In the high degrees of hypermetropia there will be but slight cause for sacrificing binocular vision when, in spite of too strong convergence, distinct retinal images cannot be obtained. In the medium and lower grades of hypermetropia there does exist the inclination to exchange binocular vision for monocular fixation when any cause makes binocular fixation less valuable, as in differences in refraction, astigmatism, corneal opacities, etc., affecting only one eye. It has also been found by Schweigger and others that the non-squinting eye does not possess full acuity of vision, and this induces squint by an effort to secure by convergence as large an image as possible.
Another contributing factor is in the causation of convergent squint is an insufficiency of the external recti muscles, which arises from the fact that in hyperopia there is a deviation inward of the visual lines, and the constant effort necessary on the part of the external recti maintain parallelism of the visual lines finally results in insufficiency, which favors strabismus convergens. As other predisposing causes we find constantly working in poor light, excessive use of the eyes for near work, weakened ciliary muscle and constant looking to one side. Convergent squint may also be found in myopic eyes, due to a preponderance of the internal recti. Macula of the cornea also cause squint from confusion of the retinal images, which cannot be suppressed while falling upon identical points of the two retinae;- hence the eye turns, in order to throw the image upon some eccentric part of the retina where it may be suppressed, and the eye is more apt to turn in on account of the greater strength of the internal rectus.
In convergent strabismus amblyopia is usually present in the squinting eye. The amblyopia is considered by some to be a consequence of the squint and by others as the cause. It is probable that both views are correct, and that in some cases it is the cause of the squint, while in others the amblyopia results from the squint. It is easy to understand that the squinting eye will become amblyopic from long disuse, and in these cases, where it is a consequence of the squint, it is called amblyopia ex anopsia; in other cases, where there may be a difference in the degree of the hypermetropia in the two eyes, the child would naturally use the best eye for vision and allow the poorer eye to turn, in order to more readily suppress the indistinct image. Hence we must conclude that a slight pre-existing amblyopia in one eye, associated with hypermetropia, will have a tendency to cause convergence, and after the squint has become established the amblyopia may increase from disuse.
Strabismus convergens is by far the most common form of squint met with, and is, in the majority of cases, a stationary monolateral squint. The degree of the strabismus can be obtained sufficiently accurate for all practical purposes by means of the strabismometer-an ivory scale shaped to fit the lower lid and graduated in millimetres on its free edge (Fig. 48). It is used by covering the good eye and fixing with the squinting eye, the O on the scale is then placed directly under the centre of the pupil, the good eye is then uncovered and the squinting eye allowed to resume its ordinary position; the number then exactly under the centre of the pupil gives the linear measure of the deviating eye in millimetres.
Strabismus Divergens is much less frequently seen than convergent squint and generally develops later, after childhood has passed. It is frequently in the beginning periodic, but usually becomes permanent later; it may also be alternating; generally, however, it is monolateral and concomitant, in that the deviation of the affected eye, or primary deviation, will be equal to the deviation of the good eye, or secondary deviation. Diplopia is usually present at the commencement of the affection, especially when periodic, but gradually disappears as the condition advances. A spontaneous cure never takes place in divergent squint.
CAUSES.-While convergent strabismus is usually associated with hypermetropia, divergence is, on the other hand, most often associated with myopia. About 65 per cent. of all the cases of divergent squint are myopic. The myope requires little or no accommodation for near vision, hence impulse for convergence is too weak. With this functional insufficiency of the interni, the increased dimensions of the myopic eye adds a mechanical impediment to convergence. Myopes therefore are predisposed to divergence and particularly when one eye has less visual power than the other. As myopia increases the demand for convergence increases owing to the approximation of the near point, but when strain upon the accommodation and the impulse to convergence decreases. The convergence finally is no longer able to answer the demand upon it and the eye turns out. This occurs first when fixating near objects and in some cases never exists except at the near point, but usually later on the eye deviates out at all times.
