Vision screening of young children: Taking a second look

Amblyopia, a developmental vision disorder, is usually reversible if detected and treated in early childhood.^1-3 In Sweden, where 99% of 4-year-old children undergo visual acuity screening, the prevalence of significant vision loss from amblyopia has been reduced from 2% to 0.2%.³ In the United States, fewer than 22% of preschoolers are estimated to receive vision screening; the prevalence of amblyopia in the general population remains approximately 2% to 3%, making it the leading cause of monocular vision loss in people aged 20 to 70 years.^4-6 Obviously, significant barriers must be overcome if we are to meet one of the Healthy People 2010 Public Health Vision Objectives to “increase the proportion of preschool children [aged 5 years and younger] who receive vision screening.”⁷ The ultimate goal of screening is to decrease the prevalence of permanent loss of vision from treatable causes.

In the United States and Great Britain, nearly every aspect of detecting and treating amblyopia is being reexamined and debated. This includes the

• Effectiveness of new treatment options^8-10
• Value of treating amblyopia to individuals and society^11,12
• Optimal age for detection and treatment^6,12
• Currently available screening tests that are acceptably sensitive and specific when used in primary care settings.¹³

The debates have stimulated much research, although some questions remain unanswered. This review provides a foundation for improving vision care for young patients—an important goal because it is during the first decade of life that most children acquire the stable, fully developed anatomy necessary for normal vision.¹⁴

The physiology of vision

Light is focused as it passes first through the cornea and then the spherical lens, before it reaches the retina. The retina converts light into nerve signals, which then travel along the optic nerve to the visual cortex.¹⁵ The eyes are aligned; smooth, symmetrical eye movements are achieved using extraocular muscles innervated by cranial nerves.¹⁶ The visual cortex receives input from both eyes, which it fuses into a single, clear, 20/20 image with good depth perception.¹⁵

There is quite a contrast between the performance of the mature visual system and the meager visual capabilities of a normal newborn, whose acuity is approximately 20/400.¹⁴ Development of the visual cortex plays a major role in this dramatic transition. In young children, high-quality visual images transmitted from the eyes are necessary to trigger normal development of corresponding areas of the visual cortex.^5,14

By far the most rapid development of the visual system occurs during the first 6 months of life, by which time visual acuity is estimated to be 20/30.¹⁴ Development of the visual cortex then continues at a slower rate and is usually complete by age 8 to 10 years.¹⁴ If one eye transmits a poorer or different image than the other during this unique period of visual development, the brain cells that correlate to that eye will be fewer and less developed than those associated with the better eye.⁵ 

Causes and treatment of amblyopia

Amblyopia refers to the decreased vision resulting from this suboptimal development of the brain's central visual pathways, when the eye itself is healthy and not affected by retinopathy.¹⁴ The severity of vision loss in the affected eye can vary from a visual acuity of 20/30 to worse than 20/200 and can also impair depth perception.^5,17 Amblyopia is generally reversible if treated during the sensitive period, which varies in length, in part depending on the underlying cause. In general, the younger the child, the better the chance that treatment will have a successful outcome.^14,18 Although amblyopia can be bilateral, it most often occurs unilaterally, as previously described. This may explain why amblyopia is frequently referred to as lazy eye.¹⁴ Any condition that prevents the transmission of two clear, aligned, fusible images along the central visual pathways during the first decade of life can cause amblyopia.¹⁴

Screening for amblyopia, in essence, is screening for a host of common vision disorders in children. Conditions that have the potential to cause the problem include refractive errors, strabismus, and disorders that block the transmission of light through the eye, causing deprivation of stimulus.⁵ These conditions frequently coexist.³