Eye Care Blog

Spectacles

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.

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