Why early vision screening matters

Why early vision screening matters

Darron A. Bacal, MD, Sepideh T. Rousta, MD, and RichardW. Hertle, MD

Most eye defects in children are correctable, but delaycan lead to permanent vision loss. Pediatricians must check young infantsand preschoolers and not wait for school exams to identify problems. Hereare the techniques to use with patients too young to read a Snellen chart.

In the practice of medicine, nothing is more disappointing than the discoveryof morbidity that could have been prevented or diminished by early screening.Amblyopia, the loss of visual acuity that results when ocular pathologyis not diagnosed and corrected in the early years, is a case in point. Routinevision screening, using simple tests geared to a child's age and level ofdevelopment, can identify ocular pathology early on, before irreversibledamage is done. The impact on a child's vision can be dramatic. Disordersthat can be detected on routine vision screening are listed in Table 1.

Understanding amblyopia

We described amblyopia, the leading cause of preventable vision lossinchildren, in detail in the June 1998 issue of Contemporary Pediatrics.¹Amblyopia is a central nervous system phenomenon, a failure of the brainto process visual cues accurately that results when a young child is notable to form a clear visual image in one or both eyes. This decrease inacuity is beyond the level expected from the pathology that causes the blurredimage, and it may persist even when the pathology has been corrected. Amblyopiaaffects between 2.0% and 2.5% of the general population,² andis the leading cause of monocular vision loss in adults between the agesof 20 and 70.³ It may be partially or fully reversible if therapyis instituted during early childhood.

The first year of life is a critical time, when ocular pathology is mostlikelyto cause amblyopia and when amblyopia is most responsive to treatment.Both the incidence of amblyopia and its responsiveness to treatment declinethrough the first decade of life (Table 2).⁴ Early correctionof amblyopia can restore useful vision to a child's eyeand allow the developmentof some degree of binocular function. Interestingly, although amblyopiausually affects only one eye, people with amblyopia are twice as likelyas those in the population at large to lose vision in their good eye dueto trauma.⁵ The most important goal of vision screening is todetect amblyopia as early as possible, so that our ability to treat it ismaximized.

Many different causes of amblyopia exist. An amblyopic eye can be structurallynormal and be unrecognized by parents and physicians on a general physicalexamination. For example, a delay in the diagnosis of anisometropia, animportant cause of amblyopia, is quite common. In anisometropia, the refractiveerror in one eye is significantly different from the error in the othereye. The brain is unable to resolve this discrepancy and the eye with thestronger refractive error remains unfocused. Decreased or absent binocularfunction and stereopsis can result, in addition to amblyopia. These children'seyes may appear perfectly aligned and show no signs of a problem unlessthe acuity of each eye is individually tested.

Strabismus, a misalignment of the two eyes, can also cause amblyopia.When the eyes are misaligned, the brain often suppresses the resulting doubleimage (diplopia), at the expense of the visual development of one eye. Evenif the strabismus is subtle or intermittent, amblyopia may occur.⁶

Other causes of amblyopia include any condition which interferes withthe normal passage of light through the visual axis. Examples include ptosis,eyelid tumors, corneal scarring, cataracts, and retinal pathology.

When to screen

The American Academy of Ophthalmology⁷ and the American Academyof Pediatrics⁸ agree on the following recommendations for visionscreening:

• Newborns should have an eye examination performed by the pediatrician or other primary care provider. Any infant who has an abnormal exam or is at increased risk for eye disease should be evaluated by an ophthalmologist. Risk factors are shown in Table 3.
• By 6 months of age, all children should be evaluated for visual behavior and ocular alignment by their pediatrician or ophthalmologist.
• By 3¹/₂ years of age, screening specifically focusing on visual acuity should be performed.
• At age 5, children should receive a repeat visual acuity screening.
• Further evaluations should be performed if routine school examinations indicate a problem or thechild has symptoms of visual difficulty.

How to do vision screening

The way vision screening is done depends on the age of the child.

Newborns. The external appearance of each eye should be evaluatedfor normal structural development. A direct ophthalmoscope can be used toevaluate the red reflex in each eye to rule out leukocoria. Under normalcircumstances, the reflection of light from the retina can appear red orlight orange. However, if the reflex is blocked by ocular pathology, suchas a cataract or tumor, it may appear white, dark, or dull (Figure 1). Evaluationto rule out a leukocoric reflex is a critical component of the vision screeningof infants.

