Pediatric Visual Acuity Testing
Mark E. Wilkinson, OD, FAAO
Posted May 9, 2021
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This is Mark Wilkinson from the University of Iowa Department of Ophthalmology and Visual Sciences. In this presentation we will discuss how to assess the visual acuity of children of all ages and functional abilities.
We will be discussing the different types of visual acuity charts to use for different ages of children. The different types of visual acuity assessments that are available for this age group. And how to test for both distance and near acuity.
There are many different charts that can be used for children.
For younger children who are visually impaired, one of the early questions to determine is whether the child is more visual oriented or more tactual oriented. Both parents and teachers want to know, what is the child's visual acuity? They want to better understand the child's visual abilities beyond their fixation pattern, which is often all that is access by a pediatric ophthalmologist. Pediatric ophthalmologist often only looks at whether the child can fix and follow along with whether their fixation is Central, Steady and Maintained. This is simply not enough information to know if the child needs early intervention services for a visual impairment.
It is important to remember that Visual Acuity Testing measures the spatial resolving capacity of the visual system. Said in another way, visual acuity testing measures the ability of the eye to see fine detail.
There are several types of Visual Acuity Testing we will review today. The different types of acuity assessment include localization acuity, detection acuity, resolution acuity and recognition acuity.
Localization Acuity is the visual awareness or perception of the location of an object or light. Examples of localization acuity include Light perception with projection and awareness of the location of people, animals or other items. Another option is the use of light boxes. With localization acuity, we are simply determining if the child has a higher level of visual acuity than light perception.
Here is an example of the use of light boxes to determine if the child can see more than just light. With the light boxes, you have a pattern on one light box and no pattern on the other one. If the child can see the pattern, they will look at the pattern, versus just the light, because the pattern is more interesting. This test is used when none of the following tests give any results.
Next is Detection Acuity, also known as Minimum Perceptible Acuity
Detection acuity measures the simple detection or awareness of objects, not their identification or naming. Examples of detection acuity include the Sheridan graded ball test, Bock candy bead test, Retrieving balls of different sizes and Object awareness.
You can use common objects to get a rough estimate of a child's visual acuity. We know that a 3 mm object is approximately equivalent to a 4/2 visual acuity size letter which is equivalent to a 20/10 acuity. A 30 mm object is equivalent to a 20/100 visual acuity and a 60 mm object is equivalent to a 20/200 visual acuity when measured at a 4-meter test distance.
An example of how this would work is as follows. A child consistently finds a 30 mm ball from a distance of 1 meter. The child's detection acuity is 1, because of the test distance of 1 meter, over 20, because as noted above, a 30 mm object is equivalent to a size 20 target at 4 meters. With 1/20, multiple both the numerator and the denominator by 20 to get a Snellen equivalent of 20/400.
Next, we have Resolution Acuity, also known as Minimum Separable Acuity.
Resolution acuity measures the threshold at which an individual can discriminate the separation between critical element of a stimulus pattern.
Examples of resolution acuity include the Landolt C, Tumbling E or Broken Wheel test as well as grating acuity or preferential looking charts by Teller or Lea. Opto-Kinetic nystagmus is another example of a resolution acuity test.
Resolution acuity is a simpler visual task than recognition acuity. That said, resolution acuity does require the child to attend to the task. Some feel resolution acuity overestimates the child's visual abilities. However, it does allow for quantification of visual acuity which allows the clinician to monitor the child's visual acuity over time, which gives everyone more information that what is known by simply assessing the child's ability to fix and follow.
A comment about Opto-Kinetic Nystagmus. This is a very gross assessment of visual acuity, which is difficult to quantify. This is because to achieve 20/20 acuity with a standard opto-kinetic drum, the test distance would need to be about 9 meters from the child.
Here are examples of Grating Acuity or Preferential Looking acuity charts. You see on the left graph that visual acuity measurements improve as the child ages. This is because no one starts out with 20/20 vision at birth. Vision is a learned sense, which is why we expect visual acuity to improve with age for all children with a non-progressive visual condition.
Also, on this chart you see a picture of a Teller acuity card in the upper center and the Lea paddles in the lower center and on the right side. Advantages of the Lea paddles is their ease of use and they are less expensive than Teller cards.
These are Teller Acuity cards. If the child can see the pattern, they will fixation on it, versus the other side of the card which is less visually interesting. This is the same strategy that is used with the Lea paddles.
Dr. Scheiman determined visual acuity ranges at different ages using Teller Cards. On this chart you see that the visual acuity of a normally sighted child ranges from 20/400 to 20/1200 at 1 month of age and improves via the visual maturation process to the 20/20 to 20/50 level at 30 months of age. As we know, most child with normal vision will reach visual maturation around 5-6 years of age, while a child with a stable visual impairment at birth will not reach visual maturation on average until around the end of the first decade of life.
