I always learn something doing mese editorials, but few have had an impact like this one. For 35 years, I mistakenly thought the primary purpose of the preschool visual acuity screen was to find refraction errors.
The American Academy of Pediatrics (AAP) now recommends that all neonates have an eye examination, and that infants be evaluated by age 6 months for fixation preference and ocular alignment.1 This examination should be repeated until visual acuity testing can be done at 3 or 4 years of age. Formal vision screening should begin at 3 years. When a child is untestable, the screen should be repeated in 4 to 6 months.
How well do we follow these recommendations? Several studies find that our preschool eye screens are deficient. For example, the Pediatric Research in Office Settings Network sampled 8,417 children 3 to 5 years old in 23 states, cared for in 102 pediatric offices.2 All were volunteers. Vision screens had been attempted in only 66% of these preschoolers and 38% of those 3 years old. The most common reasons were "too young" and "not routine." Moreover, half of the parents of the 8% who were tested but failed thought their child had passed. And, when the plan was to repeat a failed test, this was usually scheduled in 1 year. Retesting is appropriate, but a year is too long.
WHY IS THE PRESCHOOL EXAMINATION IMPORTANT?
Preschool ophthalmic alignment and acuity screens are primarily about detecting amblyopia or "lazy eye." This is because, if not treated by 5 or 6 years of age, amblyopia can become permanent visual impairment. Amblyopia occurs in approximately 3% of all children3 and is said to be the most common cause of monocular vision loss in adults.4
Amblyopia is defined as visual impairment that cannot be fully explained by an abnormality of the eye, and cannot be corrected by refraction. It is caused by visual deprivation. The pathogenesis begins with something that interferes with bilateral vision. This is usually strabismus, whereby right and left images are not centrally aligned. Less often, a difference in refraction between eyes (known as anisometropia) produces this. But, rarely, cataracts or even excessive patching of one eye can produce this deprivation. Whatever the cause, the brain suppresses the image that is less clear. The visual cortex needs stimulation to develop, so central vision will decrease on the suppressed side.
If treated early, amblyopia is reversible. But after 5 or 6 years, treatment becomes progressively less likely to restore full vision. So testing preschool visual acuity is primarily about preventing the permanent sequelae of amblyopia, through detecting either its main causes (strabismus or anisometropia) or the decreased vision caused by amblyopia itself. I knew about the importance of looking for amblyopia by symmetry of reflections on the cornea and the coveruncover tests, but this concept about why vision should be tested had not gelled until now.
Strabismus affects approximately 4% of preschool children. It may be congenital or acquired, and, if the latter, often appears between 18 months and 4 or 5 years of age (thus the need to keep looking for it). Strabismus is the most common cause of amblyopia, and one-third to half of children with this will have some degree of impairment. Strabismus can be severe and obvious, or mild and subtle. All degrees can lead to amblyopia if strabismus is continuous.
An infant can usually align his or her visual axis and have synchronized eye movements by 6 months old. At or before this age, two types of screens are used, the corneal light reflex (for corneal symmetry of a reflected light) and the cover-uncover or alternating cover test. The latter looks for eye deviation when view is obstructed for the dominant eye. More details are available in the AAP recommendations1 and a clear description of how these tests work can be found in the family medicine literature.5 A consistent deviation at any age, even birth to 6 months, is reason for referral.
PRESCHOOL VISION SCREENS ARE MOSTLY ABOUT FINDING A DIFFERENCE BETWEEN EYES
Preschool vision acuity should be tested at 10 feet using 10-foot charts. There are four main types of tests: picture charts or cards (eg, the Allen cards, which incidentally are the exception to the 10-foot recommendation); the tumbling E or HOTV (which asks the child to match the way Es point or identify only H, O, T, and V letters); vision testing machines; or the Snellen letter or number charts. These are listed in order of increasing precision, and increasing maturity required for a successful test. Approximately 70% of 3 year olds are testable by the E or picture methods.2,6 The most precise test that a child can master should be used. Vision is estimated to be 20/400 at birth, primarily because the posterior eye is smaller than the anterior segments, making the distance to the retina too short for clear focus. Triis improves as the posterior eye grows, and by 3 to 5 years old vision of 10/20 is the screening threshold; 20/30 is used after this age.
The eyes must be tested independently, and an adhesive eye patch is recommended to be sure there is no peeking. The child must recognize most (ie, 4 of 6) figures in a line to pass that line. Failure of the screen is 10/20 or worse vision, or a difference of two or more lines between the eyes, even if both eyes are within the 10/20 limits. So, 10/12.5 on one side and 10/20 on the other is a failure. Such a difference can be due to amblyopia of any cause or to anisometropia. However, this does not matter in the sense that a referral to an ophthalmologist is indicated either way.
WHY IS AMBLYOPIA MISSED IN HALF OF PRESCHOOLERS?
Several studies indicate that almost half of children with amblyopia are not diagnosed until after they begin regular school, when a full return of vision is unlikely.3 Campbell and Charney provided insight into why this happens by comparing 75 children who were diagnosed as having amblyopia after they were 5 years old (the late group) with 86 recognized before this age (early group).3 All were under the care of a pediatric ophthalmologist in Baltimore between 1983 and 1987, and 90% used a pediatrician for primary care. The child-related factors that distinguished the early group from the late group were that strabismus tended to be more common than anisometropia, and strabismus deviations greater than 10° and family history of strabismus were more likely. Pediatricians were usually not first to identify a vision problem, despite the fact that we are the only group of professionals who routinely see preschool children. Only 12% of the late and 33% of the early group were first recognized by their primary care physician. The other discoverers within the early group were a school tester for 27%, an ophthalmologist for 36%, and an optometrist for 5%. It was also interesting that the proportion of primary care physicians who indicated they did preschool vision screens by AAP guidelines was not different for the early and late groups: overall, 73% said they followed these guidelines. But, 63% of parents said their children had never had vision screening.
Parental factors that were most typical of the early diagnosis compared with the late diagnosis group were suspicion of an eye problem and a request for an eye examination. These may be explained partly by the greater family history of strabismus in the early group, and may also explain why so many parents went directly to an ophthalmologist or optometrist. The authors conclude that their data ". . . suggest that it is parents, rather than primary care physicians, who more often suspect the vision problem and then refer their children with amblyopia."3
So, there is evidence that we need better systems to screen all preschoolers for amblyopia, we seem to miss strabismus and amblyopia too often when we do screen, and we do not communicate or follow up on screening failures well. And, it does not help to fail to realize what this screening is about.
1. American Academy of Pediatrics Committee on Practice and Ambulatory Medicine, Section on Ophthalmology. Eye examination and vision screening in infants, children, and young adults. Pediatrics. 1996;98:153157.
2. Wasserman RC Croft CA, Brotherton SE. Preschool vision screening in pediatric practice: a study from the Pediatric Research in Office Settings (PROS) Network. Pediatrics. 1992;89: 834-839.
3. Campbell LR, Charney E. Factors associated with delay in diagnosis of childhood amblyopia. Pediatrics. 1991;87:178-185.
4. Simmons K. Preschool vision screening: rationale, methodology and outcome. Surv Ophthalmol. 1996;41:3-30.
5. Broderick P. Pediatric vision screening for the family physician. Am Pam Physician. 1998;58:691-700.
6. Sturner RA, Green JA, Funk SG, Jones CK, Chandler AC. A developmental approach to preschool vision screening. J Pediatr Ophthalmol Strabismus. 1981;18:61-67.