Case 1. Miss B—, æt. 22, possesses in both eyes myopia of 6·5 D. with full visual acuteness and without posterior staphyloma. A concave eyeglass of 4·5 D. is used off and on for distance, and the eyes have never been over-exerted in looking at near objects. For a long time tendency to convergent squint, which is combined with diplopia, has existed on the left side. The eyes generally have a perfectly normal position, but occasionally convergent squint occurs, remains in existence a few hours, perhaps for a whole day even, and disappears again. The deviation here amounts to 4 or 5 mm. As the patient did not wish for an operation, I have been able to observe the condition for years without any change in it or without the squint becoming permanent. The cause of periodic squint is certainly not to be sought for here, in the accommodation.
Many cases of convergent strabismus with myopia constantly offer such a peculiar phase of the defect, that one has accepted the statements which ascribe to short-sightedness a determining influence on this form of squint, without asking for further proof. It may, therefore, be useful for our purpose to cite a few cases of periodic convergent strabismus with emmetropia. For instance:
Case 2. Louise S—, æt. 6-1/2, came under treatment for follicular conjunctivitis, convergent strabismus appearing simultaneously on the right side; the investigation showed the acuity of vision of left eye = 5/12, right V. = 5/36, the ophthalmoscope, and also mydriasis by atropine, proved the presence of emmetropia. The squint had first been observed when the child was about two years old, then it disappeared spontaneously and returned again three or four months ago.
In the course of treatment, which extended over about six months, the child came repeatedly into my consulting room, sometimes with squint, sometimes without, in the periods during which correct fixation existed, no squint occurred even when working. Examination with the stereoscope showed no normal binocular fusion even during normal position of the eyes.
Case 3. Vera von K—, æt. 6; tendency to convergent strabismus, mostly on right side, has existed one and a half years. Normal position as a rule, on covering the eye immediate convergence, with a deviation of 5 mm.; with additional aid of a red glass and weak prisms deviating in a vertical direction, homonymous diplopia is very easily provoked. Visual acuteness on both sides 5/12, the left slightly better than the right; emmetropia in mydriasis by atropine. A year later a repeated examination gave the same result.
The cause of periodic squint in these cases can only be sought in the bearing of the ocular muscles; an elastic preponderance of the interni existed, which ceased, as a rule, on using the externi. A special influence of the accommodation was not traceable, which does not of course prevent this from acting differently in other cases. But in periodic squint it may frequently be observed that the deviation commences under influences which have nothing to do with the accommodation, but, on the contrary, under those which weaken the muscular energy generally, for example, fatigue, anxiety, &c.
Like convergent squint generally, the periodic form is also more frequent in hypermetropia than in emmetropia or myopia, and we admit that in hypermetropia the strain on the accommodation has more influence in producing the deviation. But as the appearance of periodic squint in emmetropia or myopia is proved without participation of the accommodation, solely on the ground of the muscular forces—so the presence of the same forces in hypermetropia ought not to be ignored.
It happens, indeed, that in considerable degrees of hypermetropia a slight convergent deviation occurs only from time to time, the cause of which, on closer investigation, can only be sought in the ocular muscles. For example:
Case 4. Paul F—, was first introduced to me in 1872 as a child of three years and two months, with a tendency to convergent strabismus on the right side of two months' standing, which was sometimes greater, sometimes less, and sometimes was not present at all. In 1877 I saw him again suffering from conjunctivitis, without perceiving any squint; no examination respecting it was made. In 1880 his elder brother came under treatment for apparent myopia, which with the ophthalmoscope proved to be hypermetropia, and my attention, being again drawn to the eyes of the family, I requested the younger brother to come for examination. At first sight the position of the eyes appeared to be quite normal, on more careful inspection slight convergent squint of the right eye showed itself occasionally. On both sides apparent emmetropia or very slight hypermetropia, acuity of vision on left side 5/9, on the right 5/18, ophthalmoscopic diagnosis of refraction was impossible on account of restless fixation.
With the addition of a red glass diplopia cannot be produced, the left field of vision is observed in the stereoscope, then the right one on covering the left eye; never both together. In mydriasis by atropine hypermetropia of high degree (about 4 dioptres) is ophthalmoscopically detected on both sides, with convex 4·5 D., V. = 5/9 with slight convergent deviation of the right eye.