Preschool-aged children typically perform worse than 20/20 on visual acuity testing in the absence of ocular or visual pathway abnormalities due to non-ocular issues, including concentration, cooperation, and/or confidence. As a result, vision screening for this age group can be a challenge. The “Rule of 8” is an easy-to-remember, highly effective mnemonic that clinicians can use to help differentiate children who are performing visual acuity testing at age-appropriate levels from those who may need further evaluation. [J Pediatric Ophthalmol Strabismus. 2017;54:e75–e76.]
The recording of visual acuity, or clarity of central vision, is among the most critical aspects of the eye evaluation. In children especially, identifying subnormal visual acuity is of utmost importance so that treatment can be initiated promptly. However, determination of visual acuity in the preschool age group can be challenging. Even children with normal ocular and visual pathway anatomy typically perform worse than 20/20 due to non-ocular issues such as concentration, cooperation, and/or confidence. As a result, it is a challenge to know what “normal vision” is in this at-risk population.
A previously published, large multi-ethnic study on visual acuity in 1,722 children aged 30 to 72 months demonstrated that, on average, a visual acuity of 20/20 is not achieved until approximately 6 years of age.1 The authors of this study recommended using specific age-stratified thresholds for visual acuity screening in preschool-aged children: 20/63 or better for 30- to 35-month-old children, 20/50 or better for 36- to 47-month-old children, either 20/40 or 20/32 or better for 48- to 59-month-old children, and 20/32 or better for 60- to 72-month-old children.
These visual acuity thresholds suggested by Pan et al. closely correlate with 2016 American Academy of Pediatrics (AAP) guidelines on “critical line” screening.2 These guidelines, derived from multiple sources including the AAP, the American Association for Pediatric Ophthalmology and Strabismus, the American Academy of Ophthalmology, and the American Association of Certified Orthoptists, also suggest using age-dependent “critical lines” for visual acuity when screening preschool-aged children. Extrapolating from these AAP guidelines and the data from the study by Pan et al., a tool to assist in interpreting the results of pediatric vision screening was devised. This is the so-called “Rule of 8” (Table 1). The premise is simple and is both verbally and graphically described as follows. For children 2 through 6 years of age, the child's age plus the first number of the denominator of the average visual acuity should equal 8. Using the Rule of 8 for a 3-year-old child, a vision screener would subtract the child's age in years (3) from the number 8, in this case yielding 5. Thus, the expected visual acuity for a 3-year-old child is 20/50. If the visual acuity is worse than 20/50, there should be a high degree of concern and this child should be referred for further evaluation.
Rule of 8
However, one important caveat to this rule is to refer any child with significant visual acuity asymmetry. Imagine in our 3-year-old patient above that the visual acuity is 20/20 in the right eye and 20/50 in the left eye. Based on the Rule of 8, the child's visual acuity would be in the acceptable range in each eye. However, there is significant asymmetry in the visual acuities of the two eyes, and thus this child should be referred for further evaluation.
The Rule of 8 is an easy-to-remember but highly effective tool for those who perform vision screening in children. It is a quick way to determine which children are performing visual acuity testing at age-appropriate levels and, most importantly, helps identify those patients who are in need of further evaluation, which is the ultimate goal of vision screening.
- Pan Y, Tarczy-Hornoch K, Cotter SA, et al. Visual acuity norms in pre-school children: the Multi-Ethnic Pediatric Eye Disease Study. Optom Vis Sci. 2009;86:607. doi:10.1097/OPX.0b013e3181a76e55 [CrossRef]
- Donahue SP, Baker CN, Committee on Practice and Ambulatory Medicine, American Academy of Pediatrics et al. Procedures for the evaluation of the visual system by pediatricians. Pediatrics. 2016;137:1–9. doi:10.1542/peds.2015-3597 [CrossRef]
Rule of 8
|Age in Years||Rule of 8||Expected Visual Acuity|
|2||8 − 2 = 6||20/60|
|3||8 − 3 = 5||20/50|
|4||8 − 4 = 4||20/40|
|5||8− 5 = 3||20/30|
|6||8 − 6 = 2||20/20|