CHAPTER V DEVIATIONS OF THE EYEBALL (Part 1)

CHAPTER V DEVIATIONS OF THE EYEBALL An eye is said to deviate when it does not maintain its normal position relative to the other. When we speak of the normal relative positions of the two eyes we mean that as long as the person is awake and conscious, their visual axes are parallel while the organs are at rest, i.e., looking into infinite space, and converge equally when looking at a finite object, though we cannot say that the upward and rarely parallel position assumed during sleep or unconsciousness is not just as normal. During waking hours this position is maintained by the balance of the tension of the extrinsic muscles, which act in unison in all movements of the eyes under the guidance of nervous impulses which start from various centers and are transmitted along certain nerve tracts. These muscles, nerve tracts and centers constitute the motor mechanism of the eyes, part of which is well understood, but we have as yet a very imperfect knowledge of many of its extremely important nerve centers and tracts. We know that when all of the parts of this complex mechanism work in harmony the normal relative positions of the eyes are maintained in a perfect balance, and that a fault at any point induces an abnormality in the ocular movements. Sometimes we can locate such a fault, and perhaps determine its nature, but in many cases we cannot. This mechanism seems to be incomplete at birth, for babies move their eyes about with a cheerful disregard of their normal relative positions, although the latter are assumed and maintained as the children grow older. Consequently we need not worry over the various deviations to be observed in a baby’s eyes unless an abnor- mal position is taken and maintained for some time. In such a case we may determine whether any of the muscles are paralyzed or not by the test suggested by Bartels. Rotate the body of the child to the right; normally the eyes remain directed to the left at first and then turn rapidly to the right; turn the body again to the left when the rotation of the eyes has stopped. If the 78 DEVIATIONS OF THE EYES 79 movement of either eye is imperfect as the body is rotated, the muscle which should have acted is paralyzed. In an older child, or an adult, a deviation of an eye may be due to strabismus, or to a paresis of one or more muscles. There is a fault in the motor mechanism in both conditions, but in the latter it is a morbid lesion, while in the former it probably is develop- mental. The differentiation between muscular paresis and _stra- bismus is to be made first of all, for in the first the deviation of the eye is a symptom of some disease which needs treatment, while in the second we have to deal with the condition itself. In some cases it is easy to make this differentiation, but in others it is quite the contrary. DIFFERENTIATION OF STRABISMUS AND MUSCULAR PARESIS We note the age of the patient, and inquire when the deviation was first noticed. Muscular paresis may occur at any age, but a convergent strabismus usually appears between the third and fifth years, often is said to have followed an attack of measles, whooping cough, or some other sickness, and never develops after binocular single vision has been established unless the sight of the deviating eye has been lost, or the eye has been drawn in by a cicatrix. Divergent strabismus usually appears about the age of ten or twelve. In children less than three years old a constant deviation is most likely to be paretic, an alternating or intermittent one to be strabismic, while irregular deviations which are neither con- stant, alternating, nor intermittent, are of no importance except as showing that the mechanism of the nervous control of the eyes is not yet perfect. In a child four years old and upward the chances are that an inward deviation is strabismic, and it certainly is so if it is alternating or intermittent, but as the age is no pro- tection against the formation of lesions which impair the motor mechanism, tests must be made to exclude paresis of the muscles. The sudden or gradual appearance of a deviation in an adult who has enjoyed binocular single vision must be due to loss of vision in the affected eye, the contraction of a cicatrix, or muscular paresis. We need to be acquainted with a number of tests, for some that are well suited to intelligent adults cannot be used satisfac- 80 DIAGNOSIS FROM OCULAR SYMPTOMS torily with children, or when intelligence is lacking, and a paresis may vary from a slight disability, which is noticeable only when the patient looks toward a certain part of the field, to a total paralysis of the affected muscle or muscles. When the deviation is well marked the simplest test is to have the patient look at some object, like the examiner’s finger, nose, or eye, measure the degree to which the affected eye deviates, cover the good eye with a hand or screen, and measure the deviation that has taken place in it. When we cover the good eye so that it can no longer see the object fixed upon the deviating eye will turn so as to fix the object instead, provided its vision is good enough to enable it to see the object, and the good eye will deviate. We call the original deviation of the affected eye the primary, that of the good one behind the screen the secondary. This test gives us two valuable diagnostic signs. If the secondary deviation is exactly equal to the primary, the case is one of strabismus; if it is greater than the primary we have to deal with muscular paresis. If the movements of the two eyes to fix are equal as each is covered alternately, the deviation is strabismic; if the movement of one is greater than that of the other, it is paretic. The commonly accepted