Amblyopia, an impairment of vision without detectable organic lesion of the eye, is often defined as a significant dissimilarity in visual acuity between the eyes based on a difference of two or more lines on Snellen testing. A diagnosis is based on such a documented discrepancy in the presence of a condition known to cause amblyopia. This can result from a variety of conditions. Approximately one third of cases are attributed to anisometropia, one third to strabismus, and one third to a combination of these conditions or to other eye disorders such as ametropia. Other studies indicate that the prevalence of amblyopia is 1 to 5 percent in children and 2.9 percent in adults. The condition is a leading cause of vision loss in adults and children. Holmes and Clarke reviewed the results of randomized clinical trials to provide an update for the treatment of amblyopia.
In children younger than 2.5 years, a squint can indicate the diagnosis. In children without a squint, the eyes must be separately assessed; however, the authors stress that most tests in use are insensitive to amblyopia in younger children. Physicians can use age-appropriate Snellen charts or specialized testing charts for older children.
Two screening studies concluded that tests for refractive errors combined with a cover test for strabismus conducted at 37 months of age would provide the best sensitivity and specificity for screening children. The studies also showed that these tests can be carried out by trained laypersons, nurses, family physicians, or ophthalmic subspecialists. Other studies indicated that screening by orthoptists is the most accurate.
Traditionally, screening was recommended for children as young as possible. However, screening at 5 to 6 years of age is considered more reasonable because the benefit of early detection on long-term outcomes has been questioned in recent randomized control trials. The authors note that if vision in the unaffected eye remains good, amblyopia may have relatively little effect on a patient’s quality of life. However, little information is available about the impact on overall health or functionality. In a 20-year study, the occurrence of vision loss in one eye was 1.75 cases per 1,000 patients; in 50 percent of these cases, the loss was a result of trauma.
Treatment of amblyopia is individualized and based on its etiology. Any source of visual deprivation, such as ptosis or cataracts, should be corrected. Associated with progressive improvement in acuity, refractive correction usually indicates some ability to adapt. Patching the eye has been difficult to study objectively because the technique varies widely. Recent studies, however, indicate that short periods of patching (one hour or more a day) may be effective. Adhering to therapy guidelines is always problematic with patching, and some experts argue that the psychological distress of the treatment outweighs the modest benefits of patching. Atropine drops also have been studied as an alternative to patching and appear to have long-term outcomes similar to patching but are more acceptable to parents.
More than 50 percent of treated children regain normal vision in the amblyopic eye. However, the treatment type and age of the child during therapy are believed to be less influential on the outcome than problems of adhering to treatment and undiagnosed, underlying ocular or cerebral pathology. Optic nerve hypoplasia also is likely to cause poor therapy response.
The authors conclude that additional studies are needed to provide evidence-based recommendations for the detection, diagnosis, therapy, and follow-up of this condition.