Schweigger on Squint Part 4

(_b_) Periodic squint 23 cases. Among them 10 cases with anisometropia of at least 2 D. Visual acuteness more than 1/7 in all 23 cases.

D. Divergent squint in myopia 2 D. to M. = 4 D.

(_a_) Permanent 17 cases. Among them 2 with anisometropia of more than 2 D. V. to 1/7 9 cases. V. < 1/7="" to="" v.="1/12" 1="" case.="" v.="">< 1/12="" to="" v.="1/36" 2="" cases.="" v.="">< 1/36="" 1="" case.="" four="" cases="" excluded="" (2="" with="" choroiditis,="" 1="" with="" congenital="" cataract,="" 1="" with="" traumatic="">

(_b_) Periodic 8 cases. Among them 4 with anisometropia of at least 2 D.

V. to 1/7 7 cases. V. 1/36 1 case.

E. Divergent squint in myopia 4 D. to M. 65 D.

(_a_) Permanent 10 cases. V. more than 1/7 in 5; V. less than 1/36 in 3 cases, 2 excluded (one on account of large anterior synechia, one on account of choroiditis of the macula lutea).

(_b_) Periodic 9 cases. Among them one with anisometropia of more than 2 D. V. more than 1/7 5 cases. V. = 1/9 1 case; 3 cases excluded on account of complications.

F. Divergent squint in myopia more than 65 D.

(_a_) Permanent 8 cases. V. more than 1/7 4 cases, 4 excluded on account of choroiditis of the macula lutea.

(_b_) Periodic 10 cases. V. to 1/7 9 cases; V. = 1/12 in one case.

_Table of Refraction and Visual Acuteness in Divergent Squint._

[Transcriber's note: Key created to make table fit page]

KEY: A: Permanent.

B: V. to 1/7.

C: V. < 1/7="" to="" v.="">

D: V. < 1/12="" to="" v.="">

E: V. <>

F: Excluded.

G: Periodic.

H: V. to 1/7.

I: V. < 1/7="" to="" v.="">

J: V. < 1/12="" to="" v.="">

K: V. <>

L: Excluded.

-------------------+---+----+----+----+----+----+----+----+----+----+----+--- A B C D E F G H I J K L -------------------+----+----+----+----+----+----+----+----+----+----+----+-- Hypermetropia 4 1 -- -- 1 2 5 3 1 1 -- -- Emmetropia 37 18 -- 10 3 6 28 27 1 -- -- -- Myopia to M. 2 D. 24 15 2 3 2 2 23 23 -- -- -- -- M. 2 D. to 4 D. 17 9 1 2 1 4 8 7 -- 1 -- -- M. 4 D. to 65 D. 10 5 -- -- 3 2 9 5 1 -- -- 3 M. more than 65 D. 8 4 -- -- -- 4 10 9 1 -- -- -- -------------------+---+----+----+----+----+----+----+----+----+----+----+--- 100 52 3 15 10 20 83 74 4 2 -- 3 -------------------+---+----+----+----+----+----+----+----+----+----+----+---

It follows then from this, that periodic absolute divergent squint is just about as frequent as the permanent form and that both become more rare as the degrees of myopia increase. As, however, in spite of this, myopia is present in about 60 per cent. of all cases, the connection can be no other than this, that myopia frequently unites itself with insufficiency of the interni and preponderance of the externi; in this respect, as in every other, myopia and hypermetropia are directly opposed.

The setting up of a "hypermetropic divergent strabismus," dependent on hypermetropia, seems to me only to show how much people have been carried away by the idea that the cause of the squint must be given by the state of refraction. Isler claims 17 to 29 per cent. of the cases for hypermetropic divergent strabismus; of these, however, the half possess only slight hypermetropia of 2 D. or less, which perfectly agrees with the fact that the same observer has also found in convergent squint a remarkably high percentage of the lower degrees of hypermetropia.

Whether squint originates in the permanent or periodic form depends chiefly on whether the movement of convergence is retained or lost.

There are cases of considerable divergent squint, in which the near point of the convergence is scarcely removed, while on the other hand, the physiological innervation for convergence may be lost, without absolute divergence ever being brought about. In a number of emmetropic or slightly myopic cases with absolute preponderance of the externi, the physiological connection between accommodation and convergence is maintained in a relaxed way; thus, for example, it is impossible to converge voluntarily to a large object, as, for instance, a pencil held in the vertical line, while accurate convergence immediately follows on reading at the same distance; in other cases accommodation can be exerted to the near point, without inducing the slightest impulse to convergence. This circ.u.mstance is worthy of consideration for the prognosis of the operation. A mere relaxing of the tie between accommodation and convergence may be strengthened by practice, but if the impulse to innervation is completely lost, it will scarcely be possible to restore it again; as after complete laying aside of absolute divergence the relative form still continues to exist.

