Part 3

In hypermetropia, if one causes fixation at about 30 cm. and then covers the eye with the hand, it frequently deviates inwards. Donders infers from this, that most hypermetropes prefer to sacrifice comfortable and clear vision in order to retain binocular vision. Now, it is easy to convince oneself that youthful hypermetropes see distinctly even without correction of their hypermetropia, and we may assume that they see comfortably if they do not complain of asthenopia; but that is by no means always the case, for the appearance of asthenopia is conditional on the relation of the degree of the hypermetropia to the amount of the accommodation, which, apart from a few other causes, depends chiefly on the age of the patient.

Just as we refer the deviation outwards of the covered eye to insufficiency of the interni or preponderance of the externi, we may conclude an inward deviation of the covered eye to be due to insufficiency of the externi or preponderance of the interni, and this all the more, as in hypermetropia the covered eye very frequently remains in fixation, and falls away exceptionally into relative divergence.

Just as in myopia even in the lesser degrees, insufficiency of the interni or preponderance of the externi is not rare, so in hypermetropia insufficiency of the externi or preponderance of the interni appears to be frequent; and if this disturbance of the muscular balance be followed even in myopia or emmetropia by convergent strabismus, this will of course happen still more easily if at the same time hypermetropia, or even without hypermetropia, the remaining favouring conditions mentioned by Donders are present. Of course I do not deny the effect of the hypermetropia and of those other favouring conditions, but only wish to draw attention to the fact with reference to them, that as a rule they do not of themselves suffice to produce convergent strabismus.

Nebulae have always been regarded as one of the causes of squint; here I quite agree with Donders that they may operate, firstly, as general causes of weak sight; secondly, through this, that the irritated condition, combined with the keratitis, may produce a spasmodic, afterwards a trophic shortening of the muscles; but this seldom happens.

Whether nebulae are found rarely or often in squint, depends in great measure on the statistic materials which are worked out. In my statistics they do not occur in any quantity worth mentioning, because in private practice purulent ophthalmia keratitis, and in short, the whole army of external inflammations of the eye is much rarer, than in that portion of the populace which fills public clinics. Further, it is to be observed that the mere occurrence of nebulae in squint proves nothing--even squinting eyes may develop keratitis. We must at least require to be assured that the squint began after the keratitis.

Among the causes which promote the occurrence of squint, Donders mentions also conditions which diminish convergence. We have ascribed a very important _role_ to the muscles, and have only to occupy ourselves here with the relation between the visual line and the axis of the cornea, which we have already mentioned on page 2. Donders has measured the angle _a_ in ten cases of hypermetropia with convergent strabismus, and from the comparison with hypermetropic non-squinting eyes draws the conclusion, that in similar degrees of hypermetropia a higher amount of _a_ specially disposes to strabismus. I will not repeat here the witty deduction by which Donders seeks to point out that a higher value of a must be followed by insufficiency of the externi and preponderance of the interni; the concession is enough that these circumstances exist and are the cause of the squint.

PERIODIC CONVERGENT SQUINT.

The opinion is prevalent that convergent strabismus usually begins in the form of periodic squint, and that a permanent deviation is developed in this way only. In many cases it may be so; on the other hand I have sometimes seen convergent strabismus arise suddenly, without a preliminary stage of periodic squint. This question, however, is of no special interest. It is more important to note that periodic squint frequently continues to exist unchanged, without ever becoming permanent.

Like the whole doctrine of strabismus, opinions on periodic squint have been governed by Donders' theory, regardless of facts, but as the accommodation frequently exercises a perceptible influence, it is judicious to consider first of all the cases in which this does not happen.

Convergent squint in myopia begins as a rule with periodic squint, and may continue to exist in this form: some patients who would not be operated upon have been under my observation for years; sometimes a correct position was retained for a long time, and sometimes strong convergent squint was present, proving that accommodation had nothing whatever to do with it. In myopia of higher degree the accommodation is scarcely used--unless concave glasses are worn; still periodic squint occurs under these circumstances. For example:

CASE 1. Miss B--, aet. 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--, aet. 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--, aet. 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.

What has here prevented the transition to permanent squint with a deviation corresponding to the great strain on the accommodation? That the accommodation was really in action is proved simply by the apparent emmetropia and the school-work, that no retention of binocular single vision took place is shown by the proved incapacity for binocular fusion of the retinal images. Nothing then remains but to accept the fact that in the ocular muscles inducement was only given for a slight periodic squint, not for a permanent one answering to the amount of accommodation used.

As further proof that periodic squint may occur even in hypermetropia quite independently of the accommodation, I should like to cite a case of intermittent convergent strabismus which a number of other oculists have seen besides myself.

