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Strabismus is the term used to describe any ocular misalignment. The term originates from the Greek strabismus, meaning to “look askance” or the “evil eye.” Ocular misalignment that is constantly present and not controlled by fusional mechanisms is termed a tropia. A phoria is defined as a latent deviation that is controlled by fusional mechanisms and only presents when 1 eye is blocked or covered. An intermittent tropia is a deviation that may sometimes be latent and controlled by fusional mechanisms (ie, a phoria) but may at other times be spontaneously manifest, often with illness or fatigue. Most children with strabismus develop a deviation that manifests mostly in 1 eye (the nondominant eye). Some strabismic children are able to switch fixation, using 1 eye at times and the other eye at other times, so that the strabismus will appear to shift from 1 eye to the other. This is termed alternating strabismus. In some children, strabismus develops as a result of poor vision in 1 eye and is termed sensory strabismus. Strabismus may also be the presenting sign of a life-threatening disease (eg, brain tumors with cranial nerve palsy) or vision-threatening conditions such as retinoblastoma or cataract.
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Strabismus affects between 1% and 4% of children. The 2 most common types of strabismus are esotropia (cross-eyes, convergent squint, inwardly turned eyes) and exotropia (wall-eyes, divergent squint, outwardly turned eyes). Strabismus has been shown to occur in families and is noted more frequently among children who have other neurologic or medical conditions, including those affected by prematurity, low birth weight, cerebral palsy, craniofacial abnormalities, Down syndrome, other genetic disorders, and intrauterine infection or drug exposure.
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Amblyopia is a monocular (or occasionally binocular) reduction of vision due to impaired visual development by the brain. It develops in as many as 50% of children with strabismus. Early diagnosis and treatment of strabismus are therefore crucial to developing optimal, binocular visual function. Brief periods of intermittent esotropia or exotropia may be commonly observed during the first 2 to 3 months of life. This does not require referral to an ophthalmologist. However, any ocular misalignment after 3 months of age should be referred to an ophthalmologist for further evaluation. Likewise, any large or constant deviation should never be considered normal.
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In addition to defining strabismus by the direction of the misalignment, it can also be categorized by the consistency of the amount of misalignment in different positions of gaze. A comitant strabismus is one in which the ocular misalignment is the same magnitude (measured by the ophthalmologist in units called prism diopters) in primary (straight-ahead gaze), left, right, up, and down gaze. All of the esotropias and exotropias discussed below are comitant. When 1 or more muscles are paretic or restricted, the amount of strabismus will differ depending on the position of gaze. This is called an incomitant strabismus. Although nearly every paretic vertical ...