Amblyopia is a reduction in best-corrected visual acuity that cannot be attributed to any structural abnormality of the eye. It is commonly unilateral but may be bilateral. It is caused by any abnormal visual experience early in life due to strabismus (squint), uncorrected refractive error or visual deprivation. These causes should be addressed as soon as possible by showing the child to an ophthalmologist. Amblyopia, depending on the severity, may be reversible if the underlying causes are identified and amblyopia treatment is started early. This is why vision screening in preschool children is important in identifying patients who are more likely to develop this condition.
The primary care physician or pediatrician should be able to detect risk factors for the development of amblyopia during the first clinic visit. In preverbal children and infants, the best method of screening is the simultaneous red reflex test or “Bruckner” test. This test will detect amblyopic refractive errors and optical media opacities such as cataracts. Older children (2 ½ - 3 years of age) should be able to cooperate with some form of visual acuity testing, usually a type of picture card, or the E game. Early detection and early referral to the pediatric ophthalmologist is critical to the treatment of amblyopia.
The first and the most important step in amblyopia therapy is to make sure that there is a clear retinal image. Significant errors of refraction should be corrected with spectacles or contact lenses, and visually significant opacities, such as cataracts, must be surgically removed. Children with visually significant cataracts are best treated during the first few weeks of life, while delaying treatment past 3 or 4 months of age carries a relatively poor visual prognosis.
The next step is to correct ocular dominance in patients with unilateral amblyopia by patching the better eye. Patching the good eye forces the brain to use the amblyopic eye to stimulate visual development. Part-time occlusion is recommended for very young children, especially those under 1 year of age, to prevent the development of occlusion amblyopia of the good eye. The earlier the intervention, the better the visual prognosis. Penalization with atropine drops is an alternative option for children who are not compliant with patching.
The response to amblyopia treatment depends on the age of the child and the severity of the amblyopia. The severity of the amblyopia depends on when the abnormal stimulus begun, the length of exposure to abnormal stimulation, and the severity of the image blur. The more severe the image blur, the earlier the onset, and the longer the duration of a poor visual stimulus, the more severe the visual loss. The first few months of life is the critical period of visual development when children are most susceptible to amblyopia. Stimulation of a severely blurred retinal image during the critical period of visual development results in dense, often irreversible, amblyopia. This is why visually significant congenital cataracts must be operated and visually rehabilitated within the first few weeks of life for best visual results. Amblyopia can occur, however, in older children. Acquired media opacity such as a cataract, or acquired strabismus, can cause some amblyopia, even up to age 7 or 8 years of age, but of lesser severity.
Occlusion therapy or patching is still the mainstay in amblyopia treatment. The optimal dosage of patching (hours of patching per day) needed for correction of amblyopia was not known until recently, when several research studies had their results published in medical literature.
The Pediatric Eye Disease Investigator Group in the United States particularly compared the improvement in visual acuity of young children undergoing different dosages of patching. They found that the improvement in visual acuity in children with severe amblyopia undergoing full time (all waking hours) and part-time (6 hours) is approximately the same. Moreover, they found out in another study on moderate amblyopes that 2 hours of daily patching produces an improvement in visual acuity that is of similar magnitude to the improvement produced by 6 hours of daily patching.
Most recently, another group of vision scientists from the United Kingdom found that the total cumulative amount of patching needed to correct amblyopia is around 200 hours. They also found out that a longer duration of patching (6 hours per day versus shorter times) resulted in a more rapid improvement in vision. Another important finding of their study was that younger children (less than 4 years old) responded much faster to patching than older children (more than 6 years old). This supports the argument for early treatment of amblyopia. Theoretically, if a child is to undergo 2 hours of patching a day, it would take approximately 3 months before an improvement in vision is seen. On the other hand, 6 hours of patching would take a relatively shorter time before a maximal improvement in vision is seen. Both patching regimens result in similar magnitude of vision improvement. Therefore, occlusion therapy should be individualized and the choice of patching regimen will ultimately depend on the severity of the amblyopia and the age of the child. Regular follow-up with the pediatric ophthalmologist despite good response to amblyopia treatment will help the parents monitor amblyopia recurrence and visual development.
However, one must be cautious in understanding these studies. None of their patients had cataract or glaucoma. The amblyopia in these disorders is different from that due to strabismus or unequal refractive error. Amblyopia with glaucoma and cataract can be much more difficult to treat especially when it starts on the first year of life. Therefore, similar studies should be done to learn if the same conclusions can be drawn about amblyopia in different kinds of childhood eye disease.
Penalization is the use of atropine drops in treating amblyopia. Topical atropine sulfate 1% dilates the pupil and paralyses accommodation (focusing) of the good eye. This forces the child to use the amblyopic eye. The blurring effect of atropine is greatest for near vision and for eyes that are hyperopic. Atropine will not significantly blur the vision if the eye is myopic or if there is no refractive error. Thus, atropine penalization will not work unless the good eye is significantly hyperopic. Most patients with amblyopia are best treated with occlusion of the good eye, but penalization may be an option for those children who are compliant to patching. A recent study showed that weekend application is as effective as daily application of atropine sulfate 1%, claiming that the magnitude of improvement in visual acuity is similar to that previously reported for patching for either 2 or 6 hours per day. This is good news for those children who show severe adverse reaction to this drug.
PEDIG. A Randomized Trial of Atropine Regimens for Treatment of Moderate Amblyopia in Children. Ophthalmology 2004; 111: 2076-2085.