SINCE we have two eyes, it is obvious that in the act of sight two pictures must be formed; and in order that these two-pictures shall be fused into one by the mind, it is necessary that there shall be perfect harmony of action between the two organs of vision. In looking at a distant object the two visual axes must be parallel, and in looking at an object at a less distance than infinity, which for practical purposes is less than twenty feet, they must converge to exactly the same degree. The absence of this harmony of action is known as squint, or strabismus and is one of the most distressing of eye defects, not only because of the lowering of vision involved, but because the want of symmetry in the most expressive feature of the face which results from it has a most unpleasant effect upon the personal appearance. The condition is one which has long baffled ophthalmological science. While the theories as to its cause advanced in the text-books seem to fit some cases, they leave others unexplained, and all methods of treatment are admitted to be very uncertain in their results.
The idea that a lack of harmony in the movements of the eye is due to a corresponding lack of harmony in the strength of the muscles that turn them in their sockets seems such a natural one that this theory was almost universally accepted at one time. Operations based upon it once had a great vogue; but to-day they are advised, by most authorities, only as a last resort. It is true that many persons have benefited by them; but at best the correction of the squint is only approximate, and in many cases the condition has been made worse, while a restoration of binocular vision – the power of fusing the two visual images into one – is scarcely even hoped for (see note 1).
The muscle theory fitted the facts so badly that when Donders advanced the idea that squint was a condition growing out of refractive errors – hypermetropia being held responsible for the production of convergent and myopia for divergent squint – it was universally accepted. This theory, too, proved unsatisfactory, and now medical opinion is divided between various theories. Hansen-Grut attributed the condition, in the great majority of cases, to a defect, not of the muscles, but of the nerve supply; and this idea has had many supporters. Worth and his disciples lay stress on the lack of a so-called fusion faculty, and have recommended the use of prisms, or other measures, to develop it. Stevens believes that the anomaly results from a wrong shape of the orbit, and as it is impossible to alter this condition, advocates operations for the purpose of neutralizing its influence.
In order to make any of these theories appear consistent it is necessary to explain away a great many troublesome facts. The uncertain result of operations upon the eye muscles is sufficient to cast suspicion on the theory that the condition is due to any abnormality of the muscles, and many cases of marked paralysis of one or more muscles have been observed in which there was no squint. Relief of paralysis, moreover, may not relieve the squint, nor the relief of the squint the paralysis. Worth found so many cases which were not benefited by training designed to improve the fusion faculty that he recommended operations on the muscles in such cases; while Donders, noting that the majority of hypermetropes did not squint, was obliged to assume that hypermetropia did not cause this condition without the aid of co-operating circumstances.
- No. 1. – Reading the Snellen test card with normal vision; visual axes parallel.
- No. 2. – The same patient making an effort to see the test card; myopia and convergent squint of the left eye have been produced.
That the state of the vision is not an important factor in the production of squint is attested by a multitude of facts. It is true, as Donders observed, that squint is usually associated with errors of refraction; but some people squint with a very slight error of refraction. It is also true that many persons with convergent squint have hypermetropia; but many others have not. Some persons with convergent squint have myopia. A person may also have convergent squint with one eye normal and one hypermetropic or myopic, or with one eye blind. Usually the vision of the eye that turns in is less than that of the eye which is straight; yet there are cases in which the eye with the poorer vision is straight and the eye with the better vision turned in. With two blind eyes, both eyes may be straight, or one may turn in. With one good eye and one blind eye, both eyes may be straight. The blinder the eye, as a rule, the more marked the squint; but exceptions are frequent, and in rare cases an eye with nearly normal vision may turn in persistently. A squint may disappear and return again, while convergent squint will change into divergent squint and back again. With the same error of refraction, one person will have squint and the other not. A third will squint with a different eye. A fourth will squint first with one eye and then with the other. In a fifth the amount of the squint will vary. One will get well without glasses, or other treatment, and another with these things. These cures may be temporary, or permanent, and the relapses may occur either with or without glasses.
