Clinical Investigations on Squint

by C. Schweigger

In contrast to paralysis of the ocular muscles stands the typical concomitant squint, in which the squinting eye normally accompanies the movements of the other. Transitional forms may thus be brought about, in some of which the paralysis recovers, with complete or almost complete restoration of movement, but with continuance of the squint. On the other hand, in concomitant strabismus, restriction of movement towards the opposite side not unfrequently develops itself.

This impairment of movement has its origin generally in a want of use. Those who squint have less need for movement, since one of their eyes is already directed obliquely. In divergent strabismus this is apparent, but in convergent strabismus the squinting eye governs the field of vision on the side to which it turns. When the fixing eye is turned towards the side of the squinting eye in convergent strabismus, the latter, it is true, makes a concomitant movement, which does not, however, bring it by a long way to the limit of the movement of which it is capable. The defect of motion is therefore generally present in both eyes, and is usually most marked in the squinting eye. Often, indeed, there is present at the same time a congenital or acquired insufficiency of the antagonistic muscle, but that want of use has also much to do with it, is shown by the improvement of mobility that often follows even short practice.

From the law of equal innervation, which governs the movements of the eyes, it follows that the fixing eye lapses into the associated deviation as soon as the squinting eye is directed straight forwards. If, for example, a convergent squinting eye is put into fixation, an innervation of the external rectus, with which just as strong an associated contraction of the internal rectus of the other eye, is called forth; the direction of the squint then, as well as the degree of deviation, is transferred from one eye to the other. It is naturally the same with divergent squint.

Squinting upwards or downwards seldom occurs as a symptom by itself; more frequently it is associated with convergent or divergent squint.

According to the law of associated movements, when an eye squinting upwards is put into fixation, the other eye should make a movement downwards, as normally both eyes move together up and down, yet this is not always the case. For example, when an upward deviation is present in convergent squint, it not uncommonly follows that the secondary deviation of the eye which usually fixes is also inwards and upwards; only exceptionally in cases of deviation in height of the squinting eye does the sympathetic movement take place without change of height. Sometimes with deviation of height, I found combined a distinct rotation of the eye, generally thus, that together with the movement upwards was combined a rotation of the vertical meridian outwards and vice versâ; in fixing the eye a rolling inwards was combined with the movement downwards. The other eye then usually showed a similar rotation (thus the meridian of both eyes rotated simultaneously to the right or left), but the deviation in height was not always the same.

The law of equal innervation requires in alternate fixation, first with one eye, then with the other, that the same degree of deviation be transferred to the non-fixing eye. When exceptions appear, and the deviation in the two eyes is unequal, it is (provided the inequality has not been caused by attempted operation, or is the result of paralysis), usually to be explained by the fact, that an accommodative movement takes place when we are expecting an associated one. For example, if there is convergent squint and hypermetropia in both eyes, but more hypermetropia in one than the other, in alternate fixation it will be found that the least hypermetropic eye always undergoes the greatest deviation, because in fixation with the more hypermetropic eye a stronger effort of accommodation unites itself with a corresponding innervation of the internal rectus, which is transferred equally to the other and non-fixing eye. Thus it happens frequently in divergent strabismus, when one eye is myopic, the other emmetropic. If the latter fixes an object stationed near the "far point" of the myopic eye, the internal recti and the accommodation act simultaneously; on the other hand if the myopic eye fixes, it wants no accommodation and the emmetropic eye sinks into divergence.

With regard to the immutability of the squint; it must not be understood that the squint angle always remains the same with the same individual; in most cases the amount of deviation varies, the squint is now less, now greater; it is desirable however, to know the bounds within which it fluctuates.

To determine the degree of the squint one can either ascertain the angle of the squint, or use v. Graefe's so-called linear measure of deviation.

The squint angle is that angle, which the visual line of the squinting eye encloses with the direction it ought normally to take—it may be measured with the aid of a perimeter. The patient's head is so placed by means of a chin rest, that the axis of the squinting eye is in the centre of the arc of the perimeter; a distant point in the centre of the field of vision is fixed. Behind the patient is a candle, the reflection of which is thrown into the squinting eye by means of a plane mirror; now slide the mirror along the arc of the perimeter, till the reflection on the cornea stands in the centre of the pupil of the eye which is under observation. The point which the mirror occupies on the arc of the perimeter, indicates the squint angle. In deviation in height of the squinting eye, bring the arc of the perimeter into the corresponding direction and so measure at the same time the degree of deviation in height. Were the method more exact than it is, one would be able to measure the angle formed by the visual line and the axis of the cornea.

To find the linear measure of the deviation, cover the fixing eye and allow the squinting eye to fix. Hold a millimetre measure close to the under lid, so that a chosen portion of it stands under the centre of the pupil; uncover the other eye and when the squinting eye returns to its deviation, it can be seen over which point the centre of the pupil stands, and the linear measure of the deviation is thus obtained. The secondary deviation of the other eye is measured of course in the same way. If, in consequence of amblyopia, the squinting eye possesses no certain fixation, the measure may be so held that the nil point of the division coincides with the lower punctum, and then in unchanged fixation the portion lying under the centre of the pupil is determined, first in the sound and then in the squinting eye.