explanation of these facts is that the innervation necessary to cause either eye to fix is the same in strabismus, while a much stronger innervation has to be sent to a paretic muscle to cause it to act than is needed by a normal one, and as the coacting muscle in the other eye re- ceives the same amount of innervation it contracts excessively and thus produces a greater degree of secondary deviation. As the eye is drawn farther aside by a greater secondary deviation its move- ment to fix must be greater than that of the other. This test is applicable in well-marked cases of both horizontal and vertical deviations in patients of all ages, provided that the examiner is sufficiently adaptable to children. Another test for well marked cases is to move the finger or a light from one side of the field of vision to the other and back again, having the patient follow it with his eyes. If one eye lags behind the other as they turn in a certain direction, the muscle that is acting is palsied, and if the eye stops in the middle line the paralysis is nearly or quite total. This test can be used in both children and adults, as the eyes of a baby will follow a small electric light, and is applicable to both horizontal and vertical deviations. DEVIATIONS OF THE EYES 81 If neither eye will fix the object the deviation is either simulated or hysterical. Diplopia is present and annoying in paretic deviations when the patient is old enough to appreciate it, has possessed binocular single vision, and the condition has not existed too long, while it is very rare in strabismus. Diplopia in which the distance between the double images increases as the eyes are turned in one direction and decreases as they move in the opposite way is almost certainly diagnostic of muscular paresis, especially when it is accompanied by vertigo and faulty orientation, though its absence is not so reliable an indication. Whenever it is possible for a patient with paresis of an ocular muscle to do away with the diplopia and secure binocular single vision by twisting his head into any posi- tion, he will assume it habitually, no matter how awkward or inconvenient such a posture may be, so a habitually distorted posi- tion of the head should be enough to suggest a possible paresis of one or more muscles of the eye. The most pronounced cases of this ocular torticollis are met with in paralysis of the superior oblique. Such a pose will not be taken unless binocular single vision can be obtained thereby, or before the development of the faculty. ; Other diagnostic points are that paretic deviations usually are constant, seldom of a fleeting nature, and never alternating, while strabismus may be constant, alternating, or intermittent. If we have an opportunity to examine the eyes during sleep we find that a strabismus has disappeared, while a paretic deviation persists. Amblyopia of the deviating eye increases the probability of strabis- mus, but does not exclude paresis. The coexistence of pareses in muscles that are not ocular, or of other symptoms which point to a lesion of the central nervous system, greatly increases the proba- bility that the deviation is paretic. Most, but not all vertical devia- tions are due to muscular paresis. A convergent strabismus in a child may decrease as he grows older until the eyes become nearly straight, or pass over into divergence, but binocular single vision never is attained and the condition cannot be affected by medica- tion, while a muscular paresis may disappear spontaneously, or as the result of medical treatment, with the restoration of binocular single vision. After we have made this differentiation a number of other obser- vations have to be made. 82 DIAGNOSIS FROM OCULAR SYMPTOMS STRABISMUS We note the direction of the deviation. If the visual line of one eye is inclined toward the nose the strabismus is convergent; if it is inclined outward the strabismus is divergent; if it inclines up or down we have a case of vertical strabismus. A great advantage of the scientific nomenclature of these conditions, which has been urged for adoption, is that the terms describe much more exactly the direction of the deviation. The general term for strabismus is heterotropia. Convergent strabismus is esotropia; divergent strabismus is exotropia; vertical strabismus in which one eye turns up is hypertropia, one in which an eye turns down is hypotropia. Vertical deviations almost always are more or less out or in as well as up or down, and according to Hansell nearly every case of esotropia is associated with more or less hypertropia; these varia- tions are indicated by such compound terms as hyperesotropia and hypoexotropia. In spite of this manifest advantage the mental effort to distinguish the meaning of words which resemble one an- other so closely, both when written and when spoken, as hyper- tropia, hypermetropia, and hypotropia, may act as a deterrent to its universal adoption. Such a mental effort hampers the close following of a line of thought, and this is the reason it is not adopted here. A symblepharon may draw an eye to one side, when the pa- tient will suffer from diplopia and its attendant symptoms, the same as in muscular paresis, if the sight has not been impaired too badly; this cause is perceived readily. A convergent or divergent deviation may follow an operation for pterygium through the contraction of cicatricial adhesions; the history and the presence of a cicatrix guide us to the correct diagnosis. We know little concerning the etiology of a vertical strabismus, except that it may be due to too short a superior or an inferior rectus. Such a case is rarely seen. The most common form of strabismus is the convergent, the divergent is met with much less often, and the