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Normal Visual Acuity in Children What is Normal? By L. E. Leguire Ph.D., MBA Program Director Ohio Amblyope Registry
We often hear that 20/20 is normal visual acuity but what is normal visual acuity for a child? As it turns-out, children don’t see as well as adults and normal visual acuity is thus different depending on the age of the child. But first, we need to review what those visual acuity numbers mean. Without getting into the details of measuring letter size per degree of visual angle (Huh? For a definition of visual angle see below note), an easy approach to understanding so-called Snellen fractions (i.e., 20/20) is to think of the fraction as the smallest letters that can be correctly identified by a normal person compared to a patient. Think of the bottom number of the fraction as the smallest letters that a normal person can see at a certain distance (e.g., 20 feet) and the top number as the distance that the patient sees the same thing. For example, 20/20 means that the normal person can identify the smallest letter of a certain size at 20 feet away the same as a patient, thus the top and bottom numbers of the fraction are the same or 20/20. Legal blindness is defined as 20/200; that is, the smallest letters that a normal person sees 200 feet away the patient has to have much closer to see at 20 feet away in order to identify the same letters. In order to drive a car in the state of Ohio, a person has to have at least 20/70 visual acuity in the better eye. Thus, what a normal person sees 70 feet away the patient has to have just 20 feet away in order to identify the same letters. As part of the Multi-Ethnic Pediatric Eye Disease study, Pan and colleagues (Optometry and Visual Science, 2009) determined normal visual acuity for children of various ages. Their findings are outlined in the table below.
As the above table illustrates, children up to 6 years of age (72 months) don’t have 20/20 vision. Visual acuity gradually improves with age from 30 to 72 months and obviously beyond. So, a 4 year old (48 months) may have 20/32 visual acuity which is actually normal for his or her age. So in the analogy used above, what a normal adult sees at 32 feet away, for example, the 6 year old child must have at 20 feet away in order to see the same thing as the adult. Knowing what is normal visual acuity for a child is very important for eye doctors, vision screeners as well as parents. When an eye exam is performed or a vision screening at a school is undertaken, it is important to know when the child has a vision problem; that is, when visual acuity is below normal for his or her age. In the table above, normal is defined as that visual acuity where 95% of normal children would pass the visual acuity test. Obviously, 20/20 visual acuity is achieved after 6 years of age. Some caution is advised in accepting these normal visual acuity values since they may vary with different types of visual acuity charts as well as vary with the procedures used to obtain the visual acuity. So in measuring visual acuity one has to compare apples to apples so to speak; otherwise, a comparison between different eye charts or procedures may not be valid or proper. It is important for visual acuity to be normal in each eye so that the eyes can work well together as, for example, with depth perception. Not only is it important for both eyes to be in the normal range (i.e., equal to or better than that listed in the above table) but that both eyes should have similar visual acuities at least within one line or so. When visual acuity differs between the eyes by about 2 lines or more is when the child should be evaluated for amblyopia. Note on visual angle and letter size in eye charts: In geometry, a circle encompasses 360 degrees. One degree is equal to 60 minutes and one minute is equal to 60 seconds. Think of yourself as being in the center of the circle. If you’re looking straight ahead, your visual field along the horizontal is about 180 degrees of visual angle – you can see all the way to the sides with both eyes open and looking straight ahead. 1/360th of a circle’s circumference is 1 degree of visual angle. Another way of understanding 1 degree of visual angle is that the size is about equal to a quarter held at arms distance. If you placed quarters side by side and they formed a circle around you at arms length, you would be able to see about 180 quarters when both eyes are open. When the size of a letter, in terms of visual angle, is calculated, the overall letter size, the stroke width of the letter and the gap between parts of the letter are critical factors. For example, the letter “C” would have a certain overall size, have a certain stroke width and have a certain spacing of the gap between the top part and bottom part of the “C”. These are the factors which determine letter size on an eye chart. In 1862, an Ophthalmologist by the name of Hermann Snellen, developed the first “optotype” letter chart, a visual acuity chart made up of specially designed letters, thus the name of commonly used eye charts today. The standard viewing distance was 20 feet (optical infinity, basically the distance from the person at which the lens of the eye no longer focuses) and at 20 feet a letter corresponding to 20/20 visual acuity would be 5 minutes (1/12 of a degree) of visual angle in total size and have a gap width and stroke width of 1 minute of visual angle.
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