I. Definition. Amblyopia, or "lazy eye," is a reduction in vision when there is a difference in the quality of the images recorded by each eye and sent to the brain. The eyes and the brain must work in conjunction for vision to develop correctly. If the pathways to the visual cortex are not properly stimulated, the visual cortex cannot mature appropriately. Thus, when the brain selectively disregards the eye with the poor visual image, the visual system for that eye develops more slowly than for the eye that sent the good image. Vision loss ranges from mild (worse than 20/25) to severe (legal blindness, 20/200 or worse).
II. Incidence. Amblyopia is a major public health problem with the estimated prevalence of 1–4% in the United States. The condition affects approximately 2 to 3 out of every 100 children. Amblyopia was shown in the Visual Acuity Impairment Survey sponsored by the National Eye Institute (2004) to be the leading cause of monocular vision loss in adults 20–70 years of age.
III. Pathophysiology. Amblyopia may be caused by any condition that affects normal visual development or use of the eyes. Animal studies and clinical studies in infants and young children support the concept of a critical period during infancy and childhood for developing amblyopia. The developing brain is sensitive, and amblyopia may occur as early as the first few weeks of life. There are three general etiologies for amblyopia in the neonate.
A. Strabismus. Strabismus, the preference of one eye when the visual axes are misaligned, is the most common contributing factor. Small, intermittent deviations commonly seen in many infants are rarely associated with amblyopia. This type of divergence decreases with time, as visual acuity and binocularity develop. Amblyopia develops in a consistently deviating eye whether it is from esotropia, exotropia, or hypertropia. This can be recognized in the first few months of life after the newborn period.
B. Refractive errors. Refractive errors, the second most common cause of amblyopia, can be difficult to detect. Amblyopia resulting from refractive errors can be divided into two types: anisometropic and isometropic. Anisometropic amblyopia results from significant inequality of the refractive errors in each eye, blunting the development of the visual pathway in the affected eye. Bilateral hyperopia (farsightedness) is the most common refractive error to cause amblyopia. Severe unilateral hyperopia, myopia (nearsightedness), and astigmatism also may cause profound vision loss. Isometric amblyopia occurs when the refractive errors in the two eyes are equal. Severe refractive errors in both eyes can cause isometropic amblyopia that is bilateral.
C. Deprivation. The least common condition, congenital or early-acquired opacity, causes deprivation amblyopia. This can be the most severe and damaging type. Cataracts, corneal lesions, or ptosis block or distort the retinal image formation. This can affect one or both eyes and may develop as early as 2–4 months. It is critical that these be diagnosed and treated as early as possible.
IV.Risk factors. These include low birthweight, prematurity, familial factors, and certain ...
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