STRABISMUS. — From the Greek (Tpai.31(y6c, to squint.
Synonyms.—Squint; heterotropia.
Definition.—The condition in which both eyes do not look toward the same point; but when one eye fixes a certain point the other is turned elsewhere.
Symptoms.—The false position of the eye that is not turned toward the object looked at is usually noticed on casual inspection, and constitutes a very dis agreeable deformity. This eye is called the deviating eye. The one which is nor mally directed is the fixing eye. The symptom of deformity may, however, prove misleading. The direction an eye is looking is judged by the direction the cornea is turned. In some eyes the visual axis pierces the cornea so far from its centre that the eye appears to deviate when in reality it is properly directed; and such an eye might really deviate when it appeared straight.
The lack of correspondence between the eyes prevents true binocular vision, if that function has already been de veloped; or prevents its development. If the patient has previously possessed normal binocular vision the deviation causes diplopia or double vision. This diplopia is distinguished from monocular diplopia by the fact that the covering of either eye removes it.
The image seen by the fixing eye is called the "true image," it being referred to the true position of the object. The image seen by the deviating eye is called the "false image," it being referred, in the consciousness of the patient, to a direction different from the real direc tion of the object. The relation of this diplopia to the deviation of the eye may be understood from the illustration here given. The visual axis R-T is properly directed toward T, the object looked at; but the other visual axis L-D deviates toward D. In the eye L, therefore, the image of T falls at t, on the nasal or inner portion of the retina; and it is referred or projected in the direction f-F as another object at F, the point f in the fixing eye corresponding to the point t in the deviating eye.
The direction of the false image is al ways the opposite of the direction in which the eye deviates. Thus, when the
eye deviates upward the false image ap pears below. When the eyes are crossed we have homonymous diplopia; and, when the eyes diverge, crossed diplopia.
Diplopia disappears when the strabis mus is corrected, or when one eye is closed. It may also disappear through extreme deviation, causing the image in the deviating eye to fall on the extreme periphery of the retina, which is com paratively insensitive. Or it may disap pear from habitually disregarding the false image, especially in early life. While, therefore, the presence of binocu lar diplopia proves the presence of stra bismus, its absence does not prove that the eyes are properly directed.
Etiology.—The normal directing of the eyes depends on an extremely deli cate system of reflex actions, which re quires sufficiently good vision in both eyes and a central co-ordinating mechan ism. The power of accurately co-ordi nating the eye movements normally de velops after birth. Arrest in its develop ment may cause strabismus. Practical blindness of one eye, especially when it depends on some lesion of the cornea that causes distortion of the retinal images or the diffusion of unfocused light within the eye, is very likely to cause that eve to deviate.
Errors of refraction are a common cause of strabismus. Hyperopia of rather high degree, 2 D. or upward, compels excessive effort of accommodation, and so brings about excessive convergence. Myopia of very high degree, 10 D. or more, is attended with elongation of the eyeball that makes it difficult to turn it in its socket. This leads to divergent squint through giving up of the effort to turn the eyes in, so strongly as would be necessary to fix both eyes upon an object so close to them. Difference of refraction between the two eyes, making it difficult or impossible for both to focus the object at the same time, also causes strabismus. Paralysis of one or more of the muscles that turn the eye disables it for certain movements and so causes strabismus. More rarely spasm of one or more of these muscles is the cause of a deviation.