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Management of Strabismic Amblyopia Infants are normally born with the ability to develop vision in each eye, and to combine this into a single picture. We now know that this developmental sequence must proceed within a fairly rigid time framework to occur normally. To provide children with the best and most normal vision development we must be prepared to recognize any abnormal factors and act appropriately to correct them, within the time sequence that our Creator has assigned to this phase of human development. Amblyopia is an interruption of the normal, gradual improvement of vision which starts at a level of about 20/1200 at full term birth, and reaches normal 20/20 vision between 3 and 6 years of age. An eye is amblyopic if its vision is reduced below the expected level, when refractive error is fully corrected, and the eye and visual pathways are normal in all other respects. Amblyopia has been classified based upon cause, and differences in behavior. In the management of strabismus doctors tend to focus most on strabismic (suppression) amblyopia. It is important to remember that refractive amblyopia (anisometropic; meridional, caused by a refractive difference between the eyes so that one eye always has a blurry image) can play a role, and that deprivation amblyopia (caused by opacities in the visual media, such as congenital cataract) may occasionally be involved. Strabismic amblyopia has become well understood over the past few years, through a variety of studies using vision physiology, anatomy, electrophysiology, clinical outcome, etc., on humans and higher primates. Its final expression is that fewer cells in the visual cortex of the brain are “driven” by optic nerve fibers from the amblyopic eye. In basic terms, there are fewer “hookups” to these cortical cells from fibers originating in the amblyopic eye. This seems to occur by an active process, where vision in the amblyopic eye not only fails to keep up with the developing vision in the other eye, but actually regresses. Treatment of strabismic amblyopia is effective, but time dependent. One must “force” the visual cortex to preferentially accept the vision signal from the amblyopic eye and thus to develop those necessary “hookups.” There is a downside, however, in that the non-amblyopic eye will suffer decreases in cortical driving, as well as vision. The most effective methods of treatment are occlusion of the sound eye with a patch, or, as an alternative, blurring the vision of the sound eye by dilating the eye with atropine drops and/or special lenses. Occlusion is the most effective method of treatment, and should be begun at the earliest possible age (after 6 months of age). Because gains in the amblyopic eye may be accompanied by loss in the sound eye, vision must be monitored at appropriate intervals (one week per year of age). When vision in the amblyopic and the sound eye become equal we stop the occlusion. If overshoot occurs and the vision reverses, we watch the acuity for a few weeks, and only use reverse occlusion if spontaneous recovery of vision fails. It is possible for other causes of amblyopia to coexist with strabismic amblyopia. Media opacities (i.e. cataract, clouded cornea, vitreous hemorrhage, etc.) can cause deprivation amblyopia. Anisometropia, especially large amounts of astigmatism in one eye, can cause refractive amblyopia and must be corrected for occlusion therapy to be effective. Although treatment effectiveness decreases after age three, patching should be tried for a period even in children over 7 years of age. It should be remembered that any child with amblyopia risk factors needs to be monitored at 6 month intervals until at least 8 years of age, as amblyopia often recurs and requires more treatment. Once vision has been equalized by amblyopia therapy, attention should be directed to achieving ocular alignment, so that binocularity can be established. We know that human infants first “turn on” the ability for binocularity between 4 ˝ months and 5 months of age. Therefore, when strabismus has been present before 6 months of age, one can assume that no binocular experience has occurred. In such infants any reasonable chance of central binocularity is lost after three years of age, and progressively greater chances of achieving binocularity occur as age of achieving alignment approaches the age of 6 months. The situation is different if alignment has been present for a time before acquiring strabismus, allowing some binocular vision development. Once acquired, the ability for binocularity tends to persist and early re-alignment of the eyes offers recovery of the best binocular function. With prolonged strabismus, previously acquired binocularity may be weakened and sensorial adaptations such as anomalous retinal correspondence (ARC) may occur. ARC is when the central vision areas of the retinas of the two eyes have two different visual directions, so that when the eyes are crossed the central vision area of the preferred eye has the same visual direction as a point in the side vision of the deviated eye. Once ARC develops occurrence of paradoxical diplopia (double vision when the eyes are straight) becomes an increasing risk when the eyes are aligned after the age of 6 years. Suppression, the ability of the brain to block out images from the deviating eye, prevents diplopia (double vision) when strabismus begins before the age of 6 years. It often cannot be reproduced for new areas of retinal correspondence caused by changes of alignment at a later age. In summary, the development of both vision and binocularity occurs gradually and abnormalities are best treated early on, preferably before the age of three years. |
