Squint (strabismus)

Condition in which the eyes do not point in the same direction when focused on an object. In order for an object to be seen properly, light reflected by it must be projected at the same time through both eyes on to a certain part of the retina (fovea centralis). If the patient has a squint, then both lines of sight do not reach this point, and thus accurate perception of perspective is not possible. Sometimes one eye deviates inwards, sometimes both. They can also be turned outwards {strabismus divergens). Squinting is the most common defect which can reduce sharpness of vision in one eye. It occurs in 5 per cent of the population (25 per cent of children). There are various types. First pseudo-squint: a child can seem to be squinting when this is not in fact the case. It can be caused by an asymmetrical face, broad nose or eyes that are set rather far apart, and usually disappears as the face develops. A simple examination reveals whether an abnormality exists. Latent squint (heterophoris) occurs only under particular conditions, for example during illness, fatigue or tension. In such cases the defect is marked and can cause discomfort such as headache. A third type is constantly present (manifest squint), and has two subdivisions, ‘concomitant’ or ‘paralytic’ squint. Concomitant squint is caused by an anatomical defect of one eye, for example cataract, incorrect refraction of light, or a tumour. The condition is often congenital. Fortunately the number of children with this complaint is very small (1 per 1,000). Paralytic squint is caused by a disorder of the eye muscles, or from a nerve disorder or a disorder of the muscular bundle itself. A new-born child does not always have normally placed eyes in the first few months of life, but if the condition persists, it is always deviant. Early detection is highly desirable, because the resultant impaired vision (amblyopia; ‘lazy eye’) must be treated at the earliest possible age. Lazy eye is an accurate description; because double vision caused by squinting is annoying, one of the images is suppressed (lazy),