With Preferential Looking Acuity there are a number of potential sources of error. Those include poor fixation, strabismus, poor attention, a lack of interest in the test, a preference for looking only to one side as well as improper placement of the child during testing.
The highest level of visual acuity testing is recognition acuity, also known as Minimum Legible Acuity. Recognition acuity is most often used clinically. I have found children able to do recognition acuity tests as young as 3 years of age. However, because recognition acuity requires the child to understand the task and give a subjective response, often the child will need to be older to accurately respond to do recognition acuity testing.
Examples of recognition acuity testing options include Lea symbols as well as Letter and Number charts.
When measuring visual acuity, we want to test not only distance acuity, but also near acuity. At near we can use single words or letters as well as standardized continuous text. For all acuity testing, we want to document the working distance the testing was done at.
Let's talk about which visual acuity testing chart you should be using for different age groups. You will find additional information about acuity testing in the adult visual acuity testing presentation.
Line pictures have been found hard to standardize for recognizability. Additionally, children may vary in their picture naming ability. I don't imagine that many children would be able to name an old-style telephone. For this reason, this type of chart should only be used as a last resort.
Similarly, the Tumbling E chart requires a sense of laterality that may not be developed in young children or children with developmental delays.
LEA symbols have been found to give the most reliable results of all recently developed symbol charts, especially for young children. One of the nice things about the Lea symbols is that when they get too small to see, they all look like circles, so the child never feels like they are failing the test.
The typical test distance will be closer than for an adult. I regularly use a 1-2 meters test distance until the child is school aged.
I find that near vision is functionally more important than distance vision for child, particularly younger children. Near vision gives a better idea about the child's functional visual abilities versus distance acuity testing which tells us the quantity of vision they have.
When testing near acuity, have the child should hold the reading card at their normal reading distance. They will do this with their reading Rx on if they have one.
Near acuity is recorded as distance in meters over the M unit acuity size. You can also record near acuity as M units @ test distance. For example, if a child can read 1M size letters at 40cm, you would record their acuity as 0.40/1M or 1M @ 40cm.
Here are examples of Near Acuity Charts. You have a Lea symbol card as well as numbers and word cards. These cards allow for visual acuity testing at different functional and age levels.
Here is an example of a child's Continuous Text Card
Here is an example of an adult Continuous Text Card.
Reading continuous text is a harder task than reading a single word or letter or identifying a symbol. Continuous text cards give a better assessment of the individual's reading abilities and need for vision enhancement.
When testing a Child's Acuity, you have to consider whether to do Monocular or Binocular testing. For very young children, where I may only get one chance to assess their acuity, I will often start with binocular acuity to establish a starting point for what their visual acuity is.
To do monocular testing, there are a variety of methods of occlusion. You could use a pirate patch, or Mom, dad or the child could cover one eye. I am not a fan of this option in most cases because of the ease by which the child can move to look around their parent's hand or their own hand. You can use a paddle or clip-on occlude if they are wearing glasses. An adhesive patch could be used, but that is often not acceptable to the child, which causes them to be distracted and reduces the likelihood you will get an accurate acuity measurement. You could also use fun glasses with one eye occluded.
Remember that fogging occlusion will result in more accurate monocular acuity findings than you will get with full occlusion when testing individuals with nystagmus. I will typically use a +4.00D lens or a frosted lens over the non-testing eye when nystagmus is present.
When choosing what acuity test to use, selection will be based on age. Are you testing Infants, Toddlers, Preschoolers or School age children? Also, what are the child's functional abilities, Developmental age and do they have any additional disabilities
For Infants: Birth to 18 months, you will use object awareness and lighted objects.
Remember, the human face is the most consistent stimulus a child will respond to early in life. You can also do preferential looking with Teller cards or Lea grating paddles.
For Toddlers: 18 months to 36 months, Preferential looking with Teller cards or Lea grating paddles will allow you to start quantifying the child's visual acuities.
As the child gets older, they will be able to do matching or even naming with the Lea symbols
For Preschooler: 3-5 years, Lea symbols can be used for both distance and near vision testing. The child can be tested using matching or by naming the symbols.
Regular optotypes can be used as well, but I find them less accurate to use for this age group.
A comment about the E chart. It is important to know that the E chart also tests laterality and directionality abilities, which may not be fully developed in this age group. For this reason, using the Lea symbols is a better option for accurate acuity measurement.
Now you know how to accurately test distance and near visual acuity for children of all ages and developmental levels. Remember, inaccurate acuity testing underestimates the child's abilities and prevents the clinician from accurately knowing the progression of disease and benefits of therapy.
Thanks for your attention. If you have any questions, you can contact me by email.