Loss of sight is something that most of us dread, as it means the loss of our ability to move around freely and deal with the objects of everyday life. Failing eyesight takes many forms, from age-related conditions that reduce acuity or field of view, to complete blindness resulting from accident or disease, and in the worst case to blindness from birth caused by mishaps in development. The eye is not the easiest organ to repair when problems arise, but much can be done to improve the eye’s optics or to stop retinal deterioration from getting worse. Even when blindness has set in, as a result of currently incurable conditions such as macular degeneration or retinitis pigmentosa, there are ways of stimulating the visual system via electrically powered implants, which can provide a useful degree of vision. Other approaches, for example the use of stem cell therapy, are beginning to show promise in animal trials. In this final chapter, I will first review the major conditions that may lead to degradation or loss of vision, and then outline some of the newer approaches to providing some useable visual input to those whose eyesight has been lost.
Conditions that threaten eyesight
In early life most visual problems are optical, for example short and long sight, as discussed in Chapter 3. About 15 per cent of children under the age of 15 have these focus problems. All are p. 89↵easily dealt with using glasses or contact lenses. Between the ages of 40 and 50 presbyopia, in which the lens becomes too stiff to accommodate to different distances, becomes a near-universal condition. Once again, glasses are the simplest answer. Later in life cataracts, in which the lens starts to crystallize and become opaque, begin to degrade vision. By age 75 about 30 per cent of people have cataracts. The remedy is an operation in which the lens is removed and replaced with a plastic one. This is a very common procedure, and nearly always successful.
Several diseases can affect vision in later life. Diabetes is one of the main causes of visual problems, and if untreated for 15 years it can lead to impaired vision in 10 per cent of cases and blindness in 2 per cent. Both Type I (insulin dependent) and Type II (late onset) diabetes result in elevated levels of blood glucose, and this in turn causes the retinal blood vessels to leak and ultimately burst. They may then start to proliferate, making things worse. The leaks cause blurring and opacities that block vision, and may also lead to retinal detachment. Treating the disease itself to reduce blood glucose levels, by the use of insulin or other drugs, is the best preventative measure. However, if the disease has taken hold the usual treatment is to destroy proliferating blood vessels using laser surgery. This is done in a grid pattern in the peripheral retina in order to keep the central retina free of blood and debris. Laser surgery is not a cure, but it can be very successful in preventing further damage.
Glaucoma is a group of diseases that is usually caused by abnormally high pressure inside the eyeball. It affects about 2 per cent of over-50s, rising to 10 per cent in over-80s. The elevated pressure has the effect of squeezing the optic nerve, damaging the fibres and restricting blood flow to the optic nerve head. The outer fibres are affected first, and as these tend to come from the more peripheral parts of the retina sight is first lost from the outer parts of the visual field (Figure 38). This loss gradually spreads towards p. 90↵
the centre, and can eventually lead to irreversible blindness. If detected early, glaucoma can be treated easily with eye drops that reduce the intra-ocular pressure. The early detection of both glaucoma and macular degeneration (see later in the chapter) has been greatly assisted in recent years by the technique of optical coherence tomography, which provides a 3-D image of the intact retina, and shows up both the normal layering, and any pathological abnormalities.
Two more sinister diseases are age-related macular degeneration (AMD) and retinitis pigmentosa (RP). In AMD damage is caused by the accumulation of plaque material, known as drusen, in the region of the macula in the centre of the retina. This build-up, behind the retina, can destroy the pigment epithelium and photoreceptors, and also cause retinal detachment. Ten per cent of sufferers have the more serious ‘wet’ form of the disease, in which blood vessels proliferate behind the retina, causing leakage and further damage to the photoreceptors; some drugs are now available that can curtail this proliferation. The macula is the region around the fovea, and is crucial for reading, face recognition, and other fine discrimination tasks, so its loss is devastating, even though peripheral vision is usually unaffected (Figure 38). The problem is that half the primary visual cortex is devoted to the macular region, and so it is very hard to use peripheral vision for tasks usually performed with central vision. Acuity is much lower outside the macula, and even with p. 91↵substantial magnification, reading is very difficult. AMD is not uncommon: 10 per cent of people between 66 and 74 will have it to some degree, and this rises to 30 per cent above the age of 75. AMD is very variable, so the degree of impairment may be minimal in some cases but debilitating in others. Although there are no effective treatments for AMD, dietary supplements can sometimes help. Recent trials in mice using replacement photoreceptors derived from human embryonic stem cells have shown promising results.
