Eye Care Blog

There are further classifications according to lens design, finish, colour, etc., which make the whole subject very confusing. If you want what is best the answer must be, ‘Whatever your practitioner advises as the best optical correction according to your eye sensitivity and required duration of wear.’ The classifications given are in fact properties of the lens, and a combination of them gives a particular kind of lens. Some types are commercially available by trade names on an `off-the-shelf’ basis, others have to be manufactured by specification. They are all made of plastics, however, though some absorb water while others do not. Most absorb and transmit gas, a necessary function to maintain the chemical equilibrium of the eye-system.

A development period of just over twenty years must be considered short by comparison with other technical advances such as the television and aeroplane. As with all technology, the advances depend almost entirely upon knowing the objectives and then having the resources to overcome the problems. What these objectives are vis-a-vis the contact lens we will come to. At first glance the contact lens appears to be a simple, one- component device and therefore not technically difficult to produce, but in fact the demands made upon size, thickness, replicability of material and lens forms, quite apart from consistency of optic definition, provide a formidable task, while the funds that have been available to individual researchers have been only a fraction of the moneys available for more obviously commercial ventures (and infinitesimal compared to world- destruct weapons systems).

Because the contact lens lies on the eye, its back surface must be perfectly compatible with the complex and sensitive tissue of the eye. Indeed, compatibility is the main prerequisite, and not a perfect, glove-like fit. In fact, for the normal eye, a perfect fit is the opposite of what is required, because the cornea can only remain clear if gases are permitted to flow backwards and forwards over its tear film. This is a form of respiration. Just as the lungs permit our red blood cells to get rid of their unwanted carbon dioxide and replenish themselves with oxygen, so too cells in the cornea take up 02 and disgorge CO,, and it does this from the gases on its surface. The cornea’s ability to do this depends upon a very complex network of metabolic cycles, which, like all complex entities, is very readily disturbed. The placing of a layer of plastic over the cornea must not be allowed to interfere with the cornea’s respiratory system. Where interference does take place, the result can be oedema, or corneal swelling. When a small lens is used (such as an ordinary hard contact lens) the swelling is localized centrally and associated with an unwanted steepening of the corneal curve, which therefore increases short sight. This is usually transitory, but in some patients can be long-lasting. When a contact lens practitioner tells you to ‘come back and see me in six months’ he is not thinking about his consultancy fee but about your health, and you’d be foolish to ignore the instruction.

There are other complications arising from the use of contact lenses. There is, for example, some likelihood of gradual distortion, common among wearers of hard contact lenses who wear their lenses day-in day-out over a period of years, especially if the lenses themselves are too thick. This creates a dependence on the contact lens for, unlike spectacles, if the lenses are abandoned for a few weeks there is very little tendency for the eye to improve of its own accord. Like a bad analyst, the hard contact lens may create problems which only it can correct. Other problems are more serious, but the majority reverse themselves once the patient stops using contact lenses. All lenses produce some surface changes to the cornea as they rub against its surface, but the degree of tolerance is a very individual matter. There are some people who simply cannot wear contact lenses, while others experience no difficulty. The damage done can only be seen by magnification and oblique illumination. By using a dye called fluorescein the injured cells on the superficial layer of the cornea can be highlighted. The cornea that has been so dyed has a green mottled appearance, the density of the dye being greater wherever the injury is most severe.

If, however, you have overworn your contact lenses, or if a significant foreign body has inserted itself between the lens and the cornea, then the corneal surface may be more seriously affected.

The syndrome of overwear is a very individual problem. Some patients are chronic overwearers. They are often wearing unsuitable lenses in bad environments, and as soon as a painful red-eye episode begins they decrease their wearing time or stop for a few days, and then continue. This chronic situation may be the result of a hyper-sensitivity to the lenses or to preparations used with the lenses. On the other hand some patients do not know they have overworn their lenses. The damage done by overwear usually takes place under the lens, and whilst the lens is worn there is no pain. Shortly after removing the lens the pain commences and in some instances becomes sufficiently severe for the patient to seek advice at a hospital (usually in the middle of the night). While the lesion to the eye is unlikely to be severe, the pain is, and leads to spasms of the lids and an inability to open or move the eye.

A less severe syndrome occurs with soft lenses. After a long period of overwear the cornea becomes cloudy, the eyes red. The lenses may hitherto have been perfectly satisfactory, but suddenly a change has occurred, either in the properties of the plastic or in the environment.

With some very rare exceptions, the problems associated with overwear are not pathologically serious, and provided you learn to recognize the symptoms at an early stage no great damage will be done. While overwear typically affects people whose jobs or social life demands that they are ‘on show’ for long stretches of time, the prolonged wear of suitable lenses, night and day, may not cause any greater problems than ordinary daily wear except for a periodic blurring of vision and a more frequent change of lenses; and if you don’t develop any of the symptoms characteristic of overwear, either while you are wearing the lenses or after you have taken them out, it is most unlikely that the eye is suffering any damage, trivial or otherwise.

The compatibility between patient and lens varies from individual to individual. Without exception it is the quality of the tears, of the eye’s lubrication system, that determines whether good contact lens wear can be achieved and maintained. But before we turn to tears, let us look at the whole media of the eye and some of the problems associated with it.

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