Many primary care practitioners, early educators, school nurses, and public health and volunteer lay screening organizations have adopted the use of autorefractors or photoscreeners in lieu of screening by testing visual acuity. These instruments are designed to identify preschool children with potentially amblyogenic refractive error. Historically, the primary outcome metric for childhood vision screening programs has been the measurement of visual acuity using optotypes (letters or shapes/pictures). As a result, the mandated screening forms used in many screening settings are designed solely to document visual acuity. This has produced some confusion because the outcome metric for preschool vision screening using autorefractors and photoscreeners is typically a “pass-refer” result based on the estimate of refractive error that is provided by the instrument.
The purpose of this statement is to clarify the difference between visual acuity and refractive error and to explain why attempts to predict a child's visual acuity from an instrument-derived estimate of that child's refractive error are not appropriate.
Visual acuity is the sharpness of vision as determined by the ability of an eye to resolve detail at a given distance. It is typically reported separately for far viewing (e.g., 10 or 20 ft, 3 or 6 m) and for near viewing (e.g., 16 in or 40 cm) and often depends on a response (verbal or matching) from the person being tested; it can be tested monocularly (with only one eye open) or binocularly (with both eyes open). Refractive error is present when the shape of the eye prevents light from focusing directly on the retina. The length of the eyeball (too long or too short) or the shape of the cornea is the primary cause of most refractive errors. The main types of refractive error are hyperopia (farsightedness), myopia (nearsightedness), and astigmatism (irregular shape of cornea or lens). Although myopia is widely recognized as causing reduced visual acuity at a far viewing distance, the relationship between other refractive errors and visual acuity is more complicated and depends on numerous factors including magnitude of the refractive error, focusing ability, and age. It is important that vision screening personnel who have access to the estimate of refractive error provided by instrument-based vision screening technology understand that there is not a simple relationship between refractive error and visual acuity. An eye can have significant refractive error yet still have good visual acuity. Conversely, an eye can have little or no refractive error but still have poor visual acuity (for example, if there is amblyopia or a structural abnormality or disease of the cornea, lens, retina, optic nerve, or brain). Visual acuity cannot be correlated with refractive error except in the context of isolated myopia, and even in that instance, the correlation is not always consistent between eyes or individuals.
Automated vision screening instruments provide only an approximation of refractive error and cannot be used to measure or estimate either distance or near visual acuity. Attempting to translate a refractive error value provided by these instruments into a visual acuity measure is not appropriate. Future efforts should instead be given to encourage the pediatric and public health communities to amend their reporting forms and electronic data capture by adding a field for a “pass-refer” outcome when autorefractor or photoscreening instruments are used in place of direct visual acuity testing for preschool vision screening.
Endorsed by the American Academy of Optometry, American Academy of Ophthalmology, American Academy of Pediatrics, American Association for Pediatric Ophthalmology and Strabismus, and National Center for Children's Vision and Eye Health at Prevent Blindness.
Susan A. Cotter, OD, FAAO
Southern California College of Optometry
Marshall B Ketchum University
Sean P. Donahue, MD, PhD
Ophthalmology and Visual Sciences
Vanderbilt University Medical Center
Bruce Moore, OD
New England College Optometry