Vision impairment changes how a child understands and functions in the world. Impaired vision can affect a child’s cognitive, emotional, neurological, and physical development by possibly limiting the range of experiences and the kinds of information a child is exposed to. Behaviors that are consistent but not pathognomonic with vision impairment include poor visual attention, the oculodigital reflex (forceful eye rubbing), photo aversion, or just the opposite, light gazing. Other blindisms, or strange behavioral mannerisms seen in children with visual impairments, include rocking, head-nodding, and exaggerated finger play.¹

The overall prevalence of visual impairment in children, including all refractive errors, is 7% to 8.2%. The prevalence of amblyopia is 2.9% to 3.9%.^18-23

Amblyopia
Amblyopia is reduced visual acuity in one or both eyes due to abnormal binocular interaction. The brain and the eye work together to produce vision. Light enters the eye and is changed into nerve signals that travel along the optic nerve to the brain. Amblyopia is the medical term used when the vision in one of the eyes is reduced because the eye and the brain are not working together properly. The eye itself looks normal, but it is not being used normally because the brain is favoring the other eye. This condition is also sometimes called lazy eye. Causes of amblyopia include ptosis, cataracts, strabismus (crossed eyes), and unequal refractive errors between the 2 eyes (one eye is more nearsighted, farsighted, or astigmatic than the other eye).⁸ One large series found that strabismus accounted for 48% of the cases of amblyopia, refractive errors for 20% of the cases of amblyopia, and mixed strabismus and refractive errors for 32% of the cases.⁹ Cataracts and ptosis are rare in children.
Amblyopia is associated with visual impairments that present early in life. If the visual impairments associated with amblyopia develop after 6 to 8 years of age of life, amblyopia usually does not arise.¹⁰ Amblyopia is considered to be a developmental disorder with an early sensitive period.^10,11

Treatment of Amblyopia
Amblyopia is unlikely to improve without therapy. Unless it is successfully treated in early childhood, amblyopia usually persists into adulthood, and is the most common cause of monocular (one eye) visual impairment among children and young and middle-aged adults. Previously, eye care professionals often thought that treating amblyopia in older children would be of little benefit. Surprising results from a nationwide clinical trial show that many children age seven through 17 with amblyopia may benefit from treatments that are more commonly used on younger children (Treatment of Amblyopia in Older Children (ATS3) Study).

Treatment for amblyopia involves correcting the underlying cause of the amblyopia and reducing or eliminating the visual suppressive effect of the non-amblyopic eye. The goal is to force the child see only with the eye with the reduced vision (weaker eye). Typically this is achieved by occluding (blocking) the better (stronger) eye with a patch. In addition to the patching, the child should spend at least 1 hour of patching time each day doing near visual activities such as crafts, coloring, tracing, cutting out objects, connecting the dots, hidden pictures and word finds, computerized video games, reading, written homework assignments, or other activities requiring eye-hand coordination.

Occlusion therapy with 'patching of the sound eye' has been the mainstay of amblyopia treatment. Compliance is a major problem because children dislike of patching owing to skin irritation and visual, social, and psychological reasons. Although less widely prescribed, 'pharmacologic penalization' is an alternative to occlusion therapy for amblyopia. This method involves the instillation of a long-acting topical cycloplegic agent, such as atropine sulfate, into the sound (better/stronger) eye. The cycloplegia prevents accommodation, blurring the sound eye at near fixation. A NEI supported clinical trial was conducted to assess whether treatment with atropine drops was as effective as patching for amblyopia treatment (patching v. atropine study). Atropine and patching produced improvement of similar magnitude, and both are appropriate modalities for the initial treatment of moderate amblyopia in children aged 3 to less than 7 years. The duration of patching or penalization required depends on many factors, including the cause of the amblyopia, the density of the amblyopia, and the age of the child. Recent findings of a clinical trial (Two v. Six Study) show that patching the unaffected eye of children with moderate amblyopia for two hours daily works as well as patching the eye for six hours.

