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. 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
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 ...