Three to 6 months of age. By 3 months of age, normal full-terminfants should not display any constant or intermittent strabismus (ocularmisalignment). They should be able to fixate on small, colorful toys andfollow the movement of these toys for brief periods of time (Figure 2).The toy should be held six to 12 inches from the baby's face while eacheye is covered briefly with the examiner's thumb. Most children with equalvision in the two eyes will remain attentive to the toy and continue tofixate with the uncovered eye. A child with poor vision in one eye may protestor reach to remove the thumb when the better eye is covered, but remaincalm and interested when the bad eye is covered. This can be an early signof amblyopia. When testing this age group, avoid toys that make a noise;a child who is blind or sees poorly can sometimes track an object by hearingalone.

Appearances can be deceiving when you are evaluating ocular alignment.Prominent epicanthal folds can give the appearance of strabismus when infact the child is perfectly normal. This is known as pseudostrabismus. However,the presence of pseudostrabismus does not rule out true strabismus, as thesetwo conditions can coexist (Figure 3). In addition, strabismus can be intermittentand go unrecognized during a brief office visit.

A useful basic test to evaluate ocular alignment is the corneal lightreflex or Hirschberg test. The examiner shines a penlight centrally ontothe child's face and notes the position of the corneal light reflexes whilethe child fixates on a small object held next to or on the penlight. Ifthe child is looking straight ahead at the examiner's light, the light reflexshould fall on the central portion of each pupil.If the light reflex iseccentric in one eye, strabismus may be present (Figure 4). Remember thatan intermittent strabismus can be missed by this method. If you suspectmisalignment but do not find it by using the light reflex test, a pediatricophthalmologist can employ other testing methods, such as alternate covertesting.

Three years of age and older. Appropriate vision screening methodsdepend on the child's developmental level and verbal ability. Familiar picturesof common objects as depicted on an Allen chart can be used (Figure 5).However, these crude figures can overestimate a child's acuity and underestimateamblyopia. Most 3- or 4-year-olds can participate in a matching game usingthe letters H,O,T,V. The child holds a card with these four letters on herlap and is asked to point out the letter that matches the one on the chart(Figure 6). This can be a fast and easy technique and is especially usefulin nonverbal children. The "tumbling E" game (Figure 7) and Landoltrings are two other acuity tests that require the child to point in thedirection the letters are oriented. These tests are cognitively a littlemore difficult than the HOTV matching test. By the time they are 5 yearsof age, most children can recognize the letters on the Snellen chart.

Vision testing with a letter eye chart such as the Snellen, HOTV, orthe tumbling Es is standardized for a 20-foot distance. Allen picture cardscan be tested at ten feet, with the acuity recorded as 10/the number ofthe line the child can read; visual acuity of 10/20 in this system is equivalentto 20/40 using a chart at 20 feet. Pediatricians can usually set up a 20-footlane by placing the chart at the end of a hallway. Testing a child's nearvision only is not an acceptable method of vision screening.

A child with amblyopia is likely to do better if you show the lettersone at a time than when he is asked to identify letters in a row; the increaseddifficulty in identifying letters in a group is called the "crowding"phenomenon.⁹ Showing only single letters can lead you to underestimatethe extent of the child's amblyopia. For information on where to obtainvision screening charts for your office, call Prevent Blindness America,800-331-2020.

Tips for successful screening

A busy pediatric office can be a difficult place for children to performwell on vision screening. Distractions abound: The child in the next roommay be wheezing from an asthma attack; the infant across the hall gets avaccination and starts to scream. Following these tips can help make screeningresults more accurate:

• Pick out the quietest, least distracting place in the office for vision screening.
• Perform testing quickly and efficiently, before the child loses interest.
• Be prepared for cheating. A child who cannot see well out of one eye will attempt to cover up the problem by memorizing the chart with the good eye or peeking through her fingers or around the occluder (Figure 8). One way to prevent this is to patch the eye you're not testing. Children enjoy the patch if they are told to pretend to be pirates.
• Make it a game, not a test. Give lots of positive feedback, and never say No when the child answers incorrectly. Children are often fearful of being wrong, and may stop trying if they feel they are being judged for "wrong" answers. Gentle coaxing will produce the most accurate results.
• Before you screen, make sure the child can recognize the pictures or letters on the chart. If you don't check this out first, you may get results that indicate lack of knowledge rather than poor acuity.
• Don't push too hard. A child who has been responsive with larger letters but seems to lose interest when you move on to the smaller ones may not be able to see them well. If gentle coaxing doesn't work, switch to the other eye. Insistence may result in the child's refusal to participate at all.

Vision screening is an important task and visual disorders can easilybe missed. Although screening can be performed by various staff members,a few guidelines should be considered before this task is delegated. Theperson doing the testing should be knowledgeable about the test and comfortableperforming it. The tester must understand the importance of vision screeningand refer any questionable or abnormal results for retesting by the pediatricianor pediatric nurse practitioner (PNPs).

