Health of the eye

Complications affecting the health of the eye are extremely rare, but are possible. During the early healing phase, the eye is susceptible to infection. You will be asked to follow certain instructions, including using antibiotic eyedrops. Carefully following these instructions will decrease the infection rate to far below 1 percent. Even if an infection does occur, use of antibiotic eyedrops should control the infection.

Steroid eyedrops are very important after excimer laser treatment, because they are used to control the healing response. However, if used improperly for too long, these drops can damage the eye by causing cataracts or glaucoma. It is very important to go to all scheduled follow-up appointments, especially if you are still using steroid eyedrops.

Decrease in best Spectacle Corrected Vision

Best spectacle corrected vision is the best vision obtainable when wearing the optimal pair of glasses. It is a measure of the best vision possible with glasses. Of course, you probably will no longer use glasses or contact lenses for distance vision after the surgery, so you may not even be aware if your best possible vision is slightly different.

For the majority of people, a mild decrease in best corrected vision is usually not noticed. A moderate loss of best corrected visual acuity would be noticed by every patient and might make it hard to work in occupations that require fine vision. Severe loss of best corrected visual acuity is exceedingly rare and in fact has not occurred in any of the FDA sponsored tests.

Some professionals, such as commercial airplane pilots, care very much about their best corrected vision. These pilots must have best corrected vision of 20/20 in both eyes to maintain their licenses, so they should carefully consider the small risk of less than 20/20 best corrected vision.

In one large FDA study, 6 percent of PRK patients experienced some decrease in best spectacle corrected vision during the first six months. This decreased to 1 percent by the end of the first year. Importantly, every patient in this study achieved 20/20 best spectacle corrected vision one year after surgery (b).

Either an irregularity or excessive haziness in the corneal surface could cause a decrease in best corrected vision. A common unevenness is known as a “central island,” which is a slightly raised area near the center of the cornea. “Central islands” usually resolve as the healing progresses. Some haziness almost always occurs in PRK as the ‘cornea heals, but it is usually minor and does not affect vision. A small number of people will form excessive haze, which will cause a decrease in the best corrected vision. In one large study, 1 percent of PRK patients had clinically important haze during the first six months after treatment, but only 0.2 percent of patients had clinically important haze two years after treatment (b). Haze is even less of a problem in LASIK patients. When haze does occur, additional steroid drops are given.

In LASIK, unevenness of the flap, or in the tissue beneath the flap, could result in some decrease in best corrected vision. Small irregularities smooth out during the initial three months after the procedure, resulting in a gradual but noticeable improvement in vision. In rare cases, the doctor may choose to lift and reposition the flap, or to apply more laser treatment under the flap.

The role of the doctor cannot be overemphasized. Some clinics would like you to think that the laser does all the work and that the surgeon is not very important. Remember—the laser is just a tool the surgeon uses to correct your vision and, like other tools, the way it is used makes all the difference. Also, the laser will not be monitoring your progress or initiating adjustments after the procedure—the doctor will.

Be wary of clinics that de-emphasize the surgeon, and make sure that you know your surgeon and feel comfortable with his or her manner, education, and experience.

Be wary also of slick advertisements. A good advertisement means nothing more than the clinic has a good advertising agency. It does not mean that the doctor is right for you.

Carefully check the credentials of the doctor. See if the doctor is board certified and make sure that you respect the doctor’s education, training, and experience. Has the doctor just started performing surgery to treat nearsightedness and astigmatism, or has the doctor been doing so for many years? As a consumer, you need to be aware that there are several ways of correcting nearsightedness and astigmatism. Does the doctor perform both major types of excimer laser surgery—PRK and LASIK—or does the doctor prefer one type? Does the doctor also perform radial keratotomy, astigmatic keratotomy, or other procedures to correct nearsightedness and astigmatism? In some cases, these other procedures may be more appropriate for you.

Does the doctor listen to you, and does the doctor clearly answer all your questions? Does the doctor seem to care about you and your individual needs?

The first step in the operation itself is the securing of the eyelids. This is usually done with a small spring speculum, which gently holds the lids open without in any way pressing on the eye.

The removal of the cataract can be achieved in a number of ways, the aim being to clear all the opacity from the pupil of the eye. Basically, there are two methods either of which may be used: (1) the intracapsular, whereby the whole lens is removed; or (2) the extracapsular, whereby the lens capsule is opened and the contents extracted, leaving the capsular bag attached to the zonular fibres.

