Amblyopia or “lazy eye” is a loss of vision which affects approximately 2.5% of all children. It results from a loss of function in the part of the brain which “sees” or responds to the image that is processed by an individual eye. It occurs when one eye does not receive input that is equal to that of the other eye. In other words, when one eye is used less or has “disuse” compared to the other. Unfortunately, amblyopia often presents without symptoms.
How do we see?
When we look at an object, called the “object of regard,” each eye forms an image of the object on the retina, the light- sensitive membrane lining the inside of the back of the eye. The image produced is termed the “image of regard.” After being processed in the retina, it is converted into electrical impulses which are transmitted along the optic nerve to the visual centers of the brain. Although the brain is presented with a pair of images of the same object, one from each eye, it fuses the two images into one. The result of this process is what we think of as our vision.
A child’s eye-brain system is amazingly plastic or pliable and is not mature until the child reaches eight or ten years of age. Even past this age, recent studies show evidence of remarkable plasticity well into adulthood. But this does not imply that a younger child’s vision cannot be normal. Indeed, the average child has 20/20 visual acuity by the age of nine months. None-the-less, subtle development does occur in this system until the child is considerably older.
Each eye has brain cells associated with it that respond only to that eye. There are other cells in the brain that respond only to stimuli from both eyes. These are termed binocular cells. Because of the plasticity of these eye-brain connections in younger children, both types of cells need continuous input to ensure proper maturation of the visual system. Any disruption in this maturation process may cause problems. Amblyopia is the term for a major interference with this visual development.
In some ways each eye is designed to be competitive with its counterpart, i.e. there is a rivalry between the two eyes for the brain’s attention. When something interferes with one eye’s imaging and processing functions, that eye can lose vision and become amblyopic. The eye itself may function normally without any permanent damage, but the brain becomes less and less attentive to it and begins to rely more and more on stimuli from the other eye. An actual loss of cells, including binocular cells, occurs in the brain area serving the amblyopic eye.
Any condition that interferes with normal retinal processing or clear vision can produce amblyopia. There are three main ways that this interference can occur:
1. Strabismus — A constantly crossed eye does not image the object of regard. Therefore, a child with esotropia, or crossed eyes, who always looks at the world with his left eye while his right eye is crossed does not receive the same visual information in each eye. The deviated right eye receives deprived information. Because of the rivalry in the brain, the visual input from the constantly deviating eye is ignored or shut off and more and more the brain depends on visual information from the straight eye. Ultimately, the constant deviation and poor visual processing in one eye leads to amblyopia in the brain cells serving that eye.
2. Deprivation — Anything that prevents a clear picture from reaching the retina can produce amblyopia. A classic example is a cataract in a child. When an adult develops a cataract, surgical treatment usually corrects the vision to 20/20 whether the cataract had been present for one month, one year, or even ten years. But in a child, even if a cataract has been present for a short time–even a matter of weeks–surgery to remove the cataract may not restore good vision. The youngster’s visual brain cells, having not received clear images through the cataract, may already have become amblyopic. Other disorders that can cause deprivation are corneal scars and opacities, and opacifications elsewhere in the system caused by a variety of eye diseases.
3. Anisometropia — This is very common, and unfortunately, a very insidious type of amblyopia because it is without any sign or symptom in a child. Anisometropia is by definition an imbalance between the refractive error of each eye. That is, one eye has a need for a stronger spectacle correction than the other eye. For instance, the right eye may have two units of farsightedness, whereas the left eye may have four units of farsightedness. Consequently, the left eye receives a more blurred image than the right. That image is ignored and the brain cells serving that eye deteriorate while the brain concentrates on the clearer image from the right eye. This process may also occur with astigmatism or nearsightedness.
The diagnosis of amblyopia requires a complete optometric exam. As noted above, a normal child does not reach 20/20 visual acuity until nine months of age; however, the vision can be checked as early as three to four months of age. The symmetry of vision rather than absolute acuity is assessed initially. This comparison between the two eyes may detect a difference in their ability to see clearly.
Ideally, most children should receive an initial visual screening from their pediatrician or family physician at approximately six months of age. If a problem is detected, or if there is a suspicion of an abnormality, a complete vision examination by an optometrist is recommended to assess the visual acuity of each eye, to look for the presence of any eye disease, such as strabismus or cataract, and to determine the refractive error or power of glasses that might be prescribed.
Another common source of amblyopia diagnosis is a screening program which may be carried out by certain organizations, clubs, or day care centers, etc. If a child goes through a screening program and an abnormality is suspected, he should receive a referral to an optometrist. Primary care physicians and group screening centers can only suspect the problem; it is the optometrist that must confirm the diagnosis and carry out the definitive treatment. Treatment
Treatment for amblyopia is twofold: correction of the underlying problem and therapy of the amblyopia itself. Obviously the treatment for the underlying disease, whether that be a strabismus, an anisometropia, or a unilateral cataract, depends on the particular condition that is present. The treatment for amblyopia is best done in a step-wise process. The child may be placed in glasses and occlusion or patch therapy to cover the good eye. Several non-prescription elliptical eye patches are commercially available. Like BandAids, they stick directly to the skin with their own adhesive. They cannot be stuck to the glasses, as the child will simply look over the top of the frames. Occluding the good eye forces the brain to rely on the amblyopic eye, slowly reversing the brain cell deterioration. Recovery usually takes several months, although it can occur in a shorter period of time in very young children. Eye patches may be worn for anywhere from several hours per day to all the waking day. Some optometrists prefer intermittent patching, i.e. two hours per day, eight hours per day, etc., while others advise full-time occlusion. This type of treatment howe ver, does very little to restore the function of the binocular cells which require input from the eyes at the same time. Visual acuity may improve but binocular vision may not be restored.
Studies in recent years have also reinforced the effectiveness of using cycloplegic eyedrops as a patching alternative. These are drops (the same as dilating drops) which “turn off” the focusing ability of the better seeing eye to help stimulate the amblyopic eye into better use. This can have a similar effect as a patch but without the cosmetic appearance issues. One study has even shown that weekend use of cycloplegic eyedrops can improve vision – allowing children to focus efficiently during the school week for studying.
The use of vision therapy continues to evolve and improve in the treatment of amblyopia. New studies now show substantial improvements in vision in patients older than nine years, the age which used to be thought that little could be done for treatment. The goal of vision therapy is to improve visual acuity by the use of activities to stimulate and develop vision in the weaker eye and to restore or improve binocular vision through the use of activities which strengthen a person’s eye-teaming abiltiy.
The most important concept in the treatment of amblyopia is the age of the child. The earlier the amblyopia is detected, the better the potential for succes with treatment. The above limitations relate to the plasticity of the brain that was mentioned earlier. The eye-brain is flexible enough to reverse the cell deterioration in the first few years of life, but after that crucial time period, the amblyopic condition becomes more difficult to remediate.
There is one other form of amblyopia treatment that is much less commonly employed and involves placing an eye drop in the good eye causing the vision to blur in that eye more than the other eye. As with patching, the idea is to stimulate the brain’s attentiveness to the amblyopic eye. Most doctors use this method sparingly.
Treatment options for amblyopia and success rates are better than ever thanks to newer techniques and recent reseach validating these methods. If treatment has been limited to only one of the above techniques then the patient should seek further treatment. Amblyopia treatment involves a commitment between the doctor, the patient and the parent to best treat the condition.