New screening techniques

The first three years of life are the most critical for visual development,but they are often the most difficult time to detect a visual disorder.Young children may not be willing to cooperate with the standard chartsused to test vision, and the results may be inaccurate. A new instrumentknown as the photoscreener may solve some of these problems. The devicephotographs the retinal reflex as the child's eyes react to a flash of light.Depending on how the reflex looks in the photo, the examiner can detectanisometropia, ametropia (refractive error), strabismus, and media opacitiescaused by tumors or cataracts. Photoscreening can be done quickly, beforethe child loses patience, and is particularly useful in preverbal children.Intermittent strabismus and certain refractive disorders can be missed,however.¹⁰ In addition, because the device is new, there aresome uncertainties about the validity of the results and the most suitableage for using it. For more on new vision screening technologies, see "Newways to screen".

Some ophthalmologists have advocated using stereoacuity tests in screening.The child wears polaroid glasses and looks at pictures that appear threedimensional if the child has high-level binocular function. Poor responseson this test can signal a disorder such as strabismic or anisometropic amblyopia.¹¹However, latent strabismus (phorias),¹² bilateral ametropia (highsymmetric refractive error in both eyes), and non-amblyopic anisometropia¹³are often missed bythis technique, and we cannot recommend its use by generalpediatricians.

Needed: A close look

The goal of vision screening should be never to miss a child with visualpathology, and--when pathology exists--to detect it as early as possible.As the only health-care professionals who have regular contact with allyoung children, pediatricians and PNPs have a crucial role to play in detectingproblems before irreparable damage is done. Children with a family historyof strabismus, amblyopia, or other visual disorders presenting in childhoodshould be watched especially closely. Any evaluation that yields even asuspicion of a problem should be referred promptly to a pediatric ophthalmologist.Waiting a few months to retest the child can damage the child's chancesof full correction. Early detection, diagnosis, and treatment give a childwith visual pathology the best opportunity for good eyesight.

THE AUTHORS:

DR. BACAL practices pediatric ophthalmology in Milford,Branford, North Haven, and Orange, CT, and is affiliated with Yale-New HavenHospital, New Haven, The Hospital of St. Raphael, New Haven, and MilfordHospital, Milford, CT.

DR. ROUSTA practices pediatric ophthalmology in East Brunswick,NJ, and is affiliated with Robert Wood Johnson University Hospital and St.Peter's Hospital, Newark, NJ.

DR. HERTLE is the pediatric ophthalmologist for the NationalEye Institute in Bethesda, MD.

REFERENCES

1. Bacal DA, Hertle RW: Don't be lazy about looking for amblyopia. ContemporaryPediatrics 1998;15(6):99

2. Von Noorden GK: Burian and von Noorden's Binocular Vision and OcularMotility. St. Louis, MO,CV Mosby Co, 1974

3. Krueger DE, Ederer R: Report on the National Eye Institute's VisualAcuity Impairment Survey Pilot Study. Bethesda, MD: Office of Biometry andEpidemiology,National Eye Institute, 1984

4. Vaegan K, Taylor D: Critical period for deprivation amblyopia in children.Trans Am Ophthal Soc UK 1979;99:432

5. Tommila V, Tarkkanan A: Incidence of loss of vision in the healthyeye in amblyopia. BrJ Ophthalmol 1981;65:575

6. Campos EC: Amblyopia. Surv Ophthalmol 1995;40:23

7. American Academy of Ophthalmology: Policy statement: Vision screeningfor infants and children. 1996

8. American Academy of Pediatrics: Eye examination and vision screeningin infants, children, and young adults. Pediatrics 1996;98(l):53

9. Rodier DW, Mayer DL, Fulton AB: Assessment of young amblyopes: Arrayvs. single picture acuities. Ophthalmology 1985;92:1197

10. Cibis GW: Video vision development assessment (VVDA): Combining theBrukner test with eccentric photorefraction for dynamic identification ofamblyogenic factors in infants and children. Trans Am Ophthalmol Soc 1994;84:643

11. Cooper J, Feldman J: Random-dot-stereogram performance by strabismic,amblyopic and ocular-pathology patients in an operant discrimination task.Am J Optom Physiol Opt 1978;55:599

12. Fern KD: A comparison of vision screening techniques in preschoolchildren (abstract). Invest Ophthalmol Vis Sci 1991;32 (suppl):962

13. Avilla CW, von Noorden GK: Limitation of the TNO random dot stereotest for visual screening. Am Orthopt J 1981;31:87

Why early vision screening matters.

Contemporary Pediatrics

1999;0:155.