Evidence of the effectiveness of treatment for children with amblyopia comes from retrospective and prospective cohort studies. Of the prospective studies in this review, between 41% and 96% of persons with amblyopia had improved visual acuity after treatment.^38-41 These studies had between 25 and 255 subjects (mean, 112). Of the retrospective studies, between 40% and 95% of those with amblyopia achieved better visual acuity.^42-50 These studies had between 51 and 961 subjects (mean, 280).
Two studies of treatment for amblyopia found that successful outcome depended on earlier treatment.^41,48 The larger of these studies, a retrospective analysis of 407 children with strabismic amblyopia, demonstrated that treatment efficacy is highest for children younger than 3 years of age.⁴⁸ Furthermore, treatment efficacy steadily decreased after 3 years of age; by 12 years of age, treatment was ineffective. The stability of the improvement for those treated with amblyopia is unclear. None of the studies included in this review followed patients into adulthood.

Consequences of amblyopia that is untreated before 5 years of age.
The literature searches yielded 1 study on the association between visual impairment and learning or intelligence.⁵¹ In this study of 10-year-old children from England, subjects with amblyopia had a statistically significant lower score on the British Ability Scales than children with normal vision (98.4 versus 100; standard deviation 15). The importance of this finding is unclear, because the absolute difference was small. Furthermore, a cross-sectional study cannot establish causality. However, a small shift in ability to learn or intelligence may have an important impact on society.
One study explored the relationship between amblyopia and quality of life.⁵² This survey was administered to 45 adults with amblyopia but without strabismus. Of the respondents, nearly 50% reported that amblyopia interfered with schooling and work and nearly 75% reported that amblyopia affected their self-image. These subjects were recruited in an academic ophthalmologic practice, raising questions about generalizability. Furthermore, this study had a response rate of 56%, which raises concern about selection bias.
Amblyopia may be a risk factor for future total blindness. A retrospective study from Finland found 23 cases in which people with amblyopia subsequently become totally blind.⁵³ The authors reviewed the prevalence of amblyopia and the population from which these cases arose to determine the risk of loss of vision in the non-amblyopic eye. The risk of vision loss (1.75 cases per 1,000 persons) among those with amblyopia was found to be greater than among the general population (0.66 cases per 1,000 persons). Accidental trauma was associated with more than half of the cases of total vision loss in those with amblyopia. This study may be biased by the assumptions regarding prevalence and population size.
Although certain jobs require normal stereoacuity (depth perception), such as piloting aircraft, we were unable to find any study detailing the relationship between amblyopia and job performance. However, the requirement for the absence of amblyopia for a given occupation does not mean that there is a basis for such a requirement. The ability to perform jobs that require near-work, however, is more likely to be affected by amblyopia.

Refractive Errors Not Associated with Amblyopia
The other common source of visual impairment is refractive error not associated with amblyopia. Examples include myopia (near-sightedness) and hyperopia (far-sightedness). These visual impairments may interfere with learning and other daily activities. Unlike amblyopia, these visual impairments remain correctable regardless of the age at which they are detected.
One challenge in evaluating refractive errors not associated with amblyopia is that refractive error often is associated with amblyopia. For example, hyperopia, the most common refractive error in children, can lead to the development of strabismus, which in turn can cause amyblopia.¹²
A second challenge in evaluating refractive errors in children is that the refractive state of the eye normally changes with the growth of the child.^13,14 The eye grows rapidly between birth and 3 years of age, after which growth slows. To maintain vision during the time of rapid eye growth, the lens changes geometry. Animal models suggest that this process, known as emmetropization, does not occur normally if the eye does not receive visual stimulus.¹³

Treatment of refractive errors not associated with amblyopia is by spectacle correction. In 1 retrospective cohort study, 98% of preschool children corrected to normal vision.⁴⁵

Is there any harm in treating vision impairment?
One study of the treatment of amblyopia found that visual acuity of the non-amblyopic eye (the better/stronger eye) can decrease on average to 20/182 as a result of patching.⁴⁸ However, this loss of visual acuity was not permanent and resolved weeks after therapy was ended.

A potential harm of early treatment visual impairment is interference with the normal development of the eye. Because the development of the eye, or emmetropization, partially depends upon the quality of image that an eye receives, some experts have suggested that interfering with this image, even by correction, may impair normal development.⁸ An analysis of the study of early treatment of hyperopia found that emmetropization did not occur in 42% of treated children compared to 31% of those not given spectacles.⁵⁴. Therefore, although studies suggest that early initiation of treatment for amblyopia results in improved outcomes, there is at least a theoretical concern that early correction of refractive error can interfere with the normal development of the eye.
1. Thompson L, Kaufman LM. The visually impaired child. Pediatr Clin North Am. 2003 Feb;50(1):225-39.

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