CASE 5. Sophie S--, aet. 7-3/4, has suffered for two years from a strong convergent squint on the left side, occurring every other day. The deviation amounts to 7 mm. (the same deviation is transferred to the left eye, when the right is put into fixation). On the intervening days the position of the eyes is quite normal, on covering one only a slight deviation takes place. The visual acuteness amounts to 5/12 on the left, 5/24 on the right, ophthalmoscopically with atropine hypermetropia of two dioptres. Quinine has been given without avail, a convex glass of 2 D. also, which has been worn for the last half year, has not affected the deviation.

Diplopia was not present--on the intervening days free from squint, with the aid of a red glass, homonymous diplopia could be detected without perceptible deviation, still it was impossible to bring about a union of the double images by prisms. In the stereoscope the left field of vision was first inspected, then both, still fusion of the fields of vision was not traceable. The statements, moreover, as indeed could not be expected otherwise in a child of such tender age, were not free from contradictions, but the existence of normal binocular vision was very doubtful. I therefore performed tenotomy of the left internal rectus, after which normal position continued to exist on the following squint days. After three quarters of a year I saw the child again; the squint was perfectly cured, even on looking down, convergence was no longer present. Whether a permanent cure was thus obtained, seems to me doubtful, owing to the rare peculiarities of this case.

Mannhardt also describes a similar case of intermittent squint; that of a girl aged eight years, in whom periodic convergent strabismus had begun four years previously, and for two years had occurred regularly every other day. On undecided vision the eyes were normally placed, but as soon as a near or distant object was fixed, a considerable deviation inwards of the left eye occurred. Under the covering hand both eyes deviated inwards equally. On the non-squinting days strabismus could in no way be produced even by fixation of the nearest objects, only under the covering hand a deviation inwards ensued. The squint could not be removed by quinine, but only by correction of the hypermetropia of 3 D. In any case, then, hypermetropia was one of the causes of the squint, but not the only one, as it cannot operate on alternate days only.

Javal, who tries to make this case coincide with his theory, accepting an intermitting paresis of accommodation as the cause of squint, is manifestly in error, as Mannhardt particularly mentions that acuity of vision, refraction and accommodation remained perfectly equal on both days.

If it is thus proved, that also in periodic inward squint the deviation may occur quite independently of the accommodation, on the other hand it is apparent, that if once a tendency to squint exists, a disproportionately strong convergence may very easily unite itself with the accommodation. Particularly of course in hypermetropes, who are able to fix nothing without using their accommodation, a remarkable fluctuation of the squint angle very frequently takes place. 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; at most we can say, that the impulse for accommodation, habitually united with the intention to see distinctly, and the too strong convergence combined with it, also takes place, though by paralysis of the accommodation the participation of the same has become impossible. As accommodative squint those cases are chiefly indicated in which the deviation only takes place when there is a claim on the accommodation. In most cases of this kind hypermetropia is present. I have occasionally seen periodic accommodative squint with emmetropia of the fixing eye.

CASE 6 may serve as an example: H. B--, aet. 15, shows a considerable and very varying periodic inward squint. Sometimes correct position is present, sometimes strong deviation, indeed the latter only occurs on looking at distant objects, while for near ones correct position of the eyes generally takes place. The examination showed for the right eye hypermetropia 1.5, for the left myopia 3.5 D.; full acuity of vision on both sides. The squint occurring in the left eye on looking at distant objects was therefore accommodative; the effort of the accommodation necessary for correcting the hypermetropia united itself to an excessively strong innervation of the interni, as the interests of binocular vision came but slightly into consideration on account of the myopia in the left eye. For near objects the myopic eye is used without accommodation and therefore also without convergent strabismus of the right. But if one caused a point about 25 cm. distant to be fixed first with the right (hypermetropic) eye while the left was covered and then caused fixation to be transferred to the left, the accommodative convergent strabismus induced was alternately transferred to the left eye and continued, although the left eye fixed without any effort of the accommodation on account of its myopia. Double tenotomy of the interni and correction of the hypermetropia effected the cure of the squint.

The clearest cases of accommodative strabismus are those in which usually a correct position and sometimes even binocular fusion is present, while squint occurs only during the strain on the accommodation necessary for distinct vision.

