When people ask what Zoe’s vision is, they’re normally expecting to hear her visual acuity – usually given as 20/20 or the metric equivalent 6/6 – and then they want to know how she compares with other children. I don’t actually know what her uncorrected acuity is, but I was curious about what you might expect a preschooler’s acuity to be.
Visual acuity is the measurement of how clearly we see at a specific distance, usually 20 feet or 6 meters, though that can vary. It is generally tested in adults with the well-known Snellen eye chart (the one with the big E on top) and with a variety of charts for children. The acuity is usually presented as two numbers that indicate how close a person will need to be to an eye chart to see the letters or symbols clearly, compared to what a person with “normal” vision would see. Let’s say my uncorrected visual acuity is 20/200 – that’s 6/60 in metric – (which it is, more or less, in my right eye). That means that I have to stand 20 feet – or 6 meters – away from the eye chart to read the big E. A person with good vision, on the other hand, could see the E at the top of an eye chart from 200 feet – or 60 meters – away.
It’s worth keeping in mind that visual acuity is not the whole picture (so to speak) when it comes to measuring vision. Visual acuity does not indicate how well the eyes work together, or peripheral vision, or even what the prescription needs to be to correct vision.
For adults, it’s pretty easy to look at the acuity numbers to see how good your vision is: if the first and second numbers are the same, 20/20 or 6/6, that means normal visual acuity. If the second number is smaller than the first, you have better than normal acuity, and if it’s larger than the first your vision is poorer. But for young children, it’s not quite as straight forward. Because children’s vision hasn’t completely developed, a child can have “normal” vision that is worse than 20/20.
In 2003, the American Academy of Pediatrics adopted a policy on Eye Examination in Infants, Children, and Young Adults by Pediatricians. According to those standards, pediatricians should refer any child under the age of 5 with a visual acuity of worse than 20/40 – that’s 6/12 in metric – to an eye doctor. At age 6 and older, they should refer any child with acuity of 20/30 – 6/9 metric – or worse.
More recently, though, a study called the Multiethnic Pediatric Eye Disease Study looked at visual acuity norms in preschool children with the goal of gaining more accurate visual acuity norms for children. The results were published in Optometry and Vision Science in June 2009 (abstract and full citation). They measured the visual acuity of 1,722 children ages 30 to 72 months with no significant refractive errors – so kids that should have good vision. From those measurements they determined the threshholds for visual acuity that would include 95% of the children tested. That means that if the test were done by pediatricians and these guidelines were followed, you’d expect that 5% of children with no refractive error would be referred on to an eye doctor for a follow up. You’d hope that all of the kids with visual problems would also be caught with the testing and also be referred on to an eye doctor. That’s not necessarily the case, the researchers note that children who have “normal” visual acuity may still have visual problems, but that’s a topic for a different post. The nice thing about this study is that it breaks out the age range in to more detail, understanding that vision is still developing significantly in children between the ages of 2 1/2 and 6.
So, according to this most recent study, normal visual acuity for preschoolers may be better defined as:
30 – 35 months: 20/63 (6/20) or better
36 – 47 months: 20/50 (6/15) or better
48 – 59 months: 20/40 (6/12) or better
60 – 72 months: 20/32 (6/10) or better