Retinitis pigmentosa is the name for a group of inherited diseases that lead to a progressive loss of photoreceptor cells, and hence visual performance. RP is fortunately rare, affecting fewer than 1 in 3,000 of the population. Unlike the conditions discussed above, RP begins to show itself before the age of 30, and becomes progressively worse. In 1969 a mutation was found in the gene that codes for rhodopsin, and since then about 150 mutations of this gene have been linked to RP. The rods degenerate first, and this leads to poor night vision and loss of peripheral vision. Loss of day vision, involving the cones, only becomes apparent in the last phases of the disease, and this may occur by the age of 50. Large doses of Vitamin A (the precursor of the chromophore retinal; see Chapter 1) are said to be helpful. Although there is currently no accepted treatment for RP there are promising developments. Gene therapy, in which the defective gene is replaced with a normal one using a viral vector, has been the subject of clinical trials since 2007. Stem cell research may also lead to a replacement for photoreceptors at some time in the future. However, because it often leads to blindness, other therapies such as the electrode-base prostheses discussed later, become realistic treatments.
Electronic prostheses
In recent years devices have become available which can supply electric signals directly to either the retina or the brain. These signals typically result in the appearance of spots of light, and are p. 92↵thus not in the form the brain is accustomed to dealing with, which means that the normal machinery for dealing with images is only partially available. For example, tasks such as resolving objects against a cluttered background are much more difficult when the brain’s input is in the form of an array of light spots, rather than a complete image. When the retinal receptors no longer function, as, for example, in retinitis pigmentosa, but the rest of the visual nervous system is intact, it may be worthwhile implanting an electrode array to provide a stimulus pattern that bears at least some resemblance to the retinal image. Three locations for such an array have been tried: the retina itself, the optic nerve, and the primary visual cortex.
Retinal implants. Probably the most successful implants, and the only ones currently licensed for general medical use, are epi-retinal prostheses. These are arrays of electrodes placed on the outer—ganglion cell—face of the retina. An advantage of such a device is that it uses the retina’s own output stage to supply information to the brain, and so preserves the geometry of the ganglion cell layout. The information provided by the array comes from an external camera. The first 16-electrode array was implanted in the eye of a patient at Johns Hopkins University in the early 1990s. Six more patients had the implant in the early 2000s, and it is claimed that from being completely blind they could now perform a range of vision-based tasks. A second generation of implants, using 60-electrode arrays and known as Argus II, was trialled in 2007 and is now approved for use in the US and Europe.
Sub-retinal prostheses are another active field of research (Figure 39). These are located behind the retina, in front of the pigment epithelium, in the space occupied by the now-defunct receptors. A team at the Eye Hospital of the University of Tübingen produced an array of photodiodes, boosted by an external power supply, which supplied electrodes that stimulate the retinal bipolar and ganglion cells. The current generation of p. 93↵
implants uses a chip with 1,500 electrodes. The photodiodes make use of the image in the eye itself, rather than an external camera, and so preserve the geometry of the visual image. Since the array moves with the eye the wearer can select where to look, as in a normal vision. In a clinical study of 11 blind patients, some were able to read letters, recognize unknown objects, and localize a plate, a cup, and cutlery. All patients had some degree of sight restored. This system shows great promise, but at the time of writing it is not in general use.
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