If we discover the existence of normal binocular fusion, squint may nevertheless have been present at a former time, for in many cases, of periodic squint particularly, the habit of binocular fusion is never quite lost.
That squint can disappear of itself is unquestionable; how often this happens it is difficult to say. The fact that in ophthalmic practice we see many more squinting children than adults is best explained by this,—that squinting children are brought to us by their parents, while adults who still squint have usually given up any desire for a cosmetic improvement, and only come under treatment accidentally or on account of other ailments; lastly, a considerable number of cases are cured by operation. If the squint has disappeared we only discover by accident that it was ever present. The fact of its previous existence may usually be determined by other signs more positive than mere statements from memory; with reference, however, to the age at which the spontaneous cure takes place we are left to depend almost entirely on the patient's statement. As far as I have been able to determine, the period from the ninth or tenth up to the sixteenth year seems to offer the most favorable conditions.
We rarely have an opportunity of watching the disappearance of squint, still I have observed two cases in which a permanent convergent squint disappeared after about a year. In both cases the squint had arisen in young people (of eight and nineteen years of age) in the course of irido-choroiditis which terminated in blindness, and disappeared with the sight. The fixing eye was emmetropic in one case, in the other the condition of error could not be determined owing to nebulæ of the cornea.
We more frequently see periodic squint disappear.
Case 33.—M—, a boy æt. 10, was first examined by me in April, 1873; the right eye has hypermetropia 4·5 D., and almost full visual acuteness, the left has convergent squint, and recognises No. 6-1/2 (Snellen) with convex 10 D.; V. = 1/18 at 1 metre. (The boy's father also squints with the left eye, which is amblyopic to a high degree (V. = 1/36), right eye has emmetropia, and full visual acuteness). The prescribed spectacles (convex, 4·5 D.) were used for working, but not continually; still three years later, in 1877, the deviation was considerably less and only occurred occasionally. In March, 1880, nothing more was seen of the squint, only slight convergence still recurred on excluding the left eye. Patient now wears convex 4·5 D. constantly.
On account of the importance which the disappearance of squint possesses in hypermetropia I will describe a few more cases which belong here.
Case 34.—Mrs. B—, æt. 32, has on the left H. 1·5 D., V. 5/9; on the right H. 1·5 D., V. 5/12, binocular vision (H. =·75 D., V. = 5/6 to 5/9). Asthenopic troubles are the cause of her present complaint. She says she squinted with the right eye as a child till her eighth or ninth year; the present position of the eyes is quite normal; ordinary type is read at the usual distance with normal fixation without glasses. Particularly keen fixation is rarely followed by squint, which may be produced by excluding the right eye; the latter then deviates about 5 mm. inwards and slightly upwards; the secondary deviation of the left eye is rather less. Only the left visual field is seen in the stereoscope.
Case 35.—Mrs. W—, æt. 31, has on the right H. 3·5 D., V. 5/9, on the left V. = 1/16 with + 4 D., single words of No. 0·8 are read (mother and aunt have also congenital weak sight in this eye). Position and movement of the eyes are perfectly normal, exclusion of the left eye is followed by slight relative divergence. In answer to my question whether she had not previously squinted, patient replied that she did not know, it had always been a matter of dispute in her family; as, however, only the right visual field was seen in the stereoscope, we may be sure that squint had been present and that binocular fusion had been lost in consequence.
Case 36.—Mrs. G—, æt. 49, report in March, 1876: On the right H. 3 D., V. 10/10, on the left H. 4 D., V. 10/40; a previously existing squint had disappeared of itself; the position of the eyes appears perfectly normal, but binocular fusion is not present; with red glass before one eye and a prism deviating in a vertical direction before the other, patient does not see double, but first with one eye and then with the other. The squint as well as its disappearance occurred however, at a time when it would have been regarded as an error to allow children to use convex glasses.
Case 37.—Miss H—, governess, æt. about 30, came under treatment for asthenopic disorders; on both sides hypermetropia 2·5 D., visual acuteness 5/18. She owns to have squinted as a child,—it had often been remarked when she was at school. The squint gradually disappeared, but still occurred sometimes on keen fixation. The usual position of the eyes appears perfectly normal, and gives no suspicion of squint; convergence occurs on exclusion, sometimes with downward deviation of the right eye. With the aid of a red glass changing fixation is easily produced even without prisms, but never diplopia. At first only the left visual field was seen with the stereoscope; then the right on exclusion of the left eye; never both at the same time. According to this the condition of binocular vision speaks entirely for the fact, that squint had existed long enough to prevent the development of a normal binocular visual act, and the squint had disappeared without the help of convex glasses in spite of the hypermetropia.