Squinting (technically, Strabismus) is a well-known and common deformity, which may be defined as a want of parallelism in the visual axes, when the patient endeavours to direct both eyes to an object at the same time. It may be due to loss of power (paralysis or paresis) of one or more of the eye muscles; and this may depend on a merely local affection, or may be a symptom of serious brain disease. But in the majority of instances, and in all ordinary cases of squint (to which this article will be confined), no such condition is present. The squint is said to be convergent when the squinting eye is directed towards the nose, and divergent when it is directed towards the temple; the convergent is much the more common. Mis- direction of one eye upwards or downwards is of rare occurrence. Only one eye can squint at a time; but if sometimes one eye and sometimes the other is misdirected, the squint is said to be alternating; if always the same eye, monocular.
Convergent squint usually comes on during childhood, most often from the second to the seventh year. If present at birth it is caused by some serious defect in the muscles. It is sometimes due to defective sight in the squinting eye, from congenital abnormality, severe inflammation, or injury; but very often no such condition is present. In a large proportion of cases it is accompanied by hypermetropia (see EYE, p. 515), and is due to the increased effort of accommodation required to see near objects, being associated with an increased and disproportionate effort of convergence. In such cases, if suitable glasses can be worn as soon as the squint begins to show itself, it may be prevented from becoming permanent. It is but seldom, however, that the surgeon is consulted at a sufficiently early stage for this treatment to have a fair chance to succeed, and even at an early stage it is by no means certain to prevent the development of a squint. In other cases the presence of a squint may be traced to the presence of worms, the irritation of teething, &c.; and it disappears when the cause is removed.
Divergent squint is very rarely present without considerable defect in the sight of the squinting eye, except where it is the result of over-correction of a convergent squint by operation. It is often associated with myopia, as the other form is with hypermetropia. When the vision of one eye is damaged by disease or injury in the adult, and takes a faulty position in consequence, divergence is much more common than convergence; in children, as has been said, the latter is the usual result.
The surgical operation for the correction of a squint consists in the division of the muscle whose excessive activity leads to the faulty position—in convergent strabismus the internal rectus, in divergent the external. It is often necessary to operate on both eyes in the same manner, even where the squint is monocular. In some cases it is requisite in addition to shorten the opposing muscle (i.e. in divergent strabismus, where this procedure is most often resorted to, the internal rectus).