The particular type of operation is determined by a number of factors, among which are the age of the patient, whether or not an intraocular implant is to be done, and the preferences of the surgeon.

Most surgeons now use a microscope during the operation. This ensures much greater accuracy when placing the stitches and is essential when implanting intraocular plastic lenses. The powerful light within the microscope should not trouble the patient being operated on under local anaesthetic because the injections he has been given will have so deadened his vision that the light will seem dim.

When the cataract has been removed, by whatever method, the wound is closed by a number of very fine sutures (stitches) and the eye protected by a pad. The sutures are so fine that they cause no irritation to the eye and do not have to be removed at a later date as is often the case with sutures used to close skin wounds.

The whole operation, from the first incision to the final stitch, takes about 1 hour. Cataract extraction is not a painful procedure, although when the anaesthetic (either local or general) has worn off a mild discomfort may be felt (the stitches sometimes irritate a little).

Some years ago both eyes were covered and the patient kept quite still for days with the head steadied with a sandbag on either side; only a very light diet was allowed. Today, with the excellent suture material available and with advanced techniques, such restraints are not necessary, and a certain amount of movement is both permitted and encouraged. Most surgeons allow their patients to be up and about the day following the operation; some, indeed, allow them up on the day of the operation, particularly if it has been performed under a local anaesthetic.

In countless films and plays operating theatres are portrayed as alarming places fraught with tension and drama. In fact, this is by no means the case. Unlike a lot of general surgeons, most eye surgeons operate sitting down; there is consequently far less movement in eye theatres than in general theatres. Moreover, many surgeons like to have some background music playing during surgery, as this encourages a relaxed atmosphere which is essential for good results.

At one time almost all intraocular surgery was performed under a local anaesthetic. For two reasons local anaesthesia is used much less commonly today. First, the whole science of general anaesthesia has greatly advanced and the likelihood of coughing has been considerably reduced. Second, methods of repairing the cataract wound are now better than they used to be. Coughing and sneezing after the operation consequently constitute less of a threat than formerly.

The decision about which type of anaesthetic is to be used is dependent largely on the patient’s personal choice provided that his overall condition does not contraindicate a general anaesthetic.

In the case of children or anxious patients a general anaesthetic is to be preferred, but in insulin-dependent diabetics a local anaesthetic may be advised so that the diabetic control is not upset. Patients with severe chest problems, heart conditions, or greatly raised blood pressure are usually better suited to local anaesthesia.

Local anaesthetic

About an hour before the operation is due to take place the patient is usually given a mild sedative, such as diazepam (Valium), which allays the anxiety that is almost inevitable at such a time. Drops of local anaesthetic and other drops to dilate the pupil are then instilled into the eye to make the operation easier to perform.

After the patient enters the operating theatre the skin around the eye and forehead is cleaned with an antiseptic solution. A number of injections are given around the eye. These have two functions: first, to deaden the area and, second, to prevent eye movements and to stop muscles around the eye from ’squeezing’. The injections are uncomfortable but not painful. One of them, given through the lower or upper lid into the tissue behind the eye, has the additional effect of making the vision in that eye very poor indeed for about an hour. The patient therefore will not be able to see details of the operation and will not be disturbed by the sight of instruments.

Although the area around the eye is completely numb, the forehead and cheek are not. The surgeon’s hands may be felt resting on the face or moving about around the area which is undergoing the operation. This has given rise to such statements as ‘They took my eye out and laid it on my cheek while they took the cataract out.’ Of course no such thing is done! It is simply that the eye itself and the surrounding tissue is completely without feeling during the operation.

After these preparatory steps have been taken all areas apart from the eye itself are covered with sterile drapes and the operation proceeds.

General anaesthetic

As with a local anaesthetic, a mild sedative may be given pre-operatively, but in this case it will be given by injection. The reason for this is that no one undergoing general anaesthesia for any reason may take anything by mouth for at least four hours before the operation because of the danger of vomiting. Vomiting while unconscious or semi-conscious is extremely dangerous.

In addition to the sedative, atropine is also given in the premedication to dry up any secretions in the mouth and throat, as these could cause problems later by making the patient cough and strain. By the time the patient reaches the operating theatre he or she is usually calm, warm, and comfortable, but has a very dry mouth.

