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 pass 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—, æt. 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—, æt 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 assume 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, 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.
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.