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; see Chapter 576). In addition to subnormal visual acuity, sophisticated testing will also reveal possible deficiency of motion sensitivity, contrast sensitivity, color sensitivity, visual field, and stereoacuity. The functional and anatomic changes in the 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 looking at the same thing at the same time. There are 3 main types of amblyopia:
Deprivational—caused by physical obstructions to vision such as congenital cataract, corneal opacification, or ptosis occluding the visual axis.
Ametropic—caused by asymmetric or, occasionally, binocular refractive error (glasses prescription).
Strabismic—caused by misalignment of the visual direction of the 2 eyes.
In deprivational amblyopia, the physical abnormality of the eye or lid itself may cause the vision to be reduced. For example, if an opacity in the lens (cataract) blurs the vision, 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.
The most common form of ametropic amblyopia is anisometropic amblyopia, caused by the 2 eyes having a different focus, such that 1 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 2 eyes. For example, if both eyes are farsighted—a very common finding in young children that usually does not require glasses—but 1 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.
There is a fundamental difference between strabismic amblyopia and the other 2 forms. Deprivational and ametropic amblyopia result from a lack of a well-focused, formed image on the retina, which leads to reduced vision. In strabismic amblyopia, the retinal images are initially in good focus, but due to the misalignment of the visual axes of the eyes, the brain suppresses the image from 1 eye (as a way to prevent diplopia), resulting in reduced vision from that eye.
Typically, deprivational amblyopia is more severe than strabismic amblyopia, which is usually more severe than amblyopia due to refractive error. Mixed strabismic and ...