Vision screening of young children: Taking a second look

There is controversy over the best ways to detect and treat amblyopia. Considering evidence from the latest research could be the first step in decreasing the preventable loss of vision.

Susan Martin Dubowy, PA-C

The author was Prevention and Education Specialist at the Susquehanna Association for the Blind and Vision Impaired, Lancaster, Pa, when she wrote this article. She now works in family practice. She has indicated no relationships to disclose relating to the content of this article.

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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

Refractive errors

Myopia (nearsightedness), hyperopia (farsightedness), and astigmatism are of concern, particularly if there is a significant difference in refractive error between the eyes, termed anisometropia.⁵ In this case, the development of the visual pathway from the eye with greater refractive error will be suppressed and the child will function using the better eye.¹⁹ Refractive amblyopia without strabismus comprises 33% to 75% of all cases of amblyopia.¹ Detecting this condition in young children—a challenging problem for clinicians—generally involves a vision screening test or refraction.¹⁹ The window of opportunity to treat refractive/anisometropic amblyopia, usually first with eyeglasses, is traditionally thought to close at about age 7 years.²⁰ After that, the child usually incurs permanent loss of vision in the amblyopic eye, which may not improve with eyeglasses or other treatment.

Hyperopia The ability of children to accommodate (focus near images by adjusting their lens) normally allows them to compensate for mild hyperopia until their eyes are fully mature. Hyperopia is problematic when it is extreme, however, or when one eye is more hyperopic than the other. This type of anisometropia is a common cause of amblyopia.¹⁹

Myopia The most common refractive error in older children, myopia affects almost 20% of them by the end of adolescence.¹⁹ Since it typically occurs after the visual system’s critical development stage, myopia rarely causes amblyopia.¹⁹

Astigmatism Also common in older children, astigmatism is usually caused by a defect in the shape of the cornea. Severe (or anisometropic) astigmatism in younger children can cause amblyopia.¹⁹  

Strabismus

Ocular misalignment, or strabismus, is frequently called crossed eyes. Many newborns have some degree of misalignment in the first few weeks of life, although strabismus that persists or develops after the age of 3 months should always prompt a referral to a qualified pediatric eye professional.¹⁴ Esotropia, in which the eye turns inward, is more common in young children than exotropia, when the eye turns outward. Strabismus affects 3% of the general population and can be the presenting sign of many different visual and neurologic disorders; thus, all cases should be referred for complete ophthalmologic evaluation.²¹ Some types of strabismus in young children include the following:

• Infantile esotropia, also called essential or congenital esotropia, usually appears by age 6 months and is often a large-angle, constant deviation that is obvious on physical examination. Treatment is eye muscle surgery, and outcomes are best if the procedure is performed before the child is 1 year old.^21,22
• Accommodative esotropia is associated with significant hyperopia; the eye turns in as it tries to achieve focus on nearby objects.¹⁹ Onset is usually at age 1.5 to 4 years, and the condition may progress from intermittent to more frequent.²¹ Eyeglasses may be the only treatment needed.
• Intermittent exotropia typically begins between 2 and 8 years of age, occurring initially only when the child is tired. Eye muscle surgery may be needed.¹⁶
• Sensory exotropia may develop in older children and adults with severe, long-standing vision loss when the brain has, in effect, stopped trying to achieve ocular alignment.²¹ 

Conditions that block the transmission of light through the eye

Congenital cataracts and abnormalities of the eyelids, such as congenital ptosis, can cause amblyopia, but these conditions are much less prevalent than refractive errors and strabismus.^14,23 Because congenital cataracts cause a profound deprivation of stimulation during the period of the most rapid development of the visual cortex, cataract surgery and visual rehabilitation with aphakic spectacles or contact lenses must be done in the first 3 months of life to be successful.^16,24 

Patching

In addition to addressing the underlying causes of the amblyopia, many cases also require resting the better eye, forcing the use of the amblyopic eye, and stimulating development of the corresponding cells in the visual cortex.⁵ Traditionally this is done by patching the better eye. Some children find the patch bothersome and embarrassing, and noncompliance is the most frequent cause of treatment failure.¹⁴    

