Many professional and service organizations concerned with the well-being of children have recognized the importance of vision screening and have advocated its wider application. Recommendations for healthrelated screening programs developed by the World Health Organization require that the disorder to be screened be common, treatable at the time of detection, and important to the individual in terms of lost function.1 Vision screening fits these requirements.
Amblyopia is a major public health problem in the United States and around the world, with a prevalence of 2% to 4%.2 By definition, amblyopia is a unilateral or bilateral reduction of corrected central visual acuity without a visible organic lesion commensurate with this loss. It is the leading cause of monocular vision loss and blindness in children and in adults 20 to 70 years old.3 Amblyopia is treatable, especially in infants and young children. After the "critical period" (typically from birth to 8 years of age), structural changes in the central nervous system render treatment with monocular occlusion or pharmacologic penalization and optical correction less effective. Some cases of amblyopia (eg, those associated with severe cataract) may become irreversible even during the first few months. Unfortunately, in most cases amblyopia is itself asymptomatic because it is usually monocular and the other eye sees normally.
Although vision screening is widely recognized as important, it is not widely performed. Fewer than half of all 3 year olds are screened in pediatric practices with current methods.46 Structural defects in the eye (eg, cataract, glaucoma, and intraocular tumors, refractive errors, and smallangle strabismus) can be more effectively treated if discovered early by careful vision screening.
HISTORY AND CONTROVERSY
Several factors have limited the application of preschool screening. First, it is difficult to measure the visual acuity of a 4 year old, not to mention a 2 or 3 year old. The child may not be able to name letters or numbers, and visual acuity tests that require matching or naming pictures or symbols may be equally difficult. Cooperation is a limiting factor with young children, especially during prolonged testing. Second, many children need screening. There are approximately 4 million preschoolers in the United States. Although semiskilled lay volunteers can be helpful, they may find evaluating the vision of young children challenging. Third, there is no universally accepted specific screening test or program.
Figure 1. A single Snellen letter with "crowding bars."
Pediatricians might have difficulty deciding what tests to perform. Vision screening has become a politically charged issue, with different professional and lay groups advancing their own points of view. Within pediatric ophthalmology, some physicians insist on presenting letters in full lines or with "crowding bars" (Fig. 1) to help detect subtle amblyopia. Others are content with single pictures. These may be more comprehensible for young children but may underdiagnose amblyopia. Optometrists tend to advocate more extensive assessments, including tests of "tracking" and visual perception, whereas ophthalmologists tend to emphasize testing visual acuity and alignment.
Among tests of visual acuity used for verbal children, the Snellen letters (Fig. 2) are the most common. The HOTV system (Fig. 3) presents these four letters, which are generally identified by the individual on a card held on his or her lap. Children who are not familiar with letters may be tested with a variety of symbols, including the Landolt "C" and the illiterate "E" (Fig. 4), which are presented in various orientations to be reproduced by children using their hands. Because children often have difficulty understanding these tests, many examiners prefer to present pictures. Two commonly used picture tests are the Lea symbols and the Allen pictures (Fig. 5).
Pictures, especially if presented singly, are easier to recognize than letters. The underdetection of amblyopia is therefore a real risk when pictures are used. On the other hand, it may be impossible for young children to participate in the more sophisticated and sensitive Snellen visual acuity test. Whatever test is used, the symbols should be presented at a distance; 20 feet (6 m) is the standard, although shorter distances can be used if necessary. Results of visual acuity testing at "near" (te, reading) distance are inaccurate for children because of variation in test distances.
The American Academy of Pediatrics standard for passing vision screening is 20/40 for children 3 to 4 years old and 20/30 for older children.7 A difference of 2 lines between the two eyes is significant for all age groups. A finding of 20/40 in one eye when the other eye is 20/20 may represent significant visual loss. At any age, it is critical that a child be unable to "peek" using the other eye. An adhesive occluder or a strip of adhesive tape is preferred over a hand or a card (Fig. 6).
Stereopsis is the use of the two eyes for depth perception. Because normal stereopsis usually signifies good vision in each eye and normal alignment of the eyes, it has been used in some preschool screening programs. Indeed, the presence of normal stereopsis alone constitutes sufficient screening, so measurement of monocular visual acuity may not be necessary.
The technique of photoscreening for preverbal children has generated considerable debate. Photoscreeners are cameras that enhance the light reflected from the pupil. The technique can detect significant refractive errors (in one eye or both eyes), strabismus, and media opacities (especially cataracts). The validity of photoscreening has been challenged on the grounds that it detects amblyogenic conditions rather than the amblyopia itself. Moreover, the interpretation of results is somewhat variable, even among eye care professionals.2 Consequently, photoscreening is not currently recommended in any screening guidelines nor is it endorsed by any professional association in the United States.8 Further research may improve the accuracy of this technique. If it does, photoscreening may represent a major advance for pediatric eye care.
Different techniques are appropriate for screening children of different ages in the pediatric office.
