What Is Amblyopia?
Amblyopia is a reduction in best-corrected visual acuity that cannot be accounted for by any structural abnormality, either in the eye or in the visual pathways (i.e., a pathway or tract of optic nerves and fibers that transmits a visual image from the eye to the brain). A commonly-used term for this condition is “lazy eye.”
A “lazy eye” refers to an amblyopic eye, not an eye that is misaligned (i.e., an eye that does not point or look in the same direction as the other eye). In some cases, however, amblyopia may be the result of a misaligned eye, or severe amblyopia may cause a misaligned eye.
In addition to a reduction in visual acuity in one or both eyes, individuals with amblyopia do not learn to use both eyes together well. As a result, they can have poor depth perception.
Understanding the Critical Period
The visual pathways continue to develop in early childhood, during “critical periods” of development. During these critical periods, if one or both eyes are not receiving sufficient visual input, or are not being used, the visual pathways fail to develop normally and vision declines.
The critical period is commonly described as the first 6-9 years of life; however, studies indicate that treatment of children aged 10-17 can be effective in reversing amblyopia, indicating that the visual pathways are still “plastic” (i.e., able to be modified) into late childhood.
These critical periods have been studied extensively, and form the basis of work that led to the 1981 Nobel Prize in Physiology or Medicine by Drs. David H. Hubel and Torsten Wiesel.
If an eye receives poor (or no) visual input due to a number of underlying conditions, or the brain ignores visual input from one eye, the visual system for that eye will not develop normally. This leads to amblyopia, or a reduction in visual acuity.
During the critical period, when the visual pathways continue to develop and change, correcting the underlying problem can lead to a “rewiring” or “rebooting” of the visual system. This rewiring can reduce (or eliminate) the degree of amblyopia and improve best-corrected visual acuity.
What Happens If Amblyopia Is Not Corrected During the Critical Period?
If amblyopia is caught too late and the visual system has already become “hard wired,” correcting the underlying problem will not fix the amblyopia.
For example, if an infant is born with a cataract in one eye, the brain learns to ignore that eye, and the visual pathways from that eye will not develop normally.
If this is caught early, the child can undergo surgery to remove the cataract, and have other treatments that “force” the child to use that eye. A combination of surgery and treatment can frequently reduce the amblyopia and improve the best-corrected visual acuity in the eye.
However, if the cataract is not diagnosed early enough, and the person’s visual system has already become hard-wired abnormally, the vision in that eye will remain permanently reduced, even if the cataract is removed at a later date.
What Causes Amblyopia?
There are number of different types and causes of amblyopia:
- Strabismic: Strabismic amblyopia is the most common type of amblyopia. Strabismus is a misalignment of the eyes, meaning that both eyes do not point or look in the same direction. This misalignment can cause visual confusion and double vision. To cope with this, individuals with strabismus may develop a preference for using one eye, while suppressing vision from the other eye.
Because the visual pathways from the suppressed eye are not being used, the person can develop amblyopia in that eye. On the other hand, people with strabismus who alternate using one eye and then the other usually do not develop amblyopia.
- Deprivational: In deprivational amblyopia, there is an obstruction in the line of vision (also called the visual axis). This is most commonly due to a cataract, but can be due to a variety of other conditions: a droopy eyelid, an eyelid lesion, a lesion or opacification (i.e., clouding) in the cornea, or bleeding in the eye.
It can also occur if the eye with amblyopia is patched for too long a period of time. As the amblyopic eye improves, full-time patching (or “occlusion”) of the “good” eye can result in deprivational amblyopia of that eye. That is why close follow-up is important for ambloyopia patients, especially those with full-time patching.
- Anisometropic: In anisometropic amblyopia, there is a significant difference in the refractive error (i.e. strength of glasses required to best correct vision) between the two eyes. If the appropriate eyeglass correction is not provided early on, the image on one of the retinas is significantly less focused than that on the other retina. The brain ignores the eye producing the less-focused image, and amblyopia develops.
- Ametropic: In ametropic amblyopia, both eyes have a similar refractive error. However, if the refractive error is severe enough, both eyes can develop a mild amblyopia.
How Is Amblyopia Treated?
Amblyopia is reversible, if caught early and treated appropriately. Many school systems and most pediatricians screen for decreased vision or reduced depth perception. In addition, persons at risk for amblyopia can be identified by observing visible abnormalities in the eyes (a cataract, for example), visible misalignment of the eyes, or behavior suggesting poor vision in one or both eyes.
Persons with amblyopia require frequent eye exams, usually with a pediatric ophthalmologist, to monitor progress and adjust glasses and therapy.
The fundamental components of amblyopia treatment include correction of the underlying problem and forced usage of the amblyopic eye. Any obstruction in the line of vision (also called the visual axis) should be removed (such as removal of a cataract or surgical correction of a droopy lid), and the appropriate eyeglass prescription should be provided.
Children with amblyopia must be encouraged to wear their glasses full-time. Children with a significant refractive error or with significant differences between the refractive errors in the left and right eyes should wear the glasses as much as possible to reduce the risk of amblyopia.
Patching (Occlusion) Treatment
The individual should be “forced” to use the amblyopic eye. Frequently, this “forcing” is done by patching (also called “occluding”) the better-seeing eye. The number of hours per day and days per week is determined by the eye doctor, based upon the degree of amblyopia, age, response to therapy, and compliance.
Patching can be performed with adhesive sticker patches, cloth patches, patches or occluders over the eyeglass lens, or opaque contact lenses.
Part-Time or Full-Time Patching/Occlusion?
Part-time patching/occlusion has been shown to be as effective as full-time occlusion. However, in some severe cases, or when the person is diagnosed very late in childhood, full-time occlusion of the good eye may be recommended.
Persons with full-time patching/occlusion must followed very closely, however, because as the amblyopic eye improves, full-time occlusion of the “good” eye can result in deprivational amblyopia of that eye.
An Alternative Approach: Penalization
Penalization degrades vision (but does not completely block vision, as in patching) in the good eye, thereby promoting use of the amblyopic eye while still allowing some binocular (i.e. involving use of both eyes) seeing.
This usually involves use of a dilating eye drop, such as atropine or homatropine, in the better eye. This allows degradation (or decrease) of vision in that eye, thereby forcing use of the amblyopic eye.
In addition to drops, penalization can also be performed by “fogging” the good eye by prescribing an excessively strong lens or placing filters/semi-opaque material over the lens of the good eye.