Until the advent of contact lenses or intraocular lens implants spectacles were the only means of achieving a finely focused image on the retina after a cataract operation. Although in many cases this is most satisfactory, there are limitations to such correction. At first some patients find that wearing thick cataract glasses causes a number of problems which arise from magnification of the image, distortion in the peripheral part of vision, and some limitation of the field of clear vision.

These all result from the spectacle lens itself rather than from the operation.

Magnification

The spectacle lens needed to replace the cataractous one has a power of approximately 12 diopters — that is to say, it is quite a powerful magnifying lens. Since it is placed in the glasses frame in front of the eye, it acts as a magnifying glass so that the image formed upon the retina is almost one-third larger than that in the normal eye.

This has three disadvantages:

Should the fellow eye have much less cataract and still achieve reasonably good vision, such will be the disparity in the size of the images presented to the two eyes that intolerable double vision will result. In such a situation it is best not to correct the acuity (central vision) in the operated eye but to rely on its fellow for detail while the operated eye provides a greatly improved field of vision upon which the ability to move about in crowded surroundings depends.

1. Familiar objects appear larger than formerly and this tends to give rise to false orientation. Difficulties occur when walking up and down stairs, for instance, and the kerb appears higher than it did before. At table, glasses tend to get knocked over and knives, forks, and spoons dropped on the floor.

A third effect — which is seldom mentioned — can be very alarming to the patient. At mealtimes normal helpings of food appear to be much larger than expected. This can have a very depressing effect on appetite.

It takes only a few weeks for the newly aphakic patient to reorientate himself to these problems of magnification, the time taken depending to a large degree on the motivation of the patient.

Distortion

All thick lenses cause some distortion of the rays of light passing through their periphery (this is known as spherical aberration). For this reason photographers taking finely detailed photographs like to use a small aperture (f- stop) in their cameras to cut down peripheral aberrations.

With cataract glasses the peripheral distortions not only cause straight lines to appear curved, but also make the curves alter dramatically when the eye is moved. The patient may consequently feel very disorientated. He commonly complains that doorways appear to be bowed in towards the middle so that it seems hardly possible to squeeze through, but that, as he approaches closer, the gap opens up to its proper width. Very soon the patient will learn to look through the centre of the lens and to move the head rather than the eyes when scanning a scene.

Field of vision

Another drawback to the use of conventional spectacle lenses is the restriction in the field of vision. Light from the very periphery of the field of vision is unrefracted, since it does not pass through the spectacle lens at all but is perceived by the eye as a blurred image.

Between the two, though, there is an area in which light passes through the very edge of the lens and is deflected as it would be by a prism. It is therefore not seen by the eye at all, so that there is a ring of blindness in this area. This can give rise to a curious jack-in-the-box effect. For example, an aphakic person could be standing on a pavement when somebody approaches from behind and passes him. As the person is passing he will be seen by the patient as a blurred image. Then suddenly he will disappear, only to reappear a little further on, having passed through this area of `scotoma’. Similarly, the party-goer may be alarmed to find faces suddenly disappearing and reappearing as other guests move about the room. Although patients soon adjust to this, it is very important to be aware of the phenomenon, particularly when driving.

Adjusting to spectacles

There are nearly always some difficulties in adjusting to the glasses at first, particularly for people who have very high visual requirements or whose occupations require accurate judging of distances or a considerable degree of manual dexterity.

Attempts to minimise these imperfections have been made over the years by altering the design of the lens itself, by using different materials, and by the use of contact lenses and intraocular lenses.

What can be done by the patient himself to overcome these initial difficulties as quickly as possible? At first it is wise to try the new glasses sitting down, for instance while watching television. Once the improved vision has been appreciated, the patient will be able to move about with increasing confidence and cope with the problems of magnification and peripheral distortion.

The restoration of manual dexterity can be assisted in a number of ways. For instance, the completing of jigsaw puzzles is useful because the handling of small objects such as jigsaw pieces may seem almost impossible at first. One American ophthalmic surgeon who had his cataracts removed readjusted to his new vision in a rather spectacular manner. He happened to live near the water’s edge, where there was a large number of soft-shell crabs. Each day he used carefully to remove the flesh from the crabs, making quite sure that none of the shell or membranes was left in the meat, a process which required a high degree of dexterity. This pursuit had two most satisfactory results: it restored his confidence in a short space of time, and he achieved remarkable popularity in his neighbourhood when it was discovered that there was a free and copious supply of surgically prepared crab meat available for the asking.

