Strabismus and amblyopia are two of the most common visual problems affecting children. Strabismus occurs in 4% and amblyopia in 4% of the population. Half of all amblyopia patients have a concomitant strabismus. Conversely, half of all amblyopia patients will have no demonstrable strabismus.
Movements of the eyes horizontally and vertically are controlled by the six muscles attached to the sclera of each eye. Movement of both eyes in unison allows vision of singular images. Through a blending process called fusion, the brain combines the two images into a single, three-dimensional image. As long as the eye muscles are able to work together, the brain can process incoming visual information. When the eye muscles are not coordinated, one eye deviates inward, outward, or upward, and the other eye remains straight. When this occurs, the brain receives a different image from each eye and cannot combine the two disparate images into a single image frame.
Misalignment of the eye muscles results in strabismus. In addition to a breakdown or absence of fusion, the causes of strabismus may include refractive errors, anatomic anomalies, and abnormal tonic innervation. Adults with acquired strabismus frequently develop double vision, but children with strabismus quickly learn to ignore or suppress the image seen by the deviated eye. As a result of suppression, the straight eye takes over most of the work of seeing, and the crossed eye develops reduced central vision because of lack of use. There are various types of strabismus and may be horizontal, vertical, or rotational (cyclotorsional). (See online text for additional information on specific types of strabismus.)
Loss of vision in a relatively normal eye is called amblyopia. The phrase "lazy eye" should not be used in diagnosing patients because it can be confused with amblyopia and strabismus. As mentioned earlier, these two distinct clinical entities
are associated with each other in only 50% of cases. In children younger than 4 years, amblyopia is the most frequent cause of unilateral vision loss (Fig. 41-12). The condition is usually unilateral, although bilateral high myopia, hyperopia, or astigmatism may occur. Unless treatment begins early, loss of vision in the affected eye may be permanent. Amblyopia is usually treatable if detected at age 3 to 4 years but is generally considered irreversible after 13 years; however, treating amblyopia in patients older than 13 years has had some success. The earlier treatment begins, the better the prognosis for the patient. The primary treatment of amblyopia includes the use of patches, glasses, or both. The better eye is occluded, and underlying conditions such as cataracts or refractive errors are treated.
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