Children are born with immature visual systems, and for normal visual development to occur, they need a clear, focused image to be projected onto the retina where it is converted to neuronal signal and then transmitted to the developing occipital cortex via the optic nerve. Uncorrected refractive errors cause image blur, impeding this process, and may result in failure of normal visual maturation (amblyopia). Amblyopia is commonly caused by anisometropia, the condition in which unequal refractive error between fellow eyes results in image blur in one eye (form vision deprivation) and/or abnormal binocular interaction via projection of dissimilar images onto the fovea of each eye . In general, anisomyopia of more than 2 diopters, anisohyperopia of more than 1 diopter, and anisoastigmatism of more than 1.5 diopters can result in amblyopia [54, 55]. Studies of anisometropic amblyopia indicate a 100% prevalence of amblyopia in patients with 4 diopters or more of uncorrected hyperopia or 6 diopters or more of uncorrected myopia [26, 50]. Anisometropic amblyopia associated with anisometropia of more than 4 diopters is also less successfully treated with traditional amblyopia therapy . The severity of amblyopia is directly related to the degree of anisometropia [13, 20, 26]. Successful treatment of anisometropic amblyopia with traditional therapy varies widely among practitioners and has been reported to be between 48 and 82% of children [16, 20, 23-26, 28, 56]. Bilateral uncorrected high refractive error can also cause amblyopia. This condition, called bilateral ametropic (or isoametropic) amblyopia, though less common than anisometropic amblyopia, is even more of a disability as it affects both eyes. Unsuccessfully treated bilateral ametropic amblyopia is typically a disorder that affects children with neurobehavioral disorders and high refractive error who are tactilely averse and refuse to wear their prescribed spectacles. It is becoming a more common problem today as more extremely premature infants are surviving with the sequelae of severe ROP and subsequent high myopia. Visual impairment in these children with multiple special needs further isolates them. Tychsen has coined the term "visual autism" to refer to the resultant severe visual isolation in these children . Traditional therapy for anisometropic amblyopia includes refractive correction with spectacles or contact lenses, minimization of aniseikonia with contact lenses, and amblyopia management with occlusion therapy and/or pharmacologic and optical penalization of the sound eye [21, 24?26, 43]. Though these treatment strategies appear simple, they are frequently problematic and unsuccessful due to induced aniseikonia or diplopia with spectacles, psychosocial stress, unacceptable cosmesis with spectacles in which one lens is much thicker than the other, impracticality of contact management, and poor compliance with occlusion therapy . Management of large magnitude bilateral ametropia is similar to anisometropia with regard to spectacles or contact lenses, though occlusion and penalization are not needed as both eyes have equal image blur. Severe amblyopia causes a lifetime of visual handicap with its associated economic and social costs. Refractive surgery is now being used with good results for severe anisometropia and ametropia in children when traditional therapy fails.
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