It is interesting to note that when the work is approached very near the eye, but convergence is not used, as in the case of watchmakers, who habitually use a strong convex glass in one eye, there is no special tendency to myopia.
Some of the more common symptoms of myopia are:—(1) Distant objects are seen indistinctly. (2) Near objects are seen distinctly, and the near point is much nearer than in the normal eye. (3) Acuteness of vision is often lowered, and especially is this the case in high myopia. (4) Eye-strain is often present, due to overuse of the muscles of convergence, and this may lead to (5) an external or divergent squint. (6) Floating black specks are often complained of, these are generally muscae volitantes, but often, especially in high myopia, may be actual opacities floating in the vitreous. (7) Myopes often stoop and become “round shouldered” from their habit of poring over their work.
A small amount of myopia, if it is stationary, is in no sense a serious defect of the eye, the possessors of it are often quite unconscious of any deficiency in vision, and in fact brag that they have better vision than their fellows. The reason of this is that they learn in early life to recognize indistinct distant objects by the aid of other senses in a way that the ordinary individual can hardly understand, and in later life they can postpone the wearing of glasses for near work for many years, and sometimes until extreme old age. Unfortunately myopia is, as a rule, not stationary; it almost always tends to increase, and if this increase leads to very high myopia such serious changes may occur in the eyes as to lower the visual acuity enormously and sometimes lead to total loss of vision.
The treatment of myopia is general and local.
General Treatment.—The most important part of this is the preventive treatment (prophylaxis), especially in its application to children. All children who have one or both parents myopic are specially “marked down” for this defect, for they have probably inherited an anatomical predisposition. Bearing in mind that excessive convergence is the most potent cause of myopia, the most rigid attention should be paid to the ophthalmic hygiene of the schoolroom. This room should be large, lofty and well ventilated, and have good-sized high windows on one wall, preferably on the north side. Each scholar should have an adjustable seat and desk so arranged that his head is upright and the work not too near his eyes. These desks should be arranged in rows so placed that the pupils sit with the light on their left. Schoolbooks must be clearly printed and the type should not be too small. The school work that needs close application of the eyes should be continued only for a short period at a time, the period alternating with other work which does not require the use of the eyes, such as mental arithmetic, black-board demonstrations, recitation, or play. Schoolmasters should teach more—that is, they should explain and impart knowledge by demonstrations and simple lectures, and reduce as much as possible the time spent in “home preparations,” which is usually work done by bad light and when the student is physically and mentally tired. Even in the nursery the greatest care should be taken. The little ones should be supplied with large toys, a large box of plain wooden bricks being the best form; picture books should be discouraged, and close work that entails undue convergence, such as sewing, threading beads, &c., ought to be forbidden. The nursery governess can teach the alphabet, small words and even simple arithmetic with the bricks. No child with a tendency to myopia, or with a myopic family history should be allowed to learn to write or draw until at least seven years old. The child's bed should not be allowed to face the window, preferably it should be back to the light. Students, or those engaged in literary or other work which entails close application for many hours a day, should be advised to regulate their work, if they are free to do so, by working for shorter periods and taking longer intervals of rest, they should be specially careful not to approach their work too near to the eyes and they should always work in a good light.
Local Treatment.—This consists in correcting the error with a concave glass. The testing must be done when the eye is under atropine in all those under 25, and under homatropine between the ages of 25 and 35 or 40. Over 40 no cycloplegic is required. Except when playing rough games the glasses must be worn always. The wearing of glasses for near work produces at first considerable rebellion in children, because they can see near work so much better without a glass. The object of enforcing this treatment is to make the muscle of accommodation do its proper work, and not only do we do this, but we also do away with the excess of convergence over accommodation, and lastly, make excessive convergence impossible, because, with the glasses on, the near work has to be held at some considerable distance from the eyes. In other words, we have practically made the eyes normal, and it is only by doing this that we can prevent the increase of myopia. Adults who have never worn their correction (especially if the myopia is high) must have a weaker glass for near work. Each case must be treated on its own merits. So-called malignant myopia, which is high myopia with serious changes in the eye, must be treated in a special manner and with the greatest care.
Astigmatism.[1]—The principal seat of astigmatism is the cornea, the curvature of one meridian being greater than that of the other. In regular astigmatism, which is the only form that can as a rule be treated by glasses, the meridians of greatest and least curvature are at right angles to each other, and the intermediate meridians pass by regular gradations from one to the other. Rays of light passing through such an astigmatic surface do not focus at one point, but form many points, with the result that the image is more or less indistinct according to the amount of the error. In uncorrected astigmatism a clock-face viewed at a distance of 4 or 5 yds. will appear to have certain figures distinct, and others (at right angles) indistinct; for instance, figures XI and V may appear quite black, while figures II and VIII are grey and indistinct. If one of the principal meridians be emmetropic the astigmatism is simple; if both be hyperopic, or if both be myopic, it is compound; and if one meridian be hyperopic and the other myopic, it is styled mixed astigmatism. Generally the vertical meridian or one near it is the most convex, and this is called direct astigmatism (astigmatism “according to the rule”). When the horizontal meridian or one near it is the most convex, the term inverse astigmatism is used (astigmatism “against the rule”). When the meridians are oblique, that is, about 45°, it is called oblique astigmatism. Low degrees of astigmatism (of the cornea) are corrected by the ciliary muscle, producing an astigmatism of the crystalline lens, the opposite of that of the cornea, and so neutralizing the defect. This work is done unconsciously, vision is generally quite good and no suspicion is entertained of anything wrong until some symptom of eye-strain shows itself (see Eye-strain, below), and the detection of it is one of the most important duties of the oculist. The only certain method of detecting and consequently correcting a low error of astigmatism, in all below the age of 50, is by paralysing the ciliary muscle with atropine or homatropine and thus preventing it from correcting the defect, and revealing the true refraction of the eye. Astigmatism is corrected by cylindrical glasses combined with spherical convex, or concave glasses if hyperopia or myopia co-exist, and the correction must be worn always in the form of rigid pince-nez or spectacles.
Presbyopia (Old Sight).—A normal-sighted child at the age of ten has his near point of accommodation 7 cms. from the eye, and as age advances this near point recedes gradually. At the age of 40 it has receded to 22 cms., in other words at this age fine print cannot be read nearer to the eye than 22 cms. Between the ages of 45 and 50 the person who has apparently enjoyed good sight up till then, both for distance and near, finds that by artificial light he cannot read the newspaper unless he holds it some distance from the eyes, and he has to give up consulting “Bradshaw” because he cannot distinguish between 3's and 8's. Another symptom often complained of is the “running together of letters,” so that the book has to be closed and the eyes rested before work can be resumed. This loss of accommodation power is due to the gradual hardening of the crystalline lens from age, and convex glasses have to take its place, in order to make reading possible and comfortable. In hyperopia the presbyopia period is earlier, and in myopia it is later than normal (see above).
It is unwise for the presbyope to select the glasses for himself, as astigmatism or anisometropia may be present and must, of course, be corrected; the eyes should be properly tested, and this testing should be repeated every two or three years, as, not only does the old sight increase, but changes in the static refraction of the eyes are probably taking place. When an error of refraction exists with the presbyopia, glasses for distance, as well as reading, have to be worn, and to avoid the trouble
- ↑ See also § Astigmatism, above.