Shorter patch times The good news that may lead to improved compliance for amblyopia patients is that shorter daily patching times have been shown to be as effective as the longer traditional times. For moderate amblyopia (20/40 to 20/80), 2 hours daily was as effective as 6 hours daily.⁹ For severe amblyopia (20/100 to 20/400), 6 hours daily was as effective as all-day patching.¹⁰ Improvement of visual acuity after 4 months of treatment was used as the measure of effectiveness. However, further studies are necessary to ensure that improvement in vision is sustained.²⁵

No patch The most exciting advance in treatment may be the ability to forgo patching altogether for some children. Using atropine drops to blur the vision in the better eye for a few hours each day has recently been shown to be an effective alternative to patching for moderate amblyopia.⁸ In this study, 75% of both the drop and patch treatment groups achieved a visual acuity in the affected eye of 20/30 or better, and/or improvement of three lines on a standard eye chart.⁸ 

Early treatment depends on early detection

A recent study in Great Britain has reinforced the primary rationale for detecting and treating amblyopia. If a person has vision loss in one eye from amblyopia, there is a 1.2% to 3.3% lifetime risk of also losing functional vision in the better eye. This is higher than previous estimates.²⁶

Successful treatment cannot take place without detection, and referring children with possible vision problems to a pediatric ophthalmologist or other eye care professional trained to treat young children is essential. Although most children with amblyopia are asymptomatic, some do have signs and symptoms.²⁷ A head tilt, light sensitivity, squinting, eye closure, learning disabilities, and holding books and toys close to the eyes all necessitate referrals.^19,27 Note that television is not a useful tool in identifying children with vision problems because many healthy preschoolers habitually sit close to the screen.¹⁹

The corneal light reflex/Hirschberg test and the red reflex test are two important components of a routine pediatric exam. Any abnormal findings in these tests should trigger prompt referral. An off-center corneal light reflex can indicate strabismus.²¹ A variety of disorders can cause abnormal or asymmetric red reflexes. A dulled red reflex may indicate a congenital cataract.²⁸ A white appearance on the red reflex test could indicate a retinoblastoma. Retinoblastoma requires special attention, not as a cause of amblyopia but because it is a rare, life-threatening malignancy of the retina that usually manifests before the age of 2 years.²⁹

Although physical examination of young children’s eyes is important, approximately 50% of those with amblyopia show no misalignment.^5,19 As a general rule, preschool children do not complain of vision problems and can function at this age using one eye, even if the vision in their other eye is severely reduced.^19,27 Thus, vision screening is recommended to identify those who would benefit from comprehensive pediatric eye examination.^4,6,19

Optimal screening

To improve screening, we need to address various barriers to it, including controversy over the optimal age to begin vision screening. In Sweden’s successful program, nurses utilize an eye chart to screen 4-year-old children; if children cannot pass the screening after three attempts, they are referred.² In 2003, the American Academy of Pediatrics (AAP) published a policy statement, “Eye Examination in Infants, Children, and Young Adults by Pediatricians,” recommending beginning visual acuity screening at age 3 years.²⁷ Later that year, a study published in the British Medical Journal showed no difference in short-term outcomes for children with moderate amblyopia whether treatment was begun before, or at, age 5 years.²⁰ 

Current practice

While we await more studies to clarify the optimal age to begin screening, it is reasonable to give particular attention to vision screening of 5-year-old children. We should start by assessing our present performance.

A study reviewing practices in North Carolina reveals much room for improvement.³⁰ This study examined the completeness of kindergarten health assessment reports (KHARs) mandated by most states. PAs did no better than physicians or other providers in documenting results of basic screening for vision, growth, etc. Specifically, 14% of the 3,952 KHARs reviewed contained no documentation of vision screening. Of even greater concern is that, of the 485 children who were screened and who met the AAP criteria at the time for referral for a complete eye examination, only 38% were identified by the primary care providers as needing follow-up.³⁰

This is not much better than findings in a previous study of practices in 23 states, “Preschool Vision Screening in Pediatric Practice: A study from the Pediatric Research in Office Settings Network.”¹⁷ Of the 8% of 3- to 5-year-old children who were screened and met criteria for referral to an eye care professional, only 21% were referred; 57% were told to repeat screening (most in a year), and for 15% no follow-up was recommended.¹⁷ Improvement is needed in two areas: 1) increasing the number of young children screened, and 2) improving follow-up care for those children whose screening reveals possible vision problems.