Those cases deserve special consideration in which emmetropia is present in one eye, in the other myopia. Slight degrees of one-sided myopia reconcile themselves with the continuance of a normal binocular act of vision. If the far point of the myopic eye lies at an inconvenient proximity even for reading, then, as a rule, the emmetropic eye is used for near as well as distant objects; if, on the contrary, the degree of myopia answers to a range of vision convenient for working, and visual acuteness is normal, then the temptation to use the emmetropic eye only for distance and the myopic one only for near objects is so overpowering, and the advantages on the other hand which would be offered by clinging to binocular vision so slight, that a convenient monocular vision is generally preferred. Even for objects which lie nearer the eye than the far point of the myopic, and at the same time farther than the near point of the emmetropic eye, for which, therefore, both eyes could secure clear retinal images, binocular vision is not used. In cases in which the patient can read with proper binocular fixation, if one covers all but one line and then makes with prisms double images standing one above another, it is the myopic eye alone which almost invariably shows a clear retinal image.

The usual result of this is, first a relaxing of binocular vision, and as together with this the motive for convergence, namely, the effort of the accommodation ceases, the conditions for the commencement of divergence are produced. Still the elastic tension of the ocular muscles decides even here; if the interni preponderate, convergent squint results, when the myopic eye is used for near objects, the emmetropic for distant ones. If the externi preponderate, then permanent or periodic divergent strabismus is caused. Nevertheless, in a remarkable minority of cases the elastic tension of the ocular muscles is so regulated that, despite relaxation of binocular fusion, neither convergent squint nor absolute divergence occurs, but simple relative divergence remains with employment of the myopic eye for near objects.

DYNAMIC SQUINT, INSUFFICIENCY OF THE INTERNI AND MUSCULAR ASTHENOPIA.

The habit of binocular single vision, when it has once reached its normal development, governs the movements of our eyes to a great degree; the desire to avoid double images makes itself continually felt; and where this is not possible, an uncomfortable feeling of uncertainty arises at every movement of the body. Double images are prevented as far as possible by movements of the eyes, which we must designate as voluntary when we are conscious of their occurrence.

If we follow a moving object with the eyes, the latter make corresponding movements in order to keep the image in the centre of both retinae. For example, if we look at a distant object which approaches in the direction of one visual axis, this eye will necessarily remain still, while the other will be put into convergence in proportion as the object advances. If this did not happen, if this eye remained also immovable, the retinal image would deviate outwards more and more from the macula lutea and diplopia would arise. In order then to avoid diplopia the macula lutea moves to where the retinal image is formed. We can, however, move the images on the retina by the aid of prisms without movement of the object. If, for example, we hold a prism before the eye so that the base lies towards the temporal side, the retinal image will be displaced towards the base of the prism, outwards then from the macula, and double images will occur, which are at once removed by a distinctly perceptible inward movement of the eye. In this way, by means of a prism applied with the base inwards, outward deviation may be produced, and even in a modified way deviations in height of the visual axes by means of prisms with the base upwards or downwards. Here the force of habit is apparent, for in the daily use of our eyes we continually practise the inward movement of the visual axes; we can also easily restore the customary degree of convergence by means of prisms with the bases outwards; physiologically indeed, it is quite immaterial whether an object is in a proximity to our eyes attainable by convergence, which causes double images until it is binocularly fixed, or whether by the aid of prisms we bring the retinal images of a distant object to parts of the retinae which do not correspond. If we look at a distant object fixed with parallel visual axes, under normal circ.u.mstances, prisms of 6 to 8 with the base inwards can be overcome, that is to say, as in weak prisms the deviation is equal to about half of the prism, an absolute divergence of the visual axes of 3 to 4 may be produced by which the double images are blended. It is immaterial whether we apply a prism of, say 8 to one eye, or prisms of 4 with the bases inwards to both. The facultative divergence thus attainable remains the same, which speaks for the fact, that this monolateral movement attainable by prisms is also combined with double innervation; and of course in the eye remaining in unmoved fixation, with impulses to innervation which are reciprocally abolished.

