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nasal side of the centre, due, of course, to the fact that the true axis and the visual axis of an eye are not identical.
As I have just said, it can be readily demonstrated that in a normal case there is some power of binocular vision as early as the sixth week. It is necessarily very feeble, and the faculty is indeed quite unstable during the first few months of life. No doubt we have all noticed a young child squint on occasions, often due to a trifling disorder of the stomach and bowels.
Ilowever, while this faculty of fusion is absent at birth, it comes into evidence, as I have said, quite early in life, and gradually increases in intensity for months ; indeed, it does not come to its highest development, and, therefore, its most stable condition, until about the sixth year.
Now this faculty, while it is a normal possession of the race, is wanting or deficient in a certain small minority of persons. It is this lack of the fusion sense that is the basic cause of squint. In those cases of alternating refractive errors, especially hypermetropia, may contribute to produce squint, but they are never essential to its appearance. In those cases of alternating strabismus in which we find sometimes one eye turning in and sometimes the other, it is quite usual to find the refraction of the eyes normal, and each eye being used in turn, the vision of each is preserved and is usually normal also. These cases are, however, much the worst from the standpoint of treatment, as it is in them that the fusion sense is quite wanting and incapable of development.
Because constant convergent squint greatly preponderates over the other varieties, I think it will be well to devote the most of our time to a consideration of its phenomena, causes and treatment. We will thereby avoid confusion, and the main purpose of our paper will be served if we succeed in obtaining a clear conception of this condition.
The term squint implies, besides the deviation of the eye (which is only the outward and visible sign of the anomaly), four other factors as being present, namely:
2. Deficiency of the fusion sense, as already mentioned.
5. There is usually present a refractive error, hypermetropia most often, which is present in both eyes.
Let us first consider the deficiency of the fusion faculty. The instrument employed for the investigation and treatment of this anomaly is known as the amblyoscope, and was devised by Claud Worth, of Moorfields Eye Hospital, London.
It is essentially a stereoscope made in the form of two tubes of about an inch and a half diameter, which are hinged at the proximal end and supplied with convex lenses, to render unnecessary any effort of accommodation in order to focus the image in the objectslides, which are at the distal end of the tubes. These tubes, instead of being straight, are in reality composed of a very short tube at the proximal end joined at an angle of 120 degrees with a longer tube, and at the junction of the two is situated a mirror.
Now, if a person with normal fusion faculty looks into this instrument and suitable object-slides be placed in position, it will be found by adjusting the direction of the tubes that a position will readily be found in which he can fuse the two images, seen separately with either eye. Moreover, it will be found that the tubes can be approximated and separated to the extent of several degrees, while fusion is still maintained. In a case of developing squint, however, it will be found that the degree to which the direction of the tubes can be changed is extremely limited.
This is a very short sketch of the method employed, but will probably be sufficient to give you an idea of it. Worth has found by this means that in all his cases of squint the faculty of fusion is limited. Moreover, another point which is very convincing as to the causal retention of this defect. He was enabled to examine a considerable number
a of younger brothers and sisters of his squinting patients, and in some of these was able to observe their cases at later periods. Of 157 children he found the fusion faculty well developed in 106. None of these have subsequently squinted. Of 37 cases which he considered doubtful, 6 have since squinted, and of 41 whom he considered very deficient 9 afterwards developed squint. Which data goes strongly to show the importance of the relation of deficiency of fusion sense to squint.
Now as to the rationale of the development of the inward deviation:
If we look at a distant object, the rays entering are parallel and the axes of our eyes are straight ahead, our accommodation being at rest. If, however, we now fix our gaze on a near object, say a book at reading distance, we have to make use of our accommodation, and at the same time we have to converge our visual axes. These two functions of accommodation and convergence have been always so closely associated that they are practically indissoluble, and it is next to impossible to perform either act without the other.
This effort of accommodation, which is equal to 3 dioptres in a case with natural refraction, means, of course, a stronger effort in hypermetropes.
In a case in which the fusion faculty is normally developed, this tendency to undue convergence induced by abnormal accommodation is kept in check, but if the fusion faculty be deficient or, as in some cases, absent, the relative directions of the eyes are to a lesser or greater extent dependent merely on their motor co-ordinating. The result is that we find in these cases, first of all, a squint manifesting itself when the child accommodates. In many of these cases, provided there is some fusion faculty present, it is found that correction of their refractive error will, by ridding the patient of the associated tendency to over-convergence, entirely correct the deviation.
The deformity, which is at first present only when the child is looking at near objects, gradually is more often seen—(a child, as a matter of fact, spends most of his waking hours playing with objects at near range)—and if the case remains untreated it will be found that after a time it persists even in distant vision.
Now, this abnormal condition existing, the two eyes looking in different directions, it naturally occurs to one that the child must be seeing double, and if we could obtain an intelligent report from him we would probably discover that such was the case, but he will not long accept the discomfort and annoyance of this state of things, and what occurs is that he in time unconsciously refuses to perceive the image of the deviating eye—what is technically known as “suppression of the vision" of this eye occurs.
