Chapter 585

Amblyopia is a monocular (or occasionally binocular) reduction of vision due to impaired visual development by the brain. It is sometimes referred to as lazy eye, although this lay term may also be applied to describe strabismus (ie, a wandering eye). In addition to the subnormal visual acuity, sophisticated testing will also reveal possible deficiency of motion sensitivity, contrast sensitivity, color sensitivity, visual field, and stereoacuity. The functional and anatomical changes in the visual brain result from disruption of normal visual experience during development. The severity and reversibility of amblyopia relate closely to the cause of the visual disturbance and the child’s age.

Normal visual development requires that both eyes are in good focus, with normal ocular structures and cerebral visual pathways. In addition, both eyes need to be look at the same thing at the same time. The three main types of amblyopia are

1. 1. Deprivational—caused by physical obstructions to vision such as congenital cataract, corneal opacification, eyelid capillary hemangioma, or ptosis occluding the visual axis;

2. Strabismic—caused by misalignment of the visual direction of the two eyes; and

3. Ametropic—caused by asymmetric monocular or, occasionally, binocular refractive error (glasses prescription).

In deprivational amblyopia, the physical abnormality of the eye or lid itself may cause the vision to be reduced. Amblyopia is visual loss in addition to the loss caused by the physical obstruction. For example, if an opacity in the lens (cataract) blurs the vision to 20/60, the brain will prefer the other normal eye and will “ignore” the eye with cataract. Amblyopia will then further impair the visual development in the eye with cataract, causing the vision to be even worse than the 20/60 the eye would have been capable of if it were not for the superimposed amblyopia.

The most common form of ametropic amblyopia is anisometropic amblyopia, caused by the two eyes having a different focus, such that one eye is out of focus when the other is in focus. The defocus can be caused by a difference in the refractive power of the two eyes. For example, if one eye does not need glasses and the other is very nearsighted, the myopic eye may develop amblyopia in addition to the blurring caused by myopia. If both eyes are farsighted—a very common finding in young children that does not require glasses, because they can focus using their own intrinsic eye muscles (accommodation)—but one eye is more farsighted than the other, the brain will prefer to view through the eye with less farsightedness. As a result, when the eye with less farsightedness is focused, the other eye is still not completely focused. The brain will prefer the better-focusing eye, and the remaining eye will become amblyopic.

Typically, deprivational amblyopia is more severe than strabismic amblyopia, which is usually more severe than amblyopia due to refractive error. Mixed strabismic and anisometropic amblyopia is usually more severe than strabismic amblyopia ...

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