C. Schweigger

“ A very simple examination suffices to determine these doubts:—Cause the patient to gaze at a certain point on the horizon and cover first one eye and then the other. If the covered eye remains stationary, no squint exists, but if it is observed that when giving one eye its freedom and covering the other, the first must make a movement in order to fix the object to [Pg 2] be looked at, it is only a question of discovering whether the squint does not simply ensue from the covering up of the eye. ”

“ If we want to draw a comparison between convergent and divergent squint, we must consider only absolute divergent strabismus, for convergent strabismus does not offer a parallel to relative divergent squint. In absolute divergent squint the direction of the visual axes is such that they would meet behind the patient's head; in the relative divergent squint the axes of vision are parallel or slightly convergent, but they do not cross at the point fixed by the one eye, but at a greater distance off. ”

“ Convergent squint as a result of paralysis of the abducens is not very often seen. It is first to be observed that a convergent squint, including the whole field of vision, occurs by no means in all cases; in about half the cases binocular fusion is retained towards the healthy side, diplopia then only occurs when the weak abducens is exerted beyond its strength. In those cases in which convergent squint is present in the whole field of vision paralysis of the abducens cannot be the sole cause, but some other cause than the most apparent one must co-operate. ”

“ The determination of the glass even, with which we believe we are able distinctly to see the fundus of the eye, is also an objective one. Whoever, for instance, is firmly convinced that convergent strabismus depends on hypermetropia, will, in doubtful cases, very easily carry his subjective conviction into the objective examination, and will still see clearly the fundus of even an emmetropic eye with a weak convex glass—the objective signs for the clearness of the image have no absolutely defined limits. ”

“ Why should a man strain his accommodation when no advantage is thereby gained?
In most cases the squinting eye has also an available power of vision and is on that account used for fixing objects which lie in the direction of its visual axis; it can also be made to fix objects in front, this occurs as soon as the other eye is covered; it remains as the fixing eye till the next blinking of the lids, or movement to another object for fixation, or till both eyes are closed for a short time, when it returns to its former deviation. ”

“ In extremely defective vision little importance should be paid to the fact of slight diversity in the statements, as where visual acuity amounts only to about 1/36, or where fingers are counted at a distance of 1 to 2 metres, it is quite immaterial, as far as the usefulness of the eyes is concerned, whether fingers are counted at a half or a whole metre, and we ought never to forget that all conclusions which we draw from the state of the visual acuity, are unreliable in proportion as the latter is lowered. ”

“ In testing with prisms it may appear doubtful as to whether binocular fusion or suppression of one eye exists; however, the stereoscope at once gives us certain information. It must not be forgotten that the altered relations between the eyes, which are always possible in squint, also appear at the same time; he who sees double with prisms, may yet be able completely to suppress the stereoscopic visual field of one eye. Binocular fusion, suppression of the squinting eye and simultaneous vision with both eyes without binocular fusion can alternate in the same individual. ”

“ Another proof that the squinting eye is really used for vision appears to me to lie in the fact that persons who squint, provided of course that the vision of the eye concerned is not very defective, do not show that uncertainty in the estimation of distance, which is apt to prove so troublesome to those who have only monocular vision. ”

“ Sometimes the deviation is exceedingly strong, sometimes so slight that it seems to be absent. It is usually impossible to determine if it is really absent, for as soon as we single out a point for fixation to make the investigation feasible, strong deviation sets in. If in such cases we perfectly atropise both eyes, restore the attainable acuity of vision by neutralisation of the hypermetropia with convex glasses, and yet, nevertheless, as is generally the case, the customary strong convergence takes place on fixation of a distant object, there can be no talk of a strain on the accommodation ”

“ Afterwards, Donders sought to explain the occurrence of convergent strabismus in emmetropia by paresis of accommodation, which must indeed, according to his theory, produce the same result as hypermetropia.
I content myself by reminding my readers, that at the age when convergent strabismus usually arises, between the second and third year of life, a determination of the near point is utterly impossible; a foundation in fact is therefore wanting to the theory. ”

“ 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. ”

“ In convergent strabismus, however, no one, at least no ophthalmologist, thinks of entertaining such fears for the eye used in fixation, and where is the physiological basis of this whole idea to be found? Is the visual purple more active in monocular than in binocular vision, or what physiological activity is thereby taxed in increased degree? ”

