DIVERGENT SQUINT.
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.
If we then only compare that which admits of comparison, we first find out that divergent squint is rarer than the convergent form, and the cause contained in the ocular muscles is here brought to light still more clearly than there.
We must next distinguish between permanent and periodic squint, and we see the latter so frequently continue as such, that we must not consider the transition from this variety to the permanent one to be the rule.
In 183 cases of absolute divergent strabismus which appeared in my private practice in the same space of time as the cases of convergent squint above discussed I have been able to obtain exact determinations of the refraction and visual acuteness. The weakness of the fixing eye was the test for classing them among the statistics, and in patients who had been long under observation, the first certain determination of refraction, which was necessary, as several children are included who came under treatment with divergent strabismus and emmetropia whilst myopia developed itself later.
A. Divergent squint with hypermetropia.
(a) Permanent 4 cases. Visual acuteness of the squinting eye more than 1/7 1 case, V. less than 1/36 1 case, 2 excluded, one on account of complication with detachment of retina, the other on account of impossibility of testing vision.
(b) Periodic squint 5 cases. Among them 3 with double hypermetropia, 2 with emmetropia in one, and hypermetropia in the other eye. Visual acuteness of more than 1/7 in 3 cases; V. = 1/9 1 case; V. = 1/36 1 case.
B. Divergent squint in emmetropia.
(a) Permanent 32 cases. Among them 10 with alternating strabismus and anisometropia of at least 2 D. And in 9 cases emmetropia in one, myopia in the other eye; once simple hypermetropic astigmatism in one, with myopic astigmatism in the other eye. Visual acuteness of both eyes in these 10 cases more than 1/7. In the 22 cases of monocular squint the visual acuteness of the squinting eye amounted 8 times to more than 1/7 -, 10 times 1/12 to 1/36 (in 1 case V. = 1/36 with nystagmus of the squinting eye when put into fixation). V. less than 1/36 in 3 cases; 6 cases excluded on account of complications.
(b) Periodic squint 28 cases. Among them 5 with anisometropia of at least 2 D. (emmetropia in one, myopia in the other eye). Visual acuteness of the squinting eye more than 1/7 in 27 cases, less than 1/7 to V. = 1/12 in 1 case.
C. Divergent squint in myopia to M. = 2 D.
(a) Permanent 24 cases (among them 6 with anisometropia of at least 2 D.). Visual acuteness of the squinting eye more than 1/7 in 15 cases. V. less than 1/7 to V. = 1/12 2; V. less than 1/12 to V. = 1/36 3; V. less than 1/36 2 cases; 2 cases excluded on account of complications (one on account of atrophy of the optic nerve, the other on account of posterior polar cataract).
(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 cataract).
(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. 6·5 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 6·5 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.
| Permanent. | V. to 1/7. | V. < 1/7 to V. = 1/12. | V. < 1/12 to V. = 1/36. | V. < 1/36. | Excluded. | Periodic. | V. to 1/7. | V. < 1/7 to V. = 1/12. | V. < 1/12 to V. = 1/36. | V. < 1/36. | Excluded. | |
| 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 6·5 D. | 10 | 5 | — | — | 3 | 2 | 9 | 5 | 1 | — | — | 3 |
| M. more than 6·5 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 circumstance 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.