CHAPTER V.

On Impaired Vision connected with Strabismus-Strabismus-Definition-Forms of Strabismus - Directions of Deviation--Apparent Strabismus-Images in Convergent and Divergent Strabismus --Concomitant Strabismus-Diplopia -Diagnosis of Concomitant and Paralytic Strabismus-Measurement of Deviation in Strabismus-Treatment-Influence of Strabismus as regards Military Service.

STRABISMUS.

Definition. Squint. Abnormal obliquity of the optic axis in one or both eyes. Malposition of the two eyes relatively to each other of such a kind that the directions of their optic axes are not in mutual accord. The visual lines do not meet together at the distance at which a given object is looked at, and true binocular vision is consequently unattainable by persons affected with strabismus.

Various Forms of Strabismus.-Two distinct forms of strabismus, due to different causes, are met with; one functional, depending directly upon irregular muscular action, originating in, or accompanying, anomalous states of refraction, or impaired vision of one or both eyes, and generally distinguished by the name of 'concomitant strabismus,' from the squinting eye accompanying the normal eye in all its movements; the other pathological, produced by partial or complete paralysis of one or more external muscles of the eyeball, or due to pressure from causes existing within the orbit. In the first of these two kinds of strabismus all the muscles act, but the normal concert of action is not maintained in the binocular movements; in the second, the movements of one or more of the muscles are impaired or entirely arrested. The term 'strabismus' is employed by many surgeons to designate only the former kind, or true concomitant strabismus; absence of concordance in the visual axes from defective innervation or complete paralysis being classed under other special headings, such as paralysis of the ocular muscle or muscles affected, or under diplopia.

Directions of Deviation in Strabismus.-Strabismus is commonly either convergent, the squinting eye being turned toward the median line; or divergent, being turned outwards. The deviation may, however, be upwards or downwards, though it is rarely so. In convergent and divergent strabismus the retinal images of an object are separated laterally, but remain in the same horizontal plane; in the other directions of deviation, they vary in altitude and inclination according as the superior and inferior recti or oblique muscles are involved.

When the subject of convergent strabismus looks at an object

one eye only is visually directed upon it, the other eye has its visual line directed inwards; in the same way, when the subject of divergent strabismus looks at an object, one eye only is directed toward it, but the other eye has its visual line directed outwards. In either case, while the sight is thus fixed upon the object the deviation is limited to the eye which is turned inwards or outwards.

The deviation may only occur at intervals, or after occasions of long-continued employment of the eyes at near objects, and under conditions of unusual ocular strain and fatigue, or general weakness; but although the occurrence of the deviation, or of the strabismus, may thus vary, the direction in which the deviation takes place, when it does occur, will always be alike in the same

eye.

Apparent Strabismus.- A condition which simulates strabismus is sometimes noticeable, and is usually referred to under the name of apparent strabismus. It depends upon the angle formed by the meeting of the visual line with the line of the optic axis at their point of intersection within the eye. (See fig. 5, p. 5.) The relative positions of the two eyes, as elsewhere mentioned, are judged of by observing the relative positions of the centres of the two cornea with respect to each other and to the middle points of the palpebral openings. When two Emc. eyes are regarding a distant object, the visual lines of the eyes are practically parallel, and as these lines pass a little internal to the centres of the corneæ, it follows that the centres of the corneæ must be turned a little outwards in respect to the middle points of the palpebral apertures. In Emc. eyes this divergence, or apparent strabismus, is so slight as to be hardly observable even on close inspection. The divergence is, however, increased in Hc. eyes, as the angle formed by the meeting of the optic axis and visual line is greater, and the apparent strabismus becomes all the more marked. In myopic eyes this angle is lessened, and the visual line may even change position and pass outside of the optic axis, when the appearance of strabismus may be presented, but with a convergent instead of a divergent direction.

Images of Objects in Convergent and Divergent Strabismus.-In convergent strabismus the two images of an object are formed on the corresponding retina; that which appears on the right side belongs to the right retina, that on the left side belongs to the left retina. In divergent strabismus the images are crossed and formed on the opposite retina; that which appears on the right side belongs to the left retina, that on the left side to the right retina. The relative positions of the images of an object in some instances of diplopia depending on slight degrees of muscular inefficiency, in which the deviation is so limited as to be hardly noticeable by ordinary observation, are almost the only means by

which a surgeon is enabled to decide what particular muscle or muscles are affected. Their respective situations in relation to the two eyes will be rendered obvious to the patient at once if the two images are seen by him under different colours, as may be effected by causing the patient to look with one eye through stained glass (see figs. 3 and 4). If the deviation be horizontal, and the images are homonymous, there is convergent displacement, and the external rectus muscle is affected; if crossed, there is divergent displacement, and the internal rectus muscle is affected.

The frequent associations of convergent strabismus with hypermetropia, and of divergent strabismus with myopia, have been already remarked on in the sections on these refractive conditions.

Concomitant Strabismus.-Concomitant strabismus may be only occasional or it may be persistent. The deviation may alternate between the two eyes, each in turn being capable of directing its visual line on the object while the other deviates; or one, or both eyes, may be permanently displaced. When the deviation alternates between the two eyes, the amount of deviation in the right eye when the visual line of the left eye is directed upon the object looked at will be equal to that of the deviation in the left eye, when the macula lutea of the right eye is in line with the object.

In the strabismus of muscular paralysis the loss of power of the eye to move in the line of impaired, or lost, action of the paralysed muscles and the deviation in the opposite direction are constant.

