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.
Moreover, this slight aid given by tenotomy has its ground not solely in the condition of the opposing recti muscles. In other practices I have seen cases enough in which tenotomy of the externi, performed on account of relative divergence, was followed by convergent squint, just as injudicious division of the interni may induce divergent squint. It is probable, therefore, that the faulty effect of simple tenotomy in permanent absolute divergent squint depends on other causes, which, in my opinion, are to be found in the obliques. The loop formed by the obliques round the posterior circumference of the eye is most stretched, when the visual line falls in with the muscular plane of the obliques in a medial direction of the eyes. On the whole, then, it is proved that the obliques are extended on turning the eyes inwards, but shortened on turning the eyes outwards by means of their muscular action. In divergent squint, if the movement inwards occurs but seldom or not at all, the obliques consequently are not extended in a normal way—it follows then that they lose in ductility, offer greater resistance to the inward movement, and by means of their elastic tension continually draw the posterior pole of the eye inwards and the cornea outwards. As in strabotomy we cannot get at the obliques, it seems all the more desirable to offer them stronger resistance by greater tension of the internus by means of advancement. Certainly tenotomy of the external rectus of the fixing eye is as a rule also necessary. A sufficient result is usually thus obtained at once; if it is much lessened in the course of one or two months there is nothing to prevent the repetition of the tenotomy of one or the other external rectus.
The innervation for the movement of convergence is not always perfectly lost; it withdraws itself from the influence of binocular fusion because this is gradually forgotten while a convergence, even if an insufficient one, unites itself with the effort of accommodation. If we ask such patients to fix a large object lying near, a pencil, for example, they cannot usually converge upon it, whilst if we ask them to read at the same distance, a distinct convergent movement occurs; large objects are sufficiently clearly recognised, even without distinct retinal images, and the supposition that an effort of accommodation is present is only justified if we employ sufficiently small objects at the examination, in order to distinguish which, clear retinal images are necessary. Of course we must have regard to the condition of refraction; myopes, who use their far point for reading, want no accommodation, therefore no convergent movement occurs, even if the impulse of innervation for it, is not yet quite lost. However, the innervation for convergence may be lost, without the internal recti losing in elastic tension. The operative importance of this relation may be illustrated by an example.
Case 51.—Bertha K—, æt. 10, has myopia 5 D. on both sides, visual acuteness 12/20, and divergent strabismus. At 4 mm. the crossed diplopia is corrected by prism 23°; a convergent movement is no longer attained, at most parallelism of the visual axes. Tenotomy of both interni on October 2nd, 1873. The immediate result was convergent squint, with a defect in movement outwards amounting to 4 to 5 mm. in both eyes. On October 9th prism 37° was still necessary to unite the homonymous double images at a distance of 4 m.; single vision existed only to about 20 cm. The area of single vision gradually extended itself; at the end of October it was restored for distance also, facultative divergence nil; however, relative divergence was present for near objects. Naturally this was not the result of muscular weakness of the interni, for they had proved their capabilities by a convergent squint, fortunately only temporary, which made one anxious, but was solely the result of a faulty innervation. The further course was also interesting. After three years, in October, 1876, the myopia of the left eye amounted to 8 D., that of the right 7 D., visual acuteness 1/2 on the right, on the left 3/4 of the normal; a posterior staphyloma measuring about 1/3 of the diameter of the optic disc was present. The left eye was used for near objects with relative divergence of the right and the occasional occurrence of diplopia; there was convergence only to about 15 cm. Facultative divergence nil.
We very frequently have the opportunity of seeing, that myopia increases even after tenotomy of the externus, and if von Graefe's assertion that the progress of myopia would be brought to a standstill by means of tenotomy still finds believers, I should like to cite one example which offers proof to the contrary.
In permanent divergent squint we shall have, as a rule, to combine shortening of the internus of the squinting eye with tenotomy of both externi, even if the convergent movement is still possible to a slight degree. The result thus obtained differs somewhat; sometimes it suffices at once, sometimes a repetition of the separation of the externi is necessary later on. Two examples may illustrate this.
Case 52.—Miss Marie M—, æt. 22, has squinted on the left side since her third year, nominally after a keratitis, which left behind in the left eye a nebula of the cornea of small circumference. The deviation amounts to 8 mm. The visual power is much worse than the opacity of the cornea leads us to suppose, with visual axes deviating inwards fingers were only counted at a distance of about 1 m.
On the right myopia 1 D., V. = 4/5. A slight convergent movement is still practicable. At the end of May, 1879, shortening of the left internal rectus, tenotomy of both externi. The next day slight convergence on viewing distant objects, correct position after four days. In January, 1880, correct position of the eyes, convergence possible to about 20 cm. While a correction of 8 mm. was immediately obtained here, the same operation does not always permanently suffice for slighter deviations.
Case 53.—Ernest Sp—, æt. 11-1/2; divergent squint had been observed as early as his second year. The deviation amounts to 5 or 6 mm., is sometimes alternating, generally the left eye deviates. No convergent movement on fixing a pencil about 25 cm. distant; the right eye is then used for reading, the left one makes a distinct, but not a sufficient, movement inwards. Emmetropia on both sides, visual acuteness nearly perfect on the right, on the left 2/3 of the normal. Even with red glass and prisms deviating in a vertical direction, double images not perceived. On October 2nd, 1879, shortening of the left internal rectus, tenotomy of both externi. A week later divergence was no longer present. When reading, the left eye makes a distinct, perhaps rather too great, movement of convergence, and yet six weeks after the operation, distinct divergent squint was again present, even if to a slighter degree than before; the left eye deviates 3 to 4 mm., the right 2 to 3 mm. outwards. The result obtained amounted then to not more than about 3 mm. In the middle of December the tenotomy of both externi was therefore repeated. A week after the operation convergent squint of 2 mm. is present with homonymous diplopia. A pencil made to approach on the middle line is seen double to about 20 cm., on approaching nearer, double images are not perceived in spite of distinct relative divergence. Double images at a distance of 4 m. are corrected by prism 25°; as, however, normal binocular vision is not present, the value of this statement is very questionable. Three weeks after the second operation the position of the eyes was normal, and the slightest convergence was perceived only on close investigation. Double images are no longer observed, however they may still be brought to view.
In periodic divergent squint, if the deviation is considerable and frequent, if at the same time the normal near point of convergence is only attained with difficulty or not at all, we can hardly combine shortening of the internus with tenotomy of the externus; more often indeed, additional tenotomy of the externus of the other eye is necessary in order to obtain a permanent cure. In exceptional cases (when it seemed to me as if the squint depended more on insufficiency of the internus than on preponderance of the externus) I have confined myself to shortening the internus without separating the externus; I will quote just one example of this.