In the physiological use of our eyes we certainly never have occasion to practise absolute divergence, but we constantly practise the transition from the inward to the outward movement of the eyes, and experiments with prisms teach us, that the innervation of the externi therewith connected may even be carried somewhat beyond the physiological limits of parallelism. Moreover, the extent of the "facultative" divergence attainable by prisms shows a considerable lat.i.tude.

The case is similar with deviation in height of the visual axes. In looking upwards or downwards the innervation of both eyes is usually precisely the same, but on looking at any point when holding the head obliquely, the difference in height of the eyes then present must be balanced by a corresponding difference in the direction of the visual axes. The same thing happens, if we hold a vertically deviating prism in front of one eye in binocular vision; prisms of 2 to 3 may then be overcome by difference in height of the eyes; rarely is a much greater difference in height of the visual axes attainable. I have seen this particularly in those cases where facultative divergence also was greater than usual.

It happens especially in myopia that prisms of considerably more than 6 to 8 are overcome by divergence, and certainly without causing any inconvenience. Among the cases presented for examination, those, of course, are most numerous where the patients have some complaint to make, even if this have quite a different cause. In any case a divergent position of the axes of vision corresponds to the balance of the muscles, and this does not generally occur, for this reason, because retaining binocular single vision necessitates a parallel or convergent position of the eyes. Frequently, however, even a slight impediment to binocular fusion, such as the application of a red gla.s.s to one eye, suffices to procure preponderance in the elastic tensions of the muscles, and to cause the fixed point to appear double. We can put a stop to binocular single vision still more surely by applying to one eye a prism with the base upwards or downwards. If the double images of a point 4 to 5 meters distant show a crossed lateral position besides the difference in height caused by the prism, we may a.s.sume that an absolute divergent position of the eyes corresponds to the elastic tension of the muscles; and the measure of the deviation will be given by those prisms which, placed with the bases inwards before one or both eyes, bring the double images perpendicularly over one another. As a rule, in these cases the degree of divergence which occurs on cessation of binocular single vision, is almost as great as the facultative divergence, which may be reached in the interest of binocular single vision.

V. Graefe designates as "dynamic squint" that condition in which the position of divergence corresponding to the state of tension of the muscles does not occur because binocular vision is retained. Without clearly defined limits these conditions pa.s.s on into periodic squint, when either diplopia occurs together with the divergence, or the habit of binocular fusion becomes less frequent or is quite forgotten, while, however, according to the varying state of the muscles sometimes normal position, sometimes divergence, is present. A correct position of the eyes is quite possible even without binocular fusion, then only the regulator is wanting, which, in the varying play of the muscular forces, ensures the balance of position and movement.

The older ophthalmologists had a parallel strabismus and probably understood by that, what we now designate as relative divergence. The connection between relative divergence and myopia, pointed out by Donders, is universally admitted; on the other hand, in more modern literature we scarcely find any intimation of the fact that a parallel squint occurs, which is quite independent of myopia, and rests solely on the fact that the impulse of innervation for convergence is lost. A few examples may explain this condition.

CASE 13.--Auguste T--, aet. 28. On the left emmetropia, V. 12/20. On the right the visual acuteness is variously given, but certainly does not amount to more than 1/5 nor less than 1/10 of the normal.

Ophthalmoscopic report normal. The left eye is naturally the fixing one, the right always remains parallel--for near objects double images are present. A convergent movement is not attained, either for near objects, or by means of prisms with bases outwards for distant ones. Prisms with the bases inwards are not overcome; with vertically deviating prisms the double images of distant objects stand perpendicularly above one another.

CASE 14.--Ludwig v. K--, aet 32, has complained of diplopia repeatedly for fifteen years. Statement in August, 1877: Convergence to a pencil held before patient on the median line is only retained to about 50 cm., nearer, crossed diplopia occurs. In reading, binocular fixation is possible with an effort at a nearer point. The facultative divergence does not amount to more than 3; even by convergence to a distance of 4 mtr. prisms of 3 only are overcome. Emmetropia and full visual acuity on both sides. In Sept., 1880, three years after, the statement remained unaltered. Patient has only used the prismatic spectacles then prescribed off and on, as the symptoms are sometimes more troublesome, sometimes less so, and he exerts his eyes but little on the whole.

