Clinical Investigations on Squint

by C. Schweigger

VISUAL ACUTENESS OF THE SQUINTING EYE.

Whether the state of refraction or the condition of the muscular equilibrium is held to be the chief cause of squint, defective vision of one eye will always have to be acknowledged as one of the most important favouring circumstances; in order to cure squint it is important to have regard to the visual acuity of both eyes, and not only to the defective condition. But this is no easy matter.

First it is to be observed, that most cases arise at an age when an objective determination of refraction is possible, but when the visual acuteness cannot be determined. Even in children who have received slight instruction, it is frequently difficult to distinguish whether imperfect knowledge of the letters or faulty visual acuteness is the cause of the non-recognition of the test-letters; when testing the vision of children it is often better to use figures than letters.

Further, in these cases it is much to be desired that the habit of determining the refraction and visual acuteness at the same time should be discontinued, particularly in reduced visual acuteness, as the test-tables only contain a few letters, which have to be recognised at a distance of 5 to 6 metres. If they have once been read with one eye it may easily happen that in testing the second eye they are repeated from memory, without being clearly recognised; even a child soon learns the few letters by heart. Therefore, when it has been a case of determining the visual acuteness I have always conducted the examination at a distance of one metre, as the choice of letters or figures which can be employed at this distance is much larger than for greater distances. In every case the reading of test-letters must be used as an additional means of examination. We must never forget that the test of vision is a perfectly subjective examination, and that we are obliged first of all to accept the statements of patients as they are given without knowing what they are worth. I have met with patients in the most highly educated classes of society who, in intra-ocular troubles, for example, hæmorrhage of the retinal artery in the macula lutea, could not distinguish the largest type in the first examination, and the next day (perhaps with slight difficulty) could read small print.

Such inaccuracies may continue to exist during repeated examinations and for long periods. One of my patients, for instance, who first came under treatment in the year 1873, had extreme myopia in the left eye with good visual acuity; with the right eye, which was also myopic, and had suffered for several years from choroiditis of the macula lutea he could read only No. 20 Snellen, and a year later 7-1/2 was read with difficulty, word by word. Choroiditis of the macula lutea gradually developed in the left eye, and in the same proportion the statements as to visual acuteness of the right eye improved, so that finally at the end of 1881, 0·5 was read with difficulty with this eye, while the left still sufficed to read 0·4 (at about 5 cm.). As I tried to comfort the patient, who was very anxious about his left eye, with the fact that the right eye had considerably improved in the course of the year, he replied that he might previously have seen just as well with the right eye if he had only taken the trouble, this was certainly my own opinion.

The attention and intelligence shown by patients during examination materially influences its results, and one should never hold the first trial of vision to be conclusive. We must always remember, however, that all conclusions drawn from visual acuteness become more unreliable in proportion as the latter is slight. We must attend to some peculiar difficulties in testing the vision of those who squint or we shall be liable to make great mistakes. When testing the squinting eye, particularly in children, it is not sufficient merely to cover the other or to hold the hand over it, for they know how to bring the usual eye into fixation by holding the head on one side or peeping between the fingers; we must keep it carefully closed with a bandage.

It is still more frequently the case that visual acuteness is stated to be less than it is in reality. The result of always using the better eye for fixation is, that fixation is not learnt with the weaker one. Even where there is no squint we see very frequently that in one-sided hypermetropia the accommodation is only used in that proportion which has become habitual to the emmetropic eye and does not therefore suffice to produce clear retinal images, while good visual acuteness is obtained by means of the correcting convex glasses. In the case of squinters (even without difference of refraction) it happens very frequently that the first statements as to the visual power are considerably below the truth. Patients who assert that they can only read the largest print with difficulty, frequently read smaller, and even the smallest type without more trouble, and we must be careful to ascertain this at first. Accurate reports are usually obtained more quickly by means of convex glasses or eserine. In any case insufficient accommodation is, according to this, one of the difficulties, but not the only one, which has to be overcome before the squinting eye can be put into fixation. We can understand that the innervation necessary for distinct vision can be set aside even without loss of visual acuteness, just as we see the movement of convergence disappear without the interni losing their capacity for contraction.

In order to explain the relation between squint and defective vision, we must first consider the question hitherto neglected, or what is worse, answered with preconceived opinion, as to whether the same form of defective sight which is so common in squint also occurs without squint. No one doubts the existence of congenital amblyopia, nevertheless it has received but little attention in the handbooks on ophthalmology. Leber, for instance (in the well-known compilation, vol. v), does not mention it at all.

A more or less considerable reduction of visual acuteness, with good field of vision, normal sense of colour and normal ophthalmoscopic condition, are characteristic of congenital amblyopia. Colour-blindness may of course be present at the same time. I also hold as probable the very rare occurrence of congenital defects of the visual field in good central vision, but I will reserve for the present the few observations I possess on the subject.