At the Oxford Ophthalmological Congress the eye surgeon Harold Ridley reported the results of eight operations that he had performed to insert a Perspex lens. He thus founded a completely new branch of ophthalmic surgery. Ridley had treated many ocular injuries suffered by members of the Royal Air Force during World War II. Aircraft windshields (made of Perspex) had shattered after explosions or the impact of bullets, and Perspex fragments had penetrated the eyes of plane crews. Ridley had noticed how inert Perspex was in the eye and how little inflammation it caused.

None the less, lens implantation was not immediately adopted by the world’s surgeons. Complications were common in the early days, mainly because of unsophisticated surgical equipment and imperfect lens design. Only a few surgeons, mostly in Holland, Italy, and Britain, retained their interest.

The first intraocular lens was an almost exact replica of the human model. The cataract was removed by the extra- capsular method and a small lenticulus (artificial lens), of the same size and power, was inserted into the space previously occupied by the cataract. The lenticulus was made from polymethylmethacrylate (Perspex), a material much heavier than the biological lens that it replaced. Positioned as it was in the very fragile capsule just in front of the vitreous face, its weight meant that it was ill-supported. There was a strong tendency for it to dislocate because it put too great a strain on the zonular fibres.

To overcome this problem a new generation of ‘anterior chamber’ implants was designed. These lenses are placed in the fore part of the eye, in front of the iris. They are supported by a number of legs which stand on the iris itself. Although the lens is thereby more stable, it may rub against the back of the cornea. This may happen during insertion, may be caused by the lens being not precisely the right size, or may be due to the iris itself having been pushed forward. In any event, the contact between lens and cornea causes loss of corneal cells, which in turn may lead to loss of transparency.

An alternative to this type of lens is the ‘iris-supported’ lens. In this case the lens may be positioned either in front of or behind the pupil. It is held in place by a number of loops which hook the lenticulus to the iris. Although this method reduces the likelihood of the lens rubbing on the cornea, it interferes with the normal functioning of the pupil.

Another solution is to implant a posterior chamber `capsular fixated’ lens. This type of lens is the one which most closely mimics a human lens. Resembling the original Ridley lens it is, however, equipped with flexible loops attached to the lens edge. Following an extra-capsular extraction, this new lens is inserted into the capsular bag. The loops are then allowed to expand in order to hold the lens firmly in place without either distorting the pupil or endangering the cornea. The lens is held rigidly in the correct position.

The image presented to the retina by any intraocular lens is almost the size of that in the normal eye. For this reason intraocular lenses are particularly appropriate for patients who retain good vision in one eye. It is most useful for those who have had one of their eyes damaged by injury — for instance, in a road-traffic accident. Indeed, this is sometimes the only acceptable method of optical correction.

Since implanted lenses mimic normal human lenses, it might seem proper to give every patient a plastic lens. However, the long-term effects of placing plastic in the eye are unknown. It is possible that the human body’s defence mechanism will attack the implanted lens, whose plastic will then deteriorate. Moreover, if the corneal cell loss (and consequent impairment of transparency) is extensive, sight will deteriorate. If there has been previous corneal damage from another eye disease (for example, glaucoma, or a long-term inflammatory condition), the danger of further cell loss is unacceptable.

Any degeneration at the back of the eye, which might lead to a detached retina in, for instance, very shortsighted people or diabetic patients who are at risk of developing retinal complications, is considered by many surgeons to contra-indicate an implant — particularly of the iris-supported type of lens. It is necessary to dilate the pupil widely in order to examine the back of the eye satisfactorily. If an intraocular lens is present, this procedure becomes difficult, and the lens itself may be dislodged during the inspection.

Despite these complications, there is undoubtedly an important place for the intraocular lens implant. The design and manufacture of the lenses are being continuously researched. Better lens materials have been developed. Ways of making the operation itself simpler and safer are being investigated. Methods of avoiding and treating subsequent problems are being explored. In the early 1970s the popularity of the operation increased rapidly. This was due largely to the introduction of the operating microscope, the development of finer surgical instruments and suture materials, and an increasing appreciation by ophthalmic physiologists of the eye’s response to the introduction of foreign materials. Today more implant operations are being undertaken, although the practice is by no means universal.