It is easy to appreciate what will now happen. If the arm of a child during this early period of development were to be strapped to his side it would not be many months before wasting of the member would be very apparent. So in the case in question, the deviating eye can do only one thing--gradually lose its power of vision. And this is what occurs. We find in all cases a greatly diminished acuity in these cases.
These more or less blind eyes are, however, if their training is taken in hand sufficiently early, capable of restoration, and this brings us to a consideration of the method of treatment.
In all cases the first thing to do is to thoroughly atropinize both eyes and measure their refraction. It will usually be found that hypermetropia, with or without astigmatism, is present, and this refractive error should be fully corrected with glasses. No child is too young to wear glasses, and, contrary to popular belief, injuries to the eyes from their breaking are extremely rare. In a certain proportion of cases, as already stated (about 30 per cent.) the deviation is corrected thereby; in any case the fusion sense is probably defective and the child will be benefitted by fusion training by means of the amblyoscope. In the majority of the cases, however, it will be found that even with these refractive errors corrected the child still squints. It will also be found that the squinting eye possesses very defective vision. Now, before anything can be done directly to correct the deviation, we must first restore the squinting eye to an equality of vision with the fixing one. There are a couple of methods commonly employed to accomplish this. One of these consists in daily bandaging the good eye for several hours, thus forcing the poorer one to exercise what function it possesses. With continued treatment over a period of several weeks or months, depending on the degree of blindness present, we will find that normal vision in this eye is regained. A child does not like a bandage, however, and most mothers do not like the responsibility of seeing that it is kept on, so that a better method to employ is the daily instillation of a drop of atropine solution into the good eye only. The child is now unable to see near objects with this eye, and the deviating eye, therefore, takes on the work of near vision, and by reason of the enforced exercise of its functions gradually returns to a normal degree of acuity. The atropinized eye is, moreover, not so subject to loss of function as though it were entirely occluded by a bandage.
We now reach a point where we have normal vision in each eye, but the deviation still exists. It is now time to undertake fusion training. Although the vision in the deviating eye may be perfect, there is usually “suppression" by it. It is, therefore, necessary in using the amblyoscope to have a separate lighting arrangement for each half of the instrument. By increasing the illumination behind the unperceived image, therefore, the squinting eye can be forced to see it.
The images used are pictures drawn on thin paper and pasted on a piece of glass which slips into the object-slides. The images used on each side are different; they are commonly two different portions of a complete picture, such as a man with a hat on his head and a stick in his hand. One slide will contain the man minus perhaps one arm, one leg and the hat and stick. The other slide will have the complementary portions. Another favorite device used is a bird on one slide and a cage on the other. Suppose the bird and the cage be put in position, the child is seated on the surgeon's knee and the instrument placed before the child's eyes. He will probably say that he sees either the bird or cage, depending on which is in front of his fixing eye. The illumination is, therefore, suppressed behind this image and increased behind the other, until he sees both the bird and the cage. The two halves of the instrument are now approximated until the child sees the bird in the cage. By alternately separating and approximating the tubes it will be found that the bird is now in, now out of the cage. The child is striving to keep the bird in, and after a time one finds that the two halves can be moved out and in several degrees and the bird still remain in. These treatments are given at intervals of several days for several weeks, and each time it is found that one can commence with the two halves of the instrument further apart, until finally the eyes assume their normal axes.
Sometimes, probably because the treatment has been undertaken too late, we find after improving the deformity considerably it remains stationary. In these cases a shortening of the external rectus of this eye, with or without a tenotomy of the internal rectus, will overcome the still existing deviation, and the patient will then take on binocular visions.
Of course, we see cases every day, in older children and in adults, for which nothing remains but operation, and the operation suitable will depend on the judgment of the surgeon. Either tenotomy or advancement of the muscles of the deviating eye or of both eyes are indicated, according to the degree of deformity. Advancement of the external rectus is undoubtedly preferable to tenotomy of the internal, and is suitable in cases of as high as 25 degrees. In cases of greater deviation than this, a tenotomy will be required in order to obviate retraction of the globe. In any case one expects nothing but a cosmetic effect from either procedure.
But what I want to suggest to you particularly this evening is the comparative innecessity of these operative measures if the case is appreciated early. Children from three to five years of age are the best subjects for fusion training; in the sixth year they are amenable to it, but with more difficulty, and after that it is very difficult often to accomplish much.
It may, perhaps, as I have described it, appear a long and tedious method of treatment, but surely the saving of an eye and the avoidance of such an ugly deformity is worth a much more difficult regime.
But remember, to get ideal results, one must have these cases young in life. The laity have no conception of the meaning of squint, therefore let every physician be a preacher when the occasion arises, and let him preach
1st. That squint is curable, if treatment be undertaken young.
2nd. That it is the exception for children to “grow out of” squints, and
3rd. Whether they grow out of it or not, there will always remain to the patient the heritage of a blind eye.