STRABISMUS

also called squint, or heterotropia failure of the eyes to align properly on the object toward which a person seeks to direct his gaze. The deviant eye may be directed inward, toward the other eye (cross-eye, or esotropia); outward, away from the other eye (walleye, or exotropia); upward (hypertropia); or downward (hypotropia). The squint is called comitant if the deviation remains constant no matter in what direction the gaze is directed; noncomitant, if the degree of misalignment varies with the direction of gaze. Strabismus is most often the result of some abnormality in the nervous controlsin the reception of images on the retina and their transmission to the higher centres of the brain or in the transmission over the motor nerves of impulses that bring into play the muscles that align the eyes. Nonalignment from defects in the muscles themselves is rare. When a baby is born his eyes operate independently and are able to distinguish only light and movement. At birth the foveas, the areas in the retinas that are used for sharp vision, are not developed. From about the third month the baby has functioning foveas and develops the ability to change the shape of his lenses for near and far vision and to direct the gaze of his two eyes symmetrically toward an object. Thus the fusional reflex develops, and the complex of nerve impulses and muscle action holds the gaze of the two eyes on an object in such a way that the images on the two retinas are merged into one impression. If any defect in the visual apparatuse.g., a scar on a cornea, a congenital clouding of a lens, or a tumourinterferes with the reception of an image, the development of the fusional reflex is impeded and the baby becomes cross-eyed or, less often, walleyed. This type of strabismusstrabismus from a defect in the sensory apparatusis comitant. If one or more of the muscles that operate to direct the eye's gaze are paralyzedthat is, fail to function because they do not receive impulses over a motor nervestrabismus of a noncomitant type occurs. In one particular position the eyes seem aligned, but, as the direction of gaze changes, the strabismus becomes apparent. The chief danger of strabismus is monocular blindness: If the two eyes are of unequal strength, so that one is always the deviant eye, the deviant eye tends to become functionally blind as a result of suppression of the unwanted image. Often, in treatment of strabismus, the good eye of the affected child is covered for a period before correctional surgery, so that the child will use the previously unused eye and build up its vision.