Several factors probably contribute to primary care providers’ inattentiveness to vision screening of young children, including

• Lack of confidence in the accuracy of the results, with little or no scientific information on the validity of the wide variety of screening methods³¹
• Inconsistent and confusing guidelines from various organizations for screening and referral³¹
• Poorly trained screeners³²
• Misunderstanding of the finite window of opportunity for treating amblyopia.¹⁹ 

VIP Study

The NIH Vision in Preschoolers (VIP) Study has provided some much-needed data about the relative accuracy of different screening methods when used by licensed eye care professionals.¹³ More than 2,500 3- to 5-year-old children attending Head Start participated. In phase I all were screened and then had full eye examinations. Phase II, now under way, will evaluate how well nurses and lay screeners utilize the best methods identified in phase I.¹³ In the interim, it seems logical that offering specific training and tools to the staff members who currently perform vision screening in most primary care offices may improve current screening practices. It is important that 5-year-old children receive special attention. The AAP’s 2003 Policy Statement, endorsed by other major organizations, offers clear guidelines for referral, including the recommendation to refer those children who cannot be screened²⁷ (see Table 1).

Child-friendly visual acuity screening tests

The AAP’s new guidelines recommend choosing one of a variety of visual acuity tests designed for young children. Two simple, relatively inexpensive tests performed well in the VIP Study: the Lea symbols visual acuity test and the HOTV test.^13,27

These recommended visual acuity screening tests each use four different simple symbols or ototypes: a circle, square, house, and apple for the Lea test, and the letters H, O, T, V, for the test which goes by that name (see Figure 1). The screener holds a chart of ototypes and points to one, asking the child to name or point to the matching ototype on his or her response card.¹³ Use of the response card reduces both the anxiety for the child and the potential for the screener to misunderstand a young child’s verbal answers. Children can do well at matching without knowing the names of shapes or letters; those with language barriers can be screened with minimal translation.

If the child is able to perform this step, one eye is then occluded with an adhesive patch. A major pitfall of vision screening is that children with significant unilateral vision problems inadvertently pass by peeking with their better eye around handheld occluders.³³

The test continues at a distance of 10 feet, with progressively smaller ototypes. The examiner then switches the patch to the other eye and repeats the screening process. Visual acuity is recorded for each eye individually.¹³  

Can technology help?

In the VIP Study, the Lea symbols visual acuity test shared the top-performing category with some technology-based screening tools¹³ (see Table 2). Two handheld screening autorefractors both quickly measure refractive error in each eye, as the child looks at a blinking light on the device.¹³ If these tests are demonstrated to be sensitive when used by nurses in the next phase of the VIP Study, the automatic refractors may become effective alternatives or adjuncts to preschool acuity tests.

Photoscreening, which entails taking and analyzing two Polaroid photos of each child’s eyes, is used in several states for large public screening programs.^34,35 This screening method had lower sensitivity rates in the VIP Study than in an earlier study.³⁶  

Improving screening, saving sight

A recent analysis calculated that the benefits of screening preschoolers exceeded the costs.³⁷ Until more data are available on the cost effectiveness of professional eye examinations for all children entering school, it seems appropriate to use the available information and resources to improve the current practice of vision screening and referral.

Detecting vision disorders in young children is challenging. However, since timely treatment can usually reduce or reverse vision loss from amblyopia, meeting this challenge is worthwhile. PAs can play a key role in overcoming many of the barriers to vision screening of young children by following the following steps:

• Obtain age-appropriate vision screening tools
• Ensure training of the screeners in their offices
• Utilize the updated referral guidelines
• Pay particular attention to vision screening results in 4- to 5-year-old children
• Be vigilant in performing examinations such as the red reflex and corneal light reflex/Hirschberg tests at well-child visits.

These actions can go a long way toward meeting the national goal of increasing screening rates and decreasing the number of children who permanently lose vision from treatable conditions. Making a difference in this way will benefit such children for a lifetime.   

The author thanks David Silbert, MD, FAAP, for his assistance in reviewing this article.

© 2007 Haymarket Media, Inc. and the American Academy of Physician Assistants. All rights reserved.
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