However slight the error of refraction, the vision of many squinting eyes is inferior to that of the straight eye, and for this condition, usually, no apparent or sufficient cause can be found in the constitution of the eye. There is a difference of opinion as to whether this curious defect of vision is the result of the squint, or the squint the result of the defect of vision; but the predominating opinion that it is, at least, aggravated by the squint has been crystallized in the name given to the condition, namely, amblyopia ex anopsia, literally dimsightedness from non-use – for in order to avoid the annoyance of double vision the mind is believed to suppress the image of the deviating eye. There are, however, many squinting eyes without amblyopia, while such a condition has been found in eyes that have never squinted.
The literature of the subject is full of the impossibility of curing amblyopia, and in popular writings persons having the care of children are urged to have cases of squint treated early, so that the vision of the squinting eye may not be lost. According to Worth, not much improvement can ordinarily be obtained in amblyopic eyes after the age of six, while Fuchs says, “The function of the retina never again becomes perfectly normal, even if the cause of the visual disturbance is done away with (see note 2).” Yet it is well known, as the translator of Fuchs points out in an editorial comment upon the above statement (see note 3), that if the sight of the good eye is lost at any period of life, the vision of the amblyopic eye will often become normal. Furthermore, an eye may be amblyopic at one time and not at another. When the good eye is covered a squinting eye may be so amblyopic that it can scarcely distinguish daylight from darkness; but when both eyes are open, the vision of the squinting eye may be found to be as good as that of the straight eye, if not better. In many cases, too, the amblyopia will change from one eye to the other.
Double vision occurs very seldom in squint, and when it does it often assumes very curious forms. When the eyes turn in the image seen by the right eye should, according to all the laws of optics, be to the right, and the image seen by the left eye to the left. When the eyes turn out the opposite should be the case. But often the position of the images is reversed, the image of the right eye in convergent squint being seen to the left and that of the left eye to the right, while in divergent squint the opposite is the case. This condition is known as paradoxical diplopia.. Furthermore, persons with almost normal vision and both eyes perfectly straight may have both kinds of double vision.
All the theories heretofore suggested fail to explain the foregoing facts; but it is a fact that in all cases of squint a strain can be demonstrated, and that the relief of the strain is in all cases followed by the cure of the squint, as well as of the amblyopia and the error of refraction. It is also a fact that all persons with normal eyes can produce squint by a strain to see. It is not a difficult thing to do, and many children derive much amusement from the practice, while it gives their elders unnecessary concern, for fear the temporary squint may become permanent. To produce convergent squint is comparatively easy. Children usually do it by straining to see the end of the nose. The production of divergent squint is more difficulty, but with practice persons with normal eyes become able to turn out either eye, or both, at will. They also become able to turn either eye upward and inward, or upward and outward, at any desired angle. Any kind of squint can, in fact, be produced at will by the appropriate kind of strain. Some persons retain the power to produce voluntary squint more or less permanently. Others quickly lose it if they do not keep in practice. There is usually a lowering of the vision when voluntary squint is produced, and accepted methods of measuring the strength of the muscles seem to show deficiencies corresponding to the nature of the squint.
1. The result obtained by the operation is, as a rule, simply cosmetic. The sight of the squinting eye is not influenced by the operation, and in only a few instances is even binocular vision restored. – Fuchs: Text-Book of Ophthalmology, p. 795.
The result of even the most successful squint operation, in long-standing strabismus, is merely cosmetic in the vast majority of cases. – Eversbusch: The Diseases of Children, edited by Pfaunder and Schlossman. English translation by Shaw and La Fetra, second edition, 1912-1914, vol. vii, p. 316.
3. Cases have been reported, some surely authentic, in which an amblyopic squinting eye has acquired good vision, either through correction of the refraction, or because loss of sight in the good eye has compelled the use of the amblyopic eye. – Ibid.