Over the next weeks the level of physical exertion can be gradually increased. Patients should not lean forward to wash their hair; rather it should be back-washed, which is less stressful to the eye. After a month or so the patient is allowed to return to the office, to light work, light weeding in the garden, and similar activities. Two to three months after the operation a return to full pre- surgery activities should be possible, and most surgeons will give an all-clear reassurance at this stage.

What, though, of the long term? In most cases the patient who has had a cataract operation is elderly and is not likely to take part in energetic sports like squash or tennis. However, cataract is by no means a condition confined to the elderly, and there is no reason why the younger patient who has had a cataract extraction should not return to his former activities. When glasses are worn rather than contact or intraocular lenses some sort of protection should be put over the spectacles while playing vigorous racket games like squash — but this, of course, should be done by any glasses-wearer anyway. The only sport which should be totally avoided is boxing, an activity in any case frowned upon by most ophthalmologists.

Older patients may return to their normal pursuits — gardening, housework, jogging, etc. — and indeed should be encouraged to do so because it is important to avoid becoming an ocular cripple, as some do, fearing that any physical activity might result in loss of vision. Of course there are certain restrictions, but this is largely a question of common sense. For example, swimmers should remember to remove their contact lenses before entering the water (if they do not, their lenses will float away). They should avoid diving or any similar activity in which a sudden and vigorous force is applied to the eyes.

Reading and television

There is widespread belief that eyes have a potential visual life that can be exhausted by reading or watching television or engaging in similar pursuits. This is quite untrue; the use of the eyes for close reading or anything else cannot in any way harm them, either before or after cataract surgery. The idea that the eyes must be rested at regular intervals is completely erroneous. Use your eyes as much as you like, for it can do them no harm.

Driving

Those who have had conventional spectacles fitted rather than a contact or intraocular lens will undoubtedly find some difficulty with driving at first because of magnification changes and the restriction in the visual field. It is wise to practise away from general traffic initially. The greatest difficulty will probably be found with reversing.

A particular problem arises in the case of holders of heavy goods vehicle (HGV) licences because under the present regulations neither conventional spectacles nor contact lenses are acceptable to the licensing authority. After cataract extraction HGV licences may be held only by those who have had a successful intraocular lens implant. The regulations state that the vision in the worse eye must be better than 6/36 without correction and better than 6/12 in the better eye with optical correction — a level of visual acuity which is exceedingly unlikely to be achieved without an intraocular lens implant. Thus the only way a driver of a heavy goods vehicle can keep his licence is to have an intraocular lens of the appropriate power inserted.

It is not generally known that some insurance policies stipulate that the insured may not drive for 21 days following a general anaesthetic. It is well worth checking one’s policy before undergoing any form of surgery.

Make-up

Provided that ordinary ocular hygiene is followed, there is no reason to stop using eye-shadow, mascara or other forms of eye make-up.

Sex

There is no reason whatever to curtail sexual activity once the eye has completely settled down after the operation. Furthermore, there is no contra-indication either to the use of the contraceptive pill or to pregnancy and child‑birth.

Air and sea travel

There is no need to avoid flying after any form of cataract surgery. However, in the first few weeks after surgery bad travellers should take precautions against motion sickness, as vomiting could be dangerous to a less than satisfactorily healed wound.

In short, whatever form of visual correction is employed, a completely normal life can be led after cataract surgery. It cannot be too strongly stressed that the use of the eyes does them no harm.

The modern, effective treatment of cataract is to remove it from the lens capsule, or, if necessary, remove the whole lens itself. From about the age of fifty onwards the ligaments which hold the crystal lens in place become weaker. The lens can therefore be quite easily removed from the eye by rupturing the ligaments, especially if they have been ‘prepared’ by appropriate weakener-enzymes. Special forceps or a ‘freezing probe’ then lift the lens out of the eye. Usually the surgeon will supply counter-pressure to facilitate this operation. Another technique makes an opening in the eye and tears out the lens capsule. The hard nucleus of the lens may be ‘broken up’ whilst it is still in the eye, and the debris then washed out.

Once the nucleus has been removed the cortex, which is softer, naturally disintegrates so that it too may be washed out.

Once the inner lens has been removed the surgeon will decide whether to replace it with an artificial lens, or not to replace it at all.

Some retinal conditions are known to be inherited, and show up in families generation after generation. It is worthwhile, therefore, to give a brief description of how the inheritance of abnormalities works.