In some cases where there is myopia of one eye and emmetropia or hypermetropia of the other a similar process ensues: the myopic eye will then usually be used for near vision, because it is impossible to secure binocular vision for reading, and as the myopic eye can be used without any exertion of the accommodation, it is almost invariably used, while the other eye is used for distant vision. Macula of the cornea may also cause divergence as well as convergence, if in such a location that suppression of the indistinct image can be more readily obtained by turning the eye outward instead of inward.
Strabismus Sursum and Deorsum Vergens are usually seen as a complication of lateral deviation and disappear when the lateral deviation is relieved. A concomitant vertical deviation, however, may occur alone, and, when it does, is increased, if an upward deviation, on looking inward, and if downward the squint is the greatest on looking outward. Diplopia is usually present in cases of vertical deviation.
TREATMENT OF STRABISMUS.-This should first be directed toward preventing the development of the squint. Whenever a tendency to squint is noticed, the child should be prevented from reading, writing, and all near use of the eyes as much as possible. As soon as the child is old enough to wear glasses the refractive error should be corrected. My rule is to wait until they are about five years old, and then to prescribe a glass of about a.25 to.50 D less than the total refractive error as shown by the ophthalmoscope, to be worn constantly. The use of atropine to paralyze the accommodation, and thus preventing near vision, if continued for several weeks, will often greatly benefit an inclination to convergent squint. If the case is one of permanent strabismus, and an early operation is not desired, the good eye should be covered for a short period daily and the child compelled to use the affected eye. In this way the vision of the squinting eye will be retained. The fact must also be borne in mind in the treatment of convergent squint that there is a tendency in some cases to a gradual disappearance of the squint as the child grows older, and, when this does occur, it is usually not earlier than the tenth year and often much later.
The use of remedies has in the early stages of many cases relieved the tendency to permanent strabismus.
Cicuta vir.-Indicated in strabismus convergens occurring in children, particularly if spasmodic in nature, or caused from convulsions, to which the child is subject.
Jaborandi.-Strabismus convergens, periodic and resulting from spasm of the internal recti; also for the return of squint after operation.
If helminthiasis has been the cause, Cina, Cyclamen or Spigelia may be required. If due to spasms, convulsions, or any intracranial disorders, Agaricus, Belladonna, Eserine, Gelsemium, Hyoscyamus, Nux or Stramonium would be first suggested to our minds.
Operative Treatment for strabismus may be by either tenotomy of the contracted muscle, advancement of the weak or opposing muscle, or by both combined. The operation for the relief of squint was first suggested by Taylor in the seventeenth century; his method, however, was unsuccessful operation for squint; the method practiced by him was to divide the belly of the muscle instead of its tendon, thereby greatly impairing the action of the muscle and often causing the eye to turn in the opposite direction. At a later period von Graefe placed the operation upon a scientific basis by suggesting the division of the tendon instead of the body of the muscle, and his operation, with some slight modifications, is the one in more common use at present. As to the time when the operation should be made, my preference is to wait until the child is ten years of age, unless it is a very pronounced permanent squint, when a tenotomy, aiming to correct only a portion of the squint, may be made at a much earlier stage.
A simple tenotomy corrects a convergent squint of three or four millimetres, but in divergent squint not more than two millimetres. As the effect of the operation in divergent squint decreases afterwards, the attempt should be made to get an over- correction. To correct a divergence it is usually necessary to make the tenotomy in both eyes and often an advancement is also needed.
Professor of Ophthalmology in the College of the New York Ophthalmic Hospital; Surgeon to the New York Ophthalmic Hospital. Visiting Oculist to the Laura Franklin Free Hospital for Children; Ex-President American Homoeopathic Ophthalmological, Otological and Laryngological Society. First Vice-President American Institute of Homoeopathy : President Homoeopathic Medical Society of the State of New York ; Editor Homoeopathic Eye. Ear and Throat Journal : Associate Editor. Department of Ophthalmology, North American Journal of Homoeopathy, etc.