A restriction of movement of the internal recti did not exist in these cases; the absence of the convergent movement is not then to be set down to the interni not possessing the proper power for acting, but only to the fact that the impulse for their simultaneous innervation was wanting. We frequently find this absence of innervation in divergent squint, and then generally consider it to be a consequence of the squint, which, however, as the above cases show, need not necessarily be the case. If preponderance of the externi is at the same time present, absolute divergence is the result, but not always permanent squint, frequently only the periodic form. The anomaly of innervation may also usually be proved in such cases, in that after the removal by operation of the absolute divergence it continues to exist in the relative form; it can indeed happen that for a few days after the operation convergent squint is present for distance, together with relative divergence for near objects.

The highest phases of this anomaly, as represented in Cases 13 and 14, are seldom seen. Slighter degrees, which, like so many other things, are usually designated as "insufficiency of the interni," are more frequently met with and are combined with asthenopia. On the one hand, in looking at near objects a tendency exists to the formation of double images, which are removed by the action of the interni; on the other hand, however, the habit of binocular single vision is relinquished on account of the frequent diplopia. In all forms of squint we see that binocular fusion is forgotten; still it seems more natural to a.s.sume this to be the result, and not the cause of the squint, as Krenchel does.

Another form of relative divergence is that which is brought about in consequence of extreme myopia. The change in form of the myopic eye diminishes its mobility, a.s.sociated 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.

Finally, however, such considerable convergence as clear vision demands in high degrees of myopia, would be difficult even for a normally movable eye. Reasons enough therefore exist for giving up binocular fixation and using only the more convenient eye for reading, without effort to the accommodation and convergence. In myopia of high degree patients almost always read with relative divergence, and these myopes do just what we must advise them to do, they avoid strain of the accommodation and convergence of the visual axes and thus keep well.

Notwithstanding that this condition necessarily results from the nature of extreme myopia, it is frequently held to be pathological, which it certainly is not in itself. At most, the short-sightedness and change in form of the eye are pathological; the relative divergence on the other hand is simply a harmless result of the above conditions.

No doubts whatever exist about this relative divergence. The theory that the demands on the working eye must be very much increased is quite unfounded. If any harmful influence were to be feared for the fixing eye, one would observe the same in convergent squint, when, as a rule, one eye only is used for fixation even after operation.

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?

I have found no confirmation of Alfred Graefe's theory that in myopia the eye chiefly used in fixation is frequently affected with choroiditis of the macula lutea, &c., but have only observed that patients to whom this happens seek the advice of a physician more eagerly than when the same intra-ocular troubles befall the other usually neglected eye in connection with myopia.

Muscular asthenopia undoubtedly occurs; it is only a question whether it is as frequent as it is diagnosed. It has its foundation in that the convergence necessary for reading, writing, &c., can only be sustained by an effort of the internal recti, which exceeds their strength, and finally results in painful fatigue of the muscles, just as accommodative asthenopia depends on painful fatigue of the muscles of accommodation.

The similarity reaches still further. We occasionally find that despite considerable degrees of hypermetropia no asthenopia occurs even in persons who strain their eyes; while, on the other hand, asthenopic troubles appear in hypermetropia which are not removed by correction of the refraction and must consequently have some other motive. Yet still more is this the case with those disorders, of which muscular asthenopia may be supposed to be the cause. Notwithstanding the existence of a considerable preponderance of the externi, muscular asthenopia may be entirely absent. If we find, for example, that as soon as we do away with binocular single vision absolute divergence occurs even on looking at a distant fixed point, and that prisms of 12 to 30 are overcome by divergence, we may safely a.s.sume that the elastic preponderance of the externi must be overcome in reading, &c., in the interest of binocular, single vision by a stronger muscular effort of the interni, which is, however, very frequently accomplished without fatigue. Asthenopic disorders are also frequently present together with preponderance of the externi, which continue to exist despite the removal of the same by operation, and must consequently have some other cause. The diagnosis of accommodative asthenopia is as a rule confirmed _ex juvantibus_; this cannot be a.s.serted for the muscular form.

For example, Case 15.--Mathilde F--, aet. 21, has suffered from asthenopic disorders for three years. The investigation at the beginning of January, 1880, shows: On the left, myopia 4 D., V. = 5/18, No. 03 is read at 10 cm.; on the right, myopia 6 D., V. 5/24, 03 is read with difficulty, cylindrical gla.s.ses cause no improvement. Patient converges to about 8 cm., on exclusion absolute divergence of 3 to 4 mm. follows, with slight upward deviation of the right eye.