“ We cannot conclude the consideration of the operative treatment of convergent squint without once more returning to the relation between the line of vision and the position of the cornea. The angle [Greek: a] still deserves mention in a few thankful words—hic mihi angulus praeter omnes ridet—it is a very useful guide in tenotomy. In tenotomy we may count as gain the apparent divergence which it causes in hypermetropes who do not squint. We obtain a perfect cosmetic result, while a convergence, objectively determinable, but not otherwise easily visible, continues to exist. ”

“ I have seen the same disturbance of vision follow severe double blenorrhœa, which destroyed one eye but left the other uninjured. These children are always of an age which renders any trial of vision impossible, and we are therefore obliged to draw conclusions as to visual power from the movements of the body. If children move as though they were blind, it need not necessarily follow that they are so in the common meaning of the word. ”

“ Care must be taken, especially when one eye has defective vision, that the corresponding visual field contains objects sufficiently large and easily recognisable, as very small objects which do not correspond to the lowered visual acuity are easily overlooked. It sometimes happens that both fields are seen at the same time, but that there is no fusion; finally it happens frequently that there is complete suppression of one visual field. ”

“ At any rate the fact is settled that squint can disappear spontaneously, and without the aid of convex glasses even in high degrees of hypermetropia.
Wecker's announcement that "this spontaneous cure goes hand in hand with the progressive decrease of the accommodation, and depends on the fact that the squinter, on the strength of this progressive decrease, renounces more and more the aid which he finds in the increased convergence during the act of accommodation," only proves to how great an extent one may be prejudiced by theories. ”

“ If permanent squint has already existed for a long time, nothing can be hoped for from the use of convex glasses; for the conditions of the muscles are then so much changed, that they are no longer influenced by such weak physiological powers. I have been able to convince myself in the case of several squinting persons, who conscientiously wore the spectacles prescribed for them elsewhere, that the squint was concealed by this means; that may suffice in some cases, but if it is a question of young girls we may well ask, which is to be preferred for appearance sake, squint or spectacles. ”

“ 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. ”

“ Of all the prevailing causes present defective vision will be the more decisive in proportion as it is of high degree; for the motive which despite the presence of favouring circumstances can prevent the real occurrence of squint, binocular vision, becomes less efficacious as the defective vision becomes more considerable. As binocular fusion takes place frequently in periodic squint, for a time at least, that is as long as proper fixation lasts, one can understand that periodic squint exists chiefly in cases where the visual faculty of both eyes is good. ”

“ The chief method for absolute divergent squint is the combination of shortening with tenotomy of the externus. If the impulse for convergence is once lost, so that an associated movement occurs in place of an accommodative one on fixation of a point situated on the middle line, a removal of the squint cannot be obtained by simple tenotomy of the externi—another proof that a change of position of the eye is by no means a necessary result of tenotomy. ”

“ The change in form of the myopic eye diminishes its mobility, associated movements of the eyes may be replaced by turning the head, but this is not possible for the movement of convergence. Further, in extreme myopia the far point is generally used for reading, &c., and sometimes even a somewhat greater distance, because on account of the close proximity of the objects the retinal images are so large that they are sufficiently clearly recognised even if they are not quite distinct. At all events accommodation certainly does not take place, hence one motive favouring convergence is removed. ”

“ If we contemplate first the largest group, that of the ordinary permanent convergent squint, the choice of the method is principally determined by the average degree of deviation, the condition of error, and the visual power, lastly by the mobility, particularly the outward movement of the eyes. ”

“ We must always remember, however, that all conclusions drawn from [Pg 77] 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 ”

“ The theory of Donders that squint is less frequent in hypermetropia of high degree because too strong convergence would not suffice to furnish clear retinal images, is scarcely tenable in the face of such cases. If indistinct retinal images are added to a visual acuteness of only 1/3 to 1/4 still, even with faulty accommodation, it is difficult to believe how a child could learn to read if it did not hold the book close to its eyes, which was not the case here, and indeed seldom happens. ”

“ Squinting upwards or downwards seldom occurs as a symptom by itself; more frequently it is associated with convergent or divergent squint.
According to the law of associated movements, when an eye squinting upwards is put into fixation, the other eye should make a movement downwards, as normally both eyes move together up and down, yet this is not always the case. For example, when an upward deviation is present in convergent squint, it not uncommonly follows that the secondary deviation of the eye which usually fixes is also inwards and upwards ”

“ If we find that binocular fusion does not exist with available power of vision on both sides, but that the same conditions of sight appear in the eyes as we have learnt to attribute to squint, there is no reason for doubting the statements about a previously existing squint. ”