Eye Care Blog

These then are the common errors of vision which may cause eye-strain. They are short- and long-sightedness, near-vision deficiency and astigmatism. Most commonly astigmatism is present with one of the other three conditions. They may all be measured by both objective and subjective methods. Objective measurement means the use of instruments, while subjective measurement requires you to state an opinion. In ordinary practice the practitioner will use both methods, and thereby establish a pattern which enables him to prescribe the right optical correction. If you repeatedly give a wrong answer to a subjective inquiry the experienced occulist will proceed to further objective (optometrical) tests. Very often a patient will deliberately devalue any visual problems he or she is having, particularly if he/she has not previously worn spectacles. `There’s nothing much the matter with my eyes, but I thought I should have them checked’ is fairly typical of a middle-aged patient having his eyes examined for the first time. Understandably there is a psychological barrier against the obvious and disconcerting fact that deteriorating eyesight is a symptom of growing old. But the practitioner will make allowances for this and proceed with a range of tests that allow him to come to his own conclusions about your sight. A good example of this is the `red-green test’. Black letters or diagrams on a red background are presented to you, and you are asked to read off similar or identical inscriptions on a green background. You will note to within a quarter of dioptre’s accuracy a difference in clarity between the two. If the green card is clearer, long-sightedness is indicated, and vice versa. The practitioner then knows in which direction your sight is deficient and can begin trying out different lenses on you, using an ordinary wall chart. You cannot ‘resist’ a stronger lens (which might be the most appropriate one) for you are not told whether the strengths of the lenses are being increased or decreased. By switching the lenses around the practitioner can very quickly adduce whether or not you are co-operating.

Obviously, many patients who have developed near sight require only a very simple prescription to correct their vision. Once it has been decided at what length you prefer to hold your reading matter the prescription can be worked out using relatively simple mathematical formulae. But if, for example, spectacles are needed to correct astigmatism as well as near vision, you will need expert examination and correction will be necessary. This is especially true if you have significantly unequal vision between each eye.

In some parts of the world patients choose their own spectacles, on an off-the-shelf basis, and even in developed countries there is currently a small trend toward ‘takeaway’ opticians. However, and it needs to be said, the aid of an expert ophthalmologist or optometrist can alone determine an accurate choice and eventual dispensing of good quality lenses and the knowledge that all else is normal.

In the majority of people the two eyes maintain a remarkable symmetry, and to within half a dioptre or a few degrees of astigmatism are the same. But nearly always one eye is marginally dominant, and ‘leads’ the other eye. It is virtually the same as being left- or right-handed, and ‘ambiocular’ eyes, to coin a phrase, are perhaps as rare as ambidextrous hands. The fused images that the person perceives as being one is nearly always orientated to just one side of the midline of the forehead. Whilst we do have two eyes, and know how to use each individually (when for example we close one eye and look through the other), we refer, when using both, to an imaginary cyclops eye. However, one eye is sometimes markedly different to the other. Several combinations are possible. For example, one eye could be normal, while the other eye is long-sighted, short-sighted or astigmatic. On the other hand, one eye could be short-sighted, while the other is long-sighted. More usually, one eye is simply significantly more short-sighted than the other. Normally, where this occurs, the weaker eye may develop a squint, become even weaker over time and not be used so much as the better eye. Early optical correction, though advised, may not be used since the child or young person sees very well using mainly his or her better eye and may not notice any improvement whilst wearing spectacles or contact lenses. But unless he or she does wear optical aids it may very well be that the ‘bad eye‘ deteriorates faster than need be.

Testing the vision of children is inherently difficult, and a significantly weaker eye may go undetected until the patient is older. With adults testing is generally more accurate. But, with the exception of reading glasses required in middle age by the patient with normal-distance sight, spectacles cannot and should not be self-prescribed.

Future Vision Tests

Whilst the eye can be measured as to its power fairly accurately this does not necessarily relate to what the patient sees. The latter is a perceptual process and depends upon many factors such as memory and interpretation. In babies, children and individuals suffering brain damage the power of the eye may have to be measured with the pupil and accommodation paralysed.

Two types of machine are being developed for testing sight and measuring the power required to correct any defect. The simplest type is a screening device that the patient manipulates to bring images into focus in various meridians and can test distance, near sight, colour vision and even the degree of binocular vision. Such machines are of great value to armed services, employment departments, schools and so on.

The other type is much more sophisticated and can measure degrees of vision not even associated with letters. Such tests appear on TV-type screens and are monitored by computers as to width and intensity of black and grey lines. These types of vision stimulus can be co-ordinated to electrical recordings of the brain, using scalp ‘pick up’ terminals. Thus the vision can be recorded without the patient thinking or speaking. Further more, such tests can be extended to measure visual fields and to select areas of investigation and thus diagnose disease.

In the foreseeable future diagnosis of disorders owing to even minute abnormalities in the tissue of eye or brain will be diagnosed by combinations of automatic machines. The task of the eye clinician will be interpretation and determination of what management procedures to follow.

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