By L. E. Leguire Ph.D., MBA
Amblyopia, commonly known as lazy eye, is the leading cause of vision loss in children and affects about 2 – 4% of the population. Every year, about 120,000 children in the United States develop amblyopia. Amblyopia is defined as a loss of vision in one, sometimes two, eye(s), that does not have an obvious reason. For example, the eye will look perfectly normal to the eye doctor but the child will have reduced vision in the lazy eye. Reduced vision includes reduced visual acuity, reduced depth perception, abnormal eye movements, poor vision for low contrast objects, and sometimes distortions in vision. Recent research also showed that the normal eye is also not normal in an amblyopic child – the stronger eye or “dominant” eye also has somewhat reduced vision!
Causes of Amblyopia
There are three main reasons why a child develops amblyopia:
There is a difference in the refractive state of the two eyes, such that one eye may be normal and not require a corrective lens, while the other eye is too nearsighted (myopic) or too farsighted (hyperopic). The condition is called Anisometropia, or difference in refractive error between the two eyes. As a consequence, the eye that needs a corrective lens will have a blurred image projected to the back of the eye (retina) and, in turn, the brain receives a blurred image from that eye. The eye (actually the brain) then becomes amblyopic. This condition is also called refractive amblyopia. Anisometropic amblyopia is very insidious; the child looks perfectly normal but one eye is really very amblyopic. Treatment is often delayed because the parents think that the child is fine and that there is no reason to see an eye doctor for a comprehensive eye exam.
There is a difference in the alignment of the eyes such that, for example, one eye points straight ahead and the other eye turns in toward the nose. When the eyes point in different directions, the child is “strabismic” and when one or sometimes both eyes appear to turn in the condition is called esotropia or crossed eyes. When the eyes are not aligned, the brain will receive two very different images from the eyes and to correct this problem of double vision (diplopia) the brain will suppress one of the images to achieve single vision. It is believed that this suppression or turning-off of one eye’s input to the brain leads to reduced vision in the eye that is suppressed and thus leads to amblyopia in the affected eye. Typically, it is the eye that turns in toward the nose that becomes the amblyopic eye.
The third type of amblyopia is often referred to as derivational amblyopia. Whenever the two eyes receive very different information or images, there is a high probability of amblyopia. For example, if a child is born with a cataract or cloudy lens such that only one eye receives a clear image and the eye with the cataract receives a blurred image, then the eye with the cataract will become amblyopic. If the infant has a problem with the cornea in one eye, or one eye has excessive matting such that the child keeps the one eye closed for a period of time or if the child has a big astigmatism in one eye – all of these conditions can lead to amblyopia.
In general, the earlier the child has the condition for the development of amblyopia the worse it may become. For example, if a child has a cataract in one eye at birth, that eye may become very amblyopia in a very short period of time. On the other hand, if a child develops a cataract at, say, 5 years of age, then the amblyopia will only gradually develop over a longer period of time. If a child develops a cataract at say 10 years of age he/she may never develop amblyopia.
Similarly, the sooner treatment is instituted the faster the amblyopia will be cured. It may only take a month of occlusion therapy to cure a 6 month old child with amblyopia but it could take a year or more of occlusion to cure a 6 year old child. So, the sooner the better in terms of treating amblyopia. The longer a parent delays treatment the worse the amblyopia becomes and the harder it is to cure the amblyopia.
In general, the main treatment for amblyopia is occlusion therapy, in which the child has to wear a patch over the stronger eye and force him/her to use of the amblyopic eye. Occlusion treatment is not an easy treatment for the child or family or for school personnel. Often, PARENTS will fail with the occlusion therapy – They’ll say something to the effect that ” he just won’t leave the patch on.” If the parents fail with occlusion, the child is often left with only one good eye. Typically, an adhesive patch is use for treating amblyopia but sometimes a patch can be placed over the child’s glasses, if he/she wears glasses.
If the child (read PARENTS) does not tolerate the occlusion therapy AND if the child’s vision isn’t too bad (generally better than 20/100 visual acuity) then sometimes the eye doctor will try eye drops in the stronger eye. The eye drops will dilate the pupil and cause blurred vision in the stronger eye. The amblyopic eye has a better chance of regaining vision because the stronger eye is now blurred. Unfortunately, the dilating drops have to be used for a long time, sometimes 6 months or more, and such “penalization” therapy only works if the amblyopia is only mild.
Although highly controversial, some eye doctors also promote “vision therapy” or eye exercises to treat amblyopia as well as other conditions. There is no set type of vision therapy, and it can include almost anything. For most patients (about 80%), particularly if the parents are compliant with therapy, the amblyopia can be cured with occlusion or penalization therapy.
The last and most recent advance in therapy for amblyopia involves an oral medication, L-dopa, combined with part time occlusion of the dominant eye. L-dopa (levodopa/carbidopa) is a drug commonly used to treat Parkinson’s disease and a number of scientists have also found that L-dopa also helps in the treatment of amblyopia. In general, L-dopa plus occlusion therapy improves vision by about 2 lines on the visual acuity chart; or about 33%. L-dopa therapy should only be used once the patient has gone through regular occlusion therapy and if the amblyopia is not cured by conventional means. See “Recent Breakthroughs” for additional information on L-dopa use in amblyopia.
Another important aspect of therapy for amblyopia is the correction of the initial cause of the amblyopia. For example, if the child is anisometropic – has a difference in refraction between the two eyes, then the child must be placed in corrective glasses. Sometimes the glasses alone will cure the amblyopia. If the child has a cataract, the cataract must be removed by surgery and a corrective lens or contact given. If the child is crossed-eyed (esotropic), then once the amblyopia is cured the child may need eye muscle surgery or glasses to correct the eye misalignment.
Some authors say that amblyopia runs in families (i.e., has a genetic component). Actually, it is not the amblyopia that runs in families but, rather, the condition that leads to amblyopia – such as crossed eyes. If the condition is detected early and treatment instituted early then the child may never become amblyopic.
Amblyopia is one of the key reasons a child needs a comprehensive eye exam. A child should have an eye exam by a eye doctor by 6 months of age, and then again at 3 years and again before entering school. So, if your child has not had an eye exam by an eye doctor, please schedule one today – you’re child’s vision may depend upon it. It is important to note that a comprehensive eye exam by an eye doctor IS NOT THE SAME as a vision screening; for example, an eye test that a school nurse performs. Statistics show that most children with amblyopia are missed by school screening programs. Add to this the fact that about 80% of parents fail to take their child to an eye doctor explains why most people with amblyopia are not detected until it is too late for treatment. If the amblyopia is not detected and treated during the first 9 years or so of life it is too late – the child will remain amblyopic for the rest of his/her life.
An adult with amblyopia has no options for treatment. He/she will have an increased chance of losing the remaining good eye thus leading to blindness. An amblyopic adult cannot qualify for certain jobs including driving an interstate commercial truck, airline pilot, and any job that requires two good eyes.
An amblyopic older child or adult is advised to wear protective glasses to protect the remaining good eye from injury. Unfortunately, few eye doctors advise such patients about protective glasses and fewer amblyopic patients comply with wearing protective glasses. This is why some amblyopic children and adults end up in the emergency room with injury to their remaining good eye. If this happens, as it does on a regular basis, the person is left with a life of visual impairment or legal blindness.