causes of these deserve separate consideration. Causes of Convergent Strabismus Some persons can simulate convergent strabismus for a time, but a careful inspection will reveal that both eyes converge, that neither looks straight forward. The same is true of hysterical convergent DEVIATIONS OF THE EYES 83 strabismus, which is caused by the spastic contraction of the two interni. When these cases have been excluded, together with those in which a deviation is produced by a cicatricial contraction, and the rare ones in which it follows the loss of sight of an eye, we may say that convergent strabismus appears in children from three to five years of age apparently as the result of several factors, no one of which seems to be sufficient alone, while the part played by each appears to differ in individual cases. Factors upon which stress has been laid are hypermetropia, amblyopia of the squinting eye, ana- tomical faults in the positions of the orbits, too short an internus, too weak an externus, spasm of the convergence, absence of the power of fusion, and the presence of esophoria either alone or com- bined with hyperphoria, or with a declination of the vertical meridian of the eye. Very few if any question the established fact that hypermetropia is present in the great majority of cases of convergent strabismus, just as myopia exists in a large proportion of persons whose eyes diverge, and this refractive error has been assumed to be the prin- cipal cause, especially when the errors in the two eyes were quite different, or the astigmatism in one rendered its vision decidedly the poorer. It cannot be doubted that this is an important factor, and one that probably is primary at times, for if we correct the errors properly under atropine we may cure the strabismus, though not commonly; in most cases it is lessened in degree, but in some it is not affected. Paralysis of the accommodation without correction often lessens and sometimes obviates the squint for the time, while in other cases it fails to do so, so the importance of this factor must vary. Most people who do not squint are hypermetropic, or have hypermetropic astigmatism, so these errors alone cannot be held to be responsible. . The question whether an amblyopia of the strabismic eye is the cause or the result of the deviation is one which has excited much controversy. It certainly is a factor in the maintenance, even though it may not be the cause of a squint, for it has been demonstrated that the visual power of such an eye sometimes can be improved by making it work, and that measures taken with this in view, together with the correction of the refractive errors in both eyes, greatly facilitate the development of binocular single vision and the doing away with the strabismus. At the same time we occasionally meet with a person who has a considerable degree of amblyopia in one 84 DIAGNOSIS FROM OCULAR SYMPTOMS eye, but has never shown any appreciable squint, so this cannot be the sole cause. Such anatomical faults as an abnormal position of the orbits, or too short an internal rectus, are possible causes, and are to be con- sidered primary when they are present, but they are not common. We should expect a strabismus due to an anatomical fault to persist during sleep or unconsciousness, but this is a very exceptional oc- currence. When an internal rectus is too short, or an external rectus too weak, the abduction is faulty, but this power is normal in a large proportion of the cases, Roemer says in eighty per cent. Wootton has shown that an insufficiency of divergence may be the main factor in some cases. He succeeded in obtaining binocular single vision in children thirteen years old who exhibited this fault, and in whom correction of the refraction did not suffice, after advancement of the external recti. It has been claimed that an esophoria may change into an esotropia, and this is understandable when the internal rectus is imervated excessively through the association of the convergence with the accommodation of a hypermetropic eye, but it seems as if a change of that nature in such a case must be subordinate either to the hypermetropia, or to the fact that the two eyes do not work together. It is difficult to understand how an esophoria, whether combined or not with a hyperphoria, or with a declination of the vertical meridian of the eye, can change into an esotropia, or con- vergent strabismus, when the eyes are emmetropic and have the power of binocular single vision. If the esophoria is reinforced by such a powerful factor as hypermetropia, or loss of the power of fusion, and strabismus is developed, the question still remains whether the esophoria is not a relatively unimportant, rather than the primary cause of the strabismus. The power of binocular single vision is that of fusing the im- pressions communicated to the brain by the images formed on the retine of the two eyes so that the object is seen as one. We have reason to believe that this power does not exist at birth, but develops later and becomes complete at some time in early childhood, prob- ably between the second and the fourth years. How it develops we do not know; we may suppose that the faculty lies dormant and awakes about this time, or that certain nerve tracts in the brain are incomplete at birth and develop gradually to make this power pos- sible when they have reached their full development. We know that older children do not necessarily possess this power, that with DEVIATIONS OF THE EYES 85 patient endeavor we may succeed in getting children in whom this power is apparently absent to observe the two images, one of which they have been accustomed to suppress, and in superimposing them without fusion taking place, and if we are persistent