At a distance, a group of naked human beings would always appear more or less similar, once allowances for differences in age, sex and race have been made. But the closer we get to such a group the more discrepancies we begin to notice. Even without using specialized equipment we can discern that every individual is different, every individual has his own particular characteristics. No two people have the same fingerprints, to use the most famous criterion, and with modern techniques of tissue analysis and cell analysis the variations from one human to another become even more apparent. And yet, within the human individual, when one cell dies and is replaced by another, the new one is identical to the old. The fertilization of the human egg by a male sperm results in a mixing up of the essential inheritance particles (genes) that determine the structure of every cell in our body so that, although one cell in a son’s body may be identical to the equivalent cell in his father’s, it is combined with other cells which derive from his mother. Everybody then is a composite (roughly fifty-fifty) of both parents’ cells, the manufacture of which is computerized from data held in the molecules of the original egg and sperm. This data is `printed’ on nucleic acid proteins (DNA), and in the normal course of events cannot be altered. The genes I have inherited from my parents (and in neither case do I inherit all their genes) are precisely and only those genes I have to give my children. But the way my genes ‘programme out’, the building that is made from the inviable blueprint, can be affected, and by different kinds of factor. The programme can be modified by external forces such as drugs, viruses and diet.

This at least is the orthodox neo-Darwinian view. Acquired characteristics cannot be inherited. If I lose my arm in an accident my children will nonetheless have two arms. Similarly, if I suffer some debilitating disease in my youth, the effects of that disease will not be passed on (unless of course I still have the disease, am a woman and pass it on to my child through the normal channels of contagion). Conversely any inherited defect I may have may very well pass down to my children, either overtly or covertly. However, recent scientific technology has explored ways of altering the blueprint itself, of restructuring the basic genetic code, using various forms of light-ray and chemical. While in the distant future this research may result in doctors being able to correct a hereditary defect, for the present it remains the case that both genes and cells naturally tend to reproduce facsimile versions of themselves. In the case of cell- production, however, modern medicine has identified many diseases which interfere with the natural process. Cancer, for example, radically alters the body’s ability to go on reproducing the same kind of cell year after year. Cell-production is also affected by age. As the body grows old it sometimes ceases to reproduce certain cells, while other cells are badly reproduced. And of course there are some cells, including many in the eye, that are only produced once (before or at birth), and which have to last a lifetime. As life expectancy increases, such cells are put under more and more pressure.

How, it may be asked, if the basic tendency of cells is to replicate themselves perfectly, did we evolve from the amoeba? The answer seems to lie in genetic mutations, which occur either spontaneously or, at an earlier period in the earth’s history, as the result of interference on the primitive genetic soup of one- cell organisms by sunrays that were able to penetrate a much thinner atmosphere. As a rule, the more complex an organism is, the greater the change effected by just one alteration in the genetic coding, for in the print-out of the genetic programme the genes act collectively, not simply. Thus among humans, for example, a gene for a longer leg tends also to mean a gene for a longer arm. The amazing thing, perhaps, is not so much the evolution of species, but the relative stability of species at any one point in their development.

The consequence of such stability is that while it ensures the inheritance of survival-fit characteristics, it also ensures the inheritance of any defects that are not absolutely unlivable- with. And of course, civilization and modern medicine often enable a defect to survive into adulthood, and therefore be able to reproduce itself, where in a primitive environment it would quickly have become extinct.

The theory of human abnormalities is complex and at times uncertain. Can behavioural defects, for example, be inherited? And then there is the question of recessive or throw-back genes. In this case a bad characteristic is clearly inherited, but often across two or more generations, so that the intervening generation is not affected. For our bodies not only contain the genes that made us, but they also contain genes that are dormant. A well-known example of this is haemophilia. Haemophilia generally occurs only in men, but it is carried by women. A haemophiliac man cannot pass the disease on to his children, but the female can. Thus the whole problem of inheritance is overlaid with an additional layer of complication: the battle between ‘X’ (female) and ‘Y’ (male) chromosomes.

