strabismus

—strabismal, strabismic, strabismical, adj. —strabismally, adv.

/streuh biz"meuhs/, n. Ophthalm.

a disorder of vision due to a deviation from normal orientation of one or both eyes so that both cannot be directed at the same object at the same time; squint; crossed eyes.

[1675-85; < NL < Gk strabismós, equiv. to strab(ós) squinting + -ismos -ISM]

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or squint or heterotropia

Failure of the eyes to align properly to focus on an object.

The affected eye may deviate in any direction, including inward (cross-eye) or outward (walleye). Problems with photoreception or the nerves that relay images to the brain cause a constant degree of deviation (comitant); defects in the nerves that control the muscles that move the eyes cause deviation that varies with the direction of gaze (noncomitant). Both types impede development of a child's ability to focus the eyes and merge images from the two retinas into one (fusional reflex). The brain suppresses the image from the deviant eye, which may become functionally blind. Treatment may involve exercises to strengthen the weak eye or surgery or both.

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▪ physiology

also called  squint 

      misalignment of the eyes (eye, human). The deviant eye may be directed inward toward the other eye (cross-eye, or esotropia), outward, away from the other eye (exotropia), upward (hypertropia), or downward (hypotropia). The deviation is called “concomitant” if it remains constant in all directions of gaze and “incomitant” if the degree of misalignment varies with the direction of gaze.

      Strabismus can be present all the time, intermittently, or brought out only by special testing. Congenital, or infantile, strabismus appears in infancy and is presumably due to defects present at birth that are poorly understood. However, given the strong tendency for strabismus to run in families, the causes undoubtedly have some genetic (genetics) component. While congenital strabismus is more common in children with birth-related problems, most affected children are otherwise neurologically normal. Acquired strabismus appears later in life and has many potential etiologies. For example, acquired strabismus can be due to diseases or trauma affecting the actual muscles responsible for moving the eye or the nerves (nervous system, human) or brain stem (brainstem) centres controlling those muscles. In addition, poor vision in one or both eyes can lead to sensory strabismus, in which the eye with the poorest vision drifts slightly over time. In children, a common contributor to acquired strabismus is farsightedness ( hyperopia), which, when severe enough, can secondarily cause the eyes to cross as the child tries to focus on an object (accommodative esotropia).

      The chief danger of strabismus in early childhood is monocular vision loss, or amblyopia, a condition that can become permanent if not treated promptly. If the brain receives two separate images because of the presence of a consistently deviating eye, the less-used eye may develop amblyopia as a result of suppression of the unwanted second image. Often in the treatment of strabismus, the preferred (“better-seeing”) eye is patched for a period of time to encourage the child to use the “weaker” eye and thereby improve the weaker eye's vision. Patching therapy is effective at younger ages but is generally not useful in older teenagers and adults. Thus, early identification and treatment of amblyopia are critical.

      Depending on the situation, important nonsurgical treatments for strabismus may include correcting any underlying nearsightedness ( myopia), farsightedness (hyperopia), or astigmatism with eyeglasses or fitting glasses with prisms. However, definitive treatment commonly requires surgical manipulation of one or more muscles that control eye movement in an effort to realign the two eyes.

Daniel M. Albert David M. Gamm

 

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