceptors in the retinalperiphery). These microphotodiodes are composed of silicium oxide,which when struck by light release a positively polarized charge.This charge travels down the implant into a series ofmicroelectrodes, which like the original photoreceptors stimulate thehorizontal and bipolar receptors, resulting in sight. As of September15, 1997, several rabbits have been given the subret implant and arecurrently under observation at the University Eye Hospital inTubingen, Germany. Functional tests are being undertaken, and earlyresults have been favorable. Also, the implants show remarkableacceptance to the rabbitsÕ ocular tissue. More in depth results havenot been made public yet.

The second form of neuroprostheses is the epiretinal implant, orepiret. A team of American scientists led by Dr. Wentai Liu use avery different approach. The epiret is implanted inside the eyebetween the ganglia cells and is exposed to the vitreous humor. It ismore alike the Cochleal Implant in that it must be powered by anexternal source. The epiret connects through the ganglia and amecrinecells into the horizontal and bipolar cells through the use ofmicroelectrodes, resulting in similar results as the subret. Onepossible means of powering this device is the use of Òlaser glassesÓ.These special eyeglasses have a tiny laser mounted on them which,when activated, direct a beam into the eye and onto a specialphotovoltaic cell, which in turn powers the chip. Results fromexperiments using the epiret have been difficult to attain, but againthe concept is quite promising.

Neuroprostheses provide hope to where there was none, but they dohave limitations. The microphotodiodes on the surface of the implantare extremely large when compared to the natural cones in the fovea,and resulting vision will be blurry at best. Also, these diodes arenot equipped to detect different wavelengths, so the subject wouldsee only in black and white. But with the advancement ofnanotechnology, these microphotodiodes will eventually become smallerand more complex. Another problem with the implant is the possibilityof circulation being cut off to healthy retinal neurons, but a newapproach using chain mail ordered structuring would open tiny ÒgatesÓin the chips, just large enough for nutrients and oxygen to passthrough.

Perhaps the biggest problem facing both groups of scientists doesnot lie in technology or availability of test subjects, but funding.In the United States, the neuroprosthesis operation is classified asÒhigh riskÓ, so no money from the U.S. government will sponsor them.German scientists are enjoying a 18 million Deutch Mark grant fromthe BMBF (the Federal Ministry of Education, Science, Research, andTechnology). This generous donation is not enough, however, to fundthe enormous research and development programs necessary to makeadvancements in this technology a reality. These problems, however,are minuscule when compared to the possible of restoring sight tothose millions afflicted with photoreceptor damage.

Looking into the future, with proper funding, scientists willfigure out how to make these microphotodiodes sensitive to certainwavelengths, allowing for color vision, and also make them smaller,increasing resolution. Compare this technology to that ofmicrocomputers over the last ten years: in 1988 the fastest homecomputer measured around 8 megahertz; in 1998, computers haveexceeded 300 megahertz. More on the edge of science fiction is thepossibility of wavelengths normally invisible to average human eyesbecoming detectable to those with retinal implants (humans beingsensitive to infrared or ultraviolet light). The possibilities arelimitless, provided funding is available.

References

8/18/97. Sub Retinal Implant Project. On-line: Web Page for SubretProject
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Johnson, Steven S. (1997). Solar cells may sub for retinalreceptors. Science News, v151. 222-224.

Ready, Tinker (1997). A vision to bring back sight. The News andObserver. Online:Web Pabe for News and Observer .

Tovee, M.J. (1996). An introduction to the visual system.Cambridge University Press: New York, NY.

Wolken, Jerome J. (1995). Light, photoreceptors, & imagingsystems in nature. Oxford Press: Oxford, NY.