Patients are often anxious to know whether their abnormalities are inherited. While the laws of inheritance can be reduced to relatively straightforward mathematical equations, it must be understood that hereditary patterns can only be known if there is a sufficient body of data stretching back over several generations, and if there are similar histories in other families sufficient to establish general tendencies. Although the principles governing genetic determinacy are well established (they were originally identified by Gregor Mendel in the latter half of the nineteenth century), the exact function of individual genes in the chromosome is as yet only a partially explored territory, and doctors are still reliant upon what the patient or members of the patient’s family can tell him in order to decide whether or not a particular problem is or is not hereditary. There are in fact relatively few instances where a case-history stretches back long enough for a pattern of inheritance to be predictable, and these mainly concern royal or aristocratic families which have been subjected to continuous record. However, such defects as squint, shaky eyes (nystagmus), albinism and diabetes are usually present in more than one member of a family, and in more than one generation, so that there can be no doubt that they are in some way inherited. In addition, it is also found that certain abnormalities are particularly rife in isolated communities (such as remote mountain villages), which can only be explained by a combination of inheritance and interbreeding. Certain types of blindness also run in families, though, like myopia, it may be that certain environmental conditions have exploited what may only be a hereditary tendency towards blindness. Again it must be remembered that because life expectancy is much greater in the modern period, and because many abnormalities are present only in a gradually progressive form, one would not necessarily expect to discover a consistent pattern among older records. A person who was going to develop diabetes may have been killed off by cholera before he reached the appropriate age.

From birth onwards the retina is very thin (less than one millimetre thick) and transparent. It consists of two layers. The outer layer is pigmented and in intimate contact with the small blood vessel layer of the choroid. The inner layer consists of the light-sensitive cells and their nerves. These nerves form connections (synapses) with a second order of nerves. Some sensory cells form a connection with only one nerve, others form a group and then synapse with one nerve. At the same time other connective cells are present, whose purposes are to support the sensory cells structurally and provide them with nutrition. Unless nutrition from the blood vessels can reach the cones and rods the retina will quickly lose its function.

The rods, disposed mainly towards the perimeters of the retina, are very good at reacting to different light levels, but are relatively poor at picking up either detail or colour. The cones, on the other hand, situated mainly in and around the centre (fovea) of the retina, are very accurate in detail and colour discrimination, and work best at high levels of light. Together, the rods and cones make up the nerve level of the retina, which lies upon the outer pigment level without being connected to it. However, nutrition certainly passes from the choroid through the pigment layer to the optic cells.

The sense of sight begins with the many thousands of cells arranged in the saucer-like hemisphere of the retina. Detailed images are momentarily printed on the retinae and then relayed down the optic nerves for co-ordination and interpretation. The retinal print is immediately blotted out, or ‘wiped’, and replaced by the next print. The brain is capable of receiving and processing many such pictures every second. In certain respects the principles are the same as for moving film. Several pictures, each slightly different, are presented to the eye and brain within a very short space of time. The brain then joins the pictures together so quickly that a sensation of perfect continuity is created. But whereas a film strip does this in a mechanical process, the retinae operate chemically. In the visual cells of the retinae the chemicals are constantly being used up and replaced. The cells are recharged again and again without the individual normally being aware of the process. In certain circumstances, however, the chemical reactions can be slowed up. Sudden exposure to very bright light, for example, can bleach all the cells, or nearly all the cells, simultaneously. Even in the healthy adult complete bleaching by very bright light can result in his being unable to see for a minute or two. The eye requires time to contract the pupil and replenish the cells with the appropriate chemicals. Snow blindness is a well-known example of this state.

As was said previously, the retina has two visual functions: first to localize the individual in the space round him, and secondly to provide him with detailed images of objects to hand. It is because of this dual purpose that the retina is twin- textured. The rods provide a large field of vision, the cones a smaller area of scrutiny. But there are still some rods towards the centre of the retina, and some cones in the outlying areas. Were it not for this, our overall sense of vision would be uncomfortably bifurcated. As it is, the system gives us a good tool for simultaneous scanning and fixation, and works very well when the two eyes are locked and synchronized together.

One of the chemicals that is the basis of seeing is called rhodopsin, another carotene. The latter is derived from vitamin A, and is particularly helpful for night vision. Thus there is some truth in the saying that eating carrots enables you to see in the dark. The rods of the retina are charged with carotene, which derives its name from the orange pigment of the root-vegetable. When the supply of this chemical in the eye has been used up, more must be made from the body’s resources. If such resources are poor, which in the case of a diet deficiency they may be, night blindness may result. The cones, too, have their own chemicals, and the superabundance and/or lack of these may account for some kinds of poor colour vision. Recent research has provided a vast amount of knowledge about colours in general and about the colour functions of individual nutrient chemicals in the eye in particular, and the relationship between the two has opened up a research field of enormous clinical and scientific interest.