The newborn examination may constitute the most important vision screening of a child's life. The external examination, using a penlight, can reveal eyelid and surface abnormalities that require urgent intervention. An eyelid that does not open adequately, as in severe ptosis or hemangioma, can induce severe amblyopia from deprivation or induced refractive error. An eyelid that does not close adequately, as in upper lid coloboma or severe proptosis, can quickly cause vision-threatening exposure, infection, and loss of clarity of the cornea.
Examination with the direct ophthalmoscope will reveal media opacities in the pupillary red reflex. Vision-threatening cataracts and lifethreatening retinoblastoma can be detected in this way. Some have advocated routine pupillary dilation by pediatricians, but this recommendation is controversial because of the potential side effects of long-term mydriatics. The same instrument can be used to assess the pupillary response to light. Although strabismus is not generally treated at this age, the pupillary light reflex can be used to detect obvious misalignments.
Children 6 Months to 3 Years Old
By this age, most children have demonstrated interest in their visual environments, especially in faces and brightly colored toys. Such targets can be used to elicit "fixation and following" movements in the office. Fixation with one eye in preference over the other is presumptive evidence of amblyopia in the nonpreferred eye, especially if strabismus is suspected. Strabismus itself can often be detected better by using the pupillary light reflex from a handheld penlight than by gross observation. At this age, pseudoesotropia is especially common, and symmetrical reflexes can help to distinguish this condition from true infantile (or congenital) esotropia (Fig. 7). On the other hand, the "test of time" is even more valuable in this regard, and repeated or persistent complaints suggesting strabismus warrant referral for ophthalmologic assessment. The red reflex test should be repeated.
Children 3 Years and Older
As mentioned previously, visual acuity should be tested monocularly as early as possible, principally because amblyopia can be treated more easily in younger children. Any of the tests described can be used. The important point is that they should be administered to all children who can be tested.
Ocular alignment can be assessed much more reliably by this age because fixation is more reliable. A cover introduced before the right eye may induce a corrective "refixation" movement of the left eye if the left eye is misaligned. Naturally, the test must be repeated on the right eye, in case it is misaligned. Results of the "cover test" are likely to be falsely positive if the child looks away from the target, so an interesting toy should be used for testing. Results of the test are likely to be falsely negative if the child does not see well enough with the strabismic eye to refixate on the toy.
Figure 6. Adhesive tape is convenient and reliably occludes one eye when the other is being tested.
Children Who Are at Risk for Eye Problems
Figure 7. Left esotropia. (A) Note the displacement of the corneal light reflex in the left eye. (B) The left eye fixes and the corneal reflex is central once the right eye is covered.
Some have advocated that pediatricians also test stereopsis at this age. As indicated earlier, stereopsis is likely to be poor if the eyes are either misaligned or amblyopic.
Children who are at risk for eye problems are listed in the table.
CONDUCTING VISION SCREENING
The following tips are important for the pediatrician who is conducting vision screening in the office:
1. Establish rapport with the child and with the parent or caregiver.
2. Position the child appropriately. Allow the child to sit on the parent's lap if possible.
3. Position the eye chart at the child's eye level and provide extra response time.
4. Use interesting fixation devices to catch the child's attention.
5. Have parents be active participants in the screening process.
6. Use positive reinforcement tactics such as applause and bribery to enhance the child's cooperation.
Vision screening can reveal a variety of vision-threatening abnormalities that can be effectively treated at an early age. This fact has been recognized by inclusion of vision screening in the HEDIS (Health Plan Employer Data Information Sent) standard and by vision screening's being reimbursed by most insurance carriers. It is a key component of routine pediatric care.
1. Wilson JMG, Jungner G. Prindples and Practice of Screening for Disease. Geneva, Switzerland: World Health Organization; 1968. Public Health Papers 34.
2. Donahue SP, Johnson TM, Leonard-Martin TC. Screening for amblyogenic factors using a volunteer lay network and the MTI photoscreener: initial results from 15,000 preschool children in a statewide effort. Ophthalmology. 2000;107:1637-1644.
3. Freedman HL, Preston KL. Polaroid photoscreening for amblyogenic factors: an improved methodology. Ophthalmology. 1992;99:1785-1795.
4. Hartmann EE, Dobson V, Hainline L, et al. Preschool vision screening: summary of a Task Force report on behalf of die Maternal & Child Health Bureau and the National Eye Institute Task Force on Vision Screening in the Preschool Child. Pediatrics. 2000;106:1105-1116.
5. Wasserman RC, Croft CA, Brotherton SE. Preschool vision screening in pediatric practice: a study from the Pediatric Research in Office Settings (PROS) Network: American Academy of Pediatrics. Pediatrics. 1992;89:834-838.
6. Marcinak JF, Yount SC. Evaluation of vision screening practices of Illinois pediatricians. Clin Pediatr (Philo). 1995;34:353-357.
7. American Academy of Pediatrics Committee on Practice and Ambulatory Medicine, Section of Ophthalmology. Eye examination and vision screening in infants, children, and young adults. Pediatrics. 1996;98:153-157.
8. Ciner EB, Dobson V, Schmidt PP, et al. A survey of vision screening policy of preschool children in the United States. Suro Ophthalmol. 1999;43:445-457.
Children Who Are at Risk for Eye Problems