CASE 7. Miss Bertha v. Pr--, aet. 27, shows strong accommodative squint of the right eye, said to have been observed by her parents when she was fifteen months old. Correct position of the eyes is generally present with indistinct vision; the endeavours to see clearly immediately causes striking convergence of the right eye. On the left hypermetropia 3.5 D., vision normal; on the right the same degree of hypermetropia, vision not more than 1/12 of the normal, no ophthalmoscopic report. On correction of the hypermetropia and with aid of a red glass crossed diplopia immediately appears, which is corrected by a prism of 5 deg. base inwards; prisms of 12 deg. with the bases inwards are overcome on fixation of an object about 12 ft. distant by divergence. The elastic tension of the ocular muscles necessitates then a preponderance of the externi, and an effort of the accommodation necessary to overcome the hypermetropia, which on account of the congenital amblyopia of the right eye unites itself with excessive convergence. Had the elastic tension of the ocular muscles made a preponderance of the interni a condition, permanent convergent squint would have been the result, and one would have called the weak sight of the right eye amblyopia from want of use.

Typical accommodative squint occurs quite independently of the will on each effort of the accommodation, and is not combined with diplopia. It is otherwise in those cases of hypermetropia of high degree in which patients voluntarily call forth convergent squint, and retain it for a short time for the purpose of distinct vision. They are then perfectly conscious of the squint, and perceive also as a rule the double images which occur at the same time; I have seen such cases in adults who could only produce the accommodation necessary for distinct vision by the aid of a too strong convergence; they, however, only now and then made use of this help. Although differing much from the typical form, these cases of voluntary accommodative squint were also included in the statistics.

In involuntary periodic (even if not accommodative) squint, the patient as a rule is not conscious of the occurrence of the false position; that exceptions to this occur Case 1 has given us an instance.

CONVERGENT SQUINT IN MYOPIA.

For the aetiology of convergent strabismus it is of interest to ascertain the age at which it is developed, and one of the first results we obtain is the exceptional position which the union of myopia with convergent strabismus takes in this category. Of the 56 cases contained in the above statistics I possess reliable information of the time of commencement in 11 cases; the squint was twice observed before the fourth year of life, once between four and ten years of age, eight times between the tenth and thirty-third years of life.

I must first state prominently with regard to the connection of myopia with convergent squint that I see no reason for holding short sight to be the cause of the squint, as v. Graefe does.

A specially severe strain of the eyes, as v. Graefe assumes, was not traceable in the cases observed by me. Excessive convergence and strain on the accommodation is often enough present in weak sight, for example, in astigmatism without the existence of squint; were short sight in general an inducement to convergent squint these cases would appear much oftener than they actually do, owing to the frequency of myopia. In my opinion the cause of their rarity lies in the fact that myopia is frequently combined with insufficiency of the interni and preponderance of the externi, but only rarely with the reverse condition of the muscles. If, however, a preponderance of the interni develops itself together with the myopia, convergent strabismus is easily produced, for without correction of the myopia by spectacles, the desire for retaining binocular single vision for everything beyond the far point is lessened by the indistinctness of the retinal images. Within the range of their field of distinct vision these squinting myopes frequently retain binocular vision, while the capacity for accepting parallel rays or retaining them for long, is lost.

Strictly speaking, the periodic squint present in these cases is of a peculiar kind, for the binocular single vision present within range of the convergence excludes the notion of squint; the latter only occurs when an object lying outside the point of convergence is fixed. Moreover, according to the common use of language, I have only used the expression periodic convergent squint for the change between a parallel direction of the visual axes and pathological convergence.

As squint in myopia usually commences at an age when binocular fusion has already become a fixed habit, diplopia regularly takes place with it, but patients become more easily accustomed to this than in paralysis of the ocular muscles, because the retinal images are indistinct and the double images in the field of vision always keep at about the same distance, while in paralysis of the ocular muscles the distance is constantly changing.

The myopia, in these cases, is not the cause of the squint, but only a favouring circumstance. If the same preponderance of the interni is developed at the same age in emmetropia, squint is not so easily caused, as the distinct retinal images present in the whole field of vision render it easy to retain binocular single vision. Therefore we see the same form of squint arise less often in emmetropia (see Case 45) when childhood is past, than in myopia. As a rule preponderance of the interni in hypermetropia leads eventually to convergent squint even in childhood.

In emmetropia and hypermetropia convergent strabismus seldom arises after the tenth year (paresis of the abducens of course excepted), therefore in my investigations as to the time of commencement of typical squint I have only considered those patients who came under my treatment before their tenth year. We must rely for the most part on the vague statements of the parents, which lose in exactness in proportion as the origin of the squint is of distant date; moreover, I have myself seen a great many of the children before they were four years old. In this way I have collected reliable information respecting the origin of the squint in 193 cases, and of these (_a_) 88 cases occurred in children one to three years old, (_b_) 53 in children three to four years old, (_c_) 35 cases in children of over four years of age. It is thus at once seen that in the great majority of cases, convergent strabismus commences in children under four years of age, who have not yet begun to read and write, and have no inducement to use their accommodation severely, and still less continuously.