When the strabismus exists only to a moderate amount, and the deviation is limited to one eye, it is sometimes not easy of detection so long as the two eyes are observed together. But it may usually be readily demonstrated, if the retinal functions of the two eyes are preserved, in the following way. The person under observation is directed to regard an object at the distance of a few feet in front of him. Each eye is then alternately covered by the hand of the observer. When the eye whose visual line is directed to the object is covered, the squinting eye may be noticed at once to change its position. It will assume a direction which will bring it visually in line with the object; and equally, on again uncovering the other eye, it will recede from the new to its former place.

Diplopia. Whenever the physiological harmony of the visual lines is interfered with in binocular vision, as happens with strabismus, the discord necessarily causes the formation of double retinal images, but does not necessarily cause diplopia, or double vision.

Diplopia does not exist in the large majority of cases of concomitant strabismus, but is constantly found in strabismus due to paralysis, or partial inefficiency of one or more of the ocular muscles.

There are several reasons why the subject of concomitant strabismus can avoid taking note of the image in the displaced eye, while the subject of paralytic strabismus is unable to do so. In the

former, the eye whose visual line is properly directed on the object has its image formed on the macula lutea, and it is of course relatively sharp and distinct; while the image in the displaced eye is pictured on a part of the retina away from the macula lutea, and is, therefore, more or less dull and indistinct. The relative. displacement of the two eyes, or the strabismal angle, remains constant, for in concomitant strabismus there is no interference with the range of the muscular movements of the globe of the eye, as in paralytic strabismus, but only with the position of the range over which the movements take place. In consequence, the indistinct image on the retina of the displaced eye is always formed at a fixed part of the retina; just as much so as the sharp image is formed on the macula lutea of the normally directed eye. The retinal sensitiveness of the part where the relatively dull or spectral image occurs becomes lessened at first from exhaustion and subsequently from disuse, and the suppression of mental attention to the image by the subject of concomitant strabismus in the efforts which he instinctively makes to rid himself of the troubles of double vision, becomes easily established. On the other hand, in strabismus due to paralysis of one or more muscles, the relative positions of the two retina are constantly changing as objects in different posi-. tions are looked at, and corresponding changes constantly occur in the situations of the retinal images. If, for example, the external rectus of one eye be paralysed, causing a certain amount of inward deviation of this eye, as the sound eye turns inwards, or in a direction toward the paralysed muscle of the deviated eye, the strabismus is rendered more and more obvious-the strabismal angle becomes greater, and the distance of separation of the double images in the two eyes is increased. If the sound eye turn outwards, or in a direction away from the paralysed muscle of the deviated eye, the two eyes in the case supposed will assume a direction more in concert, and at a certain point the visual lines may even both centre in the object, and the diplopia disappear. The strabismus is thus seen not to be constant, as it is in concomitant. strabismus, but varies according to the position of the object looked at, and consequently the position of the image in the displaced eye is equally shifting and inconstant. There is little to interfere with the retina retaining its sensitiveness over its whole area, and under these circumstances the strabismal subject finds himself unable to suppress perception of the second image when it ever occurs.

When concomitant strabismus happens in very early life, the suppression of the images in the squinting eye seems to take place with the utmost facility, amblyopia is established, and if the strabismus be not rectified, the retinal sensitiveness becomes lost beyond recovery.

Diplopia, due to slight muscular paralysis, may be noticed by a patient before strabismus is visible to an observer. It may amount

to little more than mere haziness from overlapping of the two images at varying distances, but may still be accompanied by a difficulty of co-ordinating the touch with the sight of objects, as well as by the vertigo and other subjective symptoms to which diplopia usually leads. If two separate images are noticed, the sharper image produced on the macula lutea of the sound eye, and the weaker image on the retina of the deviated eye, may be distinguished by the aid of the colour test elsewhere mentioned, if they are not otherwise recognisable; and the recognition as soon as established will indicate not only which eye is disordered, but also the affected muscle or muscles. Iftwo images of a bright object, as of a lighted candle, placed at a distance, are seen, and one image is coloured by the intervention of coloured glass before one of the patient's eyes, the patient will readily point out to the observer the situation of the image belonging to the eye before which the coloured screen is placed and that belonging to the other eye. The observer will at once see, from the relative positions of the two images, in which of the patient's eyes the true image is formed. The affected eye is thus made known, and the situation of the displaced image, relatively to that of the true image, whether it be to its right or left hand, or above or below, indicates the muscle or muscles which are at fault in this eye. The displaced image always appears on the side opposite to the ocular deviation, so that if the images are homonymous, convergent strabismus is shown to exist, while, if they are crossed, divergent strabismus is indicated. If, for example, it be the muscle concerned in convergence which is affected, say of the right eye, leading to divergent displacement, the spectral image will be crossed to the left hand of the true image as seen by the patient; if it be the muscle of divergence which is affected, leading to convergent displacement, the false image will be to the right side of the true image as seen by the patient and indicated by him to the observer. (See figs. 3 and 4, p. 4.)

Diagnosis of Concomitant and Paralytic Strabismus.-The conditions already mentioned form sufficient means of distinguishing between these two forms of strabismus, but one other difference between them, which is useful for diagnosis, may be presently mentioned. The characteristics of concomitant strabismus, which have been already referred to, are: 1. The affected eye retains its full natural range of movement, but this range is displaced. Its movements do not take place in the normal arc, the eye turning more in one direction, and less in the opposite direction, than the normal eye; 2. The deviation of the affected eye, and the amount of disagreement in direction of the visual lines of the two eyes, at whatever distance the object may be placed which is looked at by them, are constant; and 3. The displaced, or spectral image, formed in the affected eye is mentally ignored, so that diplopia does not occur. On the other band, in paralytic strabismus, 1. The