Frequently Asked Questions About Vision Screening
If a child between the ages of 3 and 6 does not pass vision screening, should I refer that child to an optometrist, an ophthalmologist, or a pediatric ophthalmologist?
You should refer the child to an eye care professional that enjoys seeing young children, is set up to see young children, and accepts the parents/caregivers insurance.
What is an ETDRS eye chart?
ETDRS refers to the format of optotypes in inverted pyramid form. ETDRS charts have 7 advantages:
- Standardized and considered gold standard in vision research
- Regular geometric progression of size and spacing of optotypes
- Use logarithmic scale in steps of 0.1 log units for more precise visual acuity measurements
- Equal number of optotypes in each row
- No serifs
- Use Sloan optotypes
ETDRS charts come in different optotypes, such as LEA Symbols, Sloan letters, and HOTV letters.
My eye chart has a 20/30 line and my colleague's eye chart has a 20/32 line. What is the difference?
20/30 is a Snellen visual acuity measurement fraction or notation (Hermann Snellen introduced letter chart in 1862). 20/30 is on charts using a visual acuity scale in Snellen notations. 20/32 is based on a more recent and accurate measurement known as logMAR.
What does 20/100 mean?
The top number represents the testing distance in feet from the child's eyes to the eye chart. The bottom number represents the smallest line that the child can read. The higher the bottom number, the worse the eyesight. For 20/100, 20 = testing distance from child's eyes to the eye chart; 100 = smallest line child could clearly identify. The child could not clearly identify optotypes on lines below the 20/100 line on an eye chart. Think about it this way: the child must stand 20 feet from an eye chart to correctly identify what a person with "normal" vision can see standing 100 feet from the chart.
My eye chart says to screen at 10 feet, but it has 20/XX numbers on the side. What does this mean?
We are accustomed to describing visual acuity in 20 ft notations: 20/20, 20/32, 20/40 etc. Thus:
- 10/10 on a 10 ft chart is described as 20/20
- 10/16 on a 10 ft chart is described as 20/32
- 10/20 on a 10 ft chart is described as 20/40
What is critical line screening?
Critical line screening represents the line on an eye chart that children should pass according to their age. According to the American Academy of Pediatrics, children aged 3, 4, and 5 must pass the majority of symbols on the 20/40 line; children 6 and older must pass the majority of the symbols on the 20/30 or 20/32 line.
What is threshold screening?
The screener begins at the top of the chart, asks child/student to identify the first optotype on each line until the child/student misses the optotype, moves up one line and asks child/student to identify the full line of optotypes beginning with the 2nd optotype on that line, and continues moving up one line until child correctly identifies the majority of optotypes on that line.
What does "crowding phenomenon" mean?
Optotypes (letters, symbols, etc.) are easier to identify when isolated, which is NOT what one wants to occur during vision screening. Crowding - an optotype flanked by an adjacent bar or letters, for example - causes an optotype to be more difficult to identify than the same optotype presented in isolation. This is known as the "crowding phenomenon".
What does "linear spacing" on an eye chart mean?
Linear spacing refers to equal spacing between optotypes on a line, but not between lines.
What does "proportional spacing" on an eye chart mean?
Proportional spacing refers to spaces between optotypes on a line equal to the size of the optotype on the line (1 letter width apart). Additionally, the space between lines is equal to the size of optotypes on the next line down.
Many options are available for vision screening charts. Is one better than another?
No, single, perfect eye chart exists. You want to select charts that are backed by science, or evidence-based. Proportionally spaced charts in ETDRS format with LEA Symbols or HOTV letters should be appropriate for children from age 3 through 2nd grade. For older children, Tumbling E or proportionally spaced charts in ETDRS format with Sloan letters should be appropriate for children older than 3rd grade.
What do different charts test and what are the differences?
Visual acuity charts are recognition charts that test how well students/children can clearly identify optotypes at a certain distance. Some charts inadvertently test cognitive development instead of visual acuity. For example, the Tumbling E requires direction and orientation skills, which are developmental skills that do not emerge at the same time. Up and down makes sense; left and right is abstract. You may want to avoid Tumbling E for children younger than 3rd grade.
What is the best way to cover children's eyes during vision screening?
The child's hand is not the correct answer. When you give young children responsibility for occluding their own eyes, they are likely to peek, especially if one eye has amblyopia or blurred vision. Consider using special glasses occluders.
What is the difference between Snellen and Sloan letters?
Herman Snellen (Dutch ophthalmologist) introduced his block letters with SERIFS in 1862. Though considered a standard screening chart, a Snellen chart has 6 critical design flaws:
- Unequal numbers of optotypes per line - if child/student is required to accurately identify the majority of letters on each line, the definition of "majority" is not the same for each line.
- Irregular progression in letter size from one line to another - gain or loss of one line of vision does not have same value in different parts of chart.
- Unequal spacing between letters and rows.
- Large gaps between visual acuity levels (20/80 - 20/200).
- Some letters, because of serifs, are easier to identify than others.
- No standardization and manufacturers may use different fonts, letters, and spacing ratios.
Louise Sloan, PhD (Wilmer Eye Institute at Johns Hopkins), introduced her 10 letters in 1952: C D H K N O R S V Z. Sloan letters have 5 advantages:
- No serifs.
- Roughly equal to each other in difficulty/ease to identify.
- Based on the Landolt rings, international standard, in terms of recognition difficulty.
- Used in the ETDRS chart, which is the gold standard for clinical studies.
- More accurate than Snellen letters.
Do you have suggestions for screening young students who have developmental delays and/or severely multiply delayed?
Consider 4 versions of LEA Symbols for children who cannot tolerate a full line on an eye chart, and LEA's flicker wand to get a sense of a child's visual fields.
You would begin with the full eye chart, then move to the crowded book if the child cannot tolerate the full eye chart. If the child cannot do the crowded book, you would use the single book. If the child cannot participate with the single book, use the single presentation flash cards. Finally, if the child cannot do the flash cards, you could use the LEA Symbols Cards, 40M and 60M. You would not need to go through each test; you would begin with the test you think would work for the child.
LEA Symbols Crowded Symbol Book
LEA Symbols Single Symbols Book
LEA Symbols Single Presentation Flash Cards
LEA Symbols Cards, 40M and 60M (from Good-Lite site, but can be special ordered through School Health by calling 866-323-5465)
LEA Flicker Wand
Do you have suggestions for screening children that cannot talk or match? Or children who have autism or severely multiply disabled?
Go to Dr. Lea's web site. From the menu on the left side, select English and then Instructions. Scroll down the page to "252700 LEA Symbols Flash Cards". Watch her video.
Why are the recommendations regarding Snellen and Sloan eye charts different? Which should be followed?
Sloan letter charts adhere to eye chart design recommendations from 1 national and 2 international groups:
The World Health Organization (2003), the International Council of Ophthalmology in collaboration with optometrists (1984), and the United States National Academy of Sciences - National Research Council (1980) recommend the following 5 eye chart design guidelines to achieve standardization:
- Optotypes of almost equal legibility. This ensures that optotypes or letters are almost equal in the ability to recognize. Some letters on the Snellen chart are more difficult to recognize than others.
- Same number of optotypes per line (5 is the recommended number). Some lines on the Snellen chart have 2 optotypes, others have 6, 7, 8, or 9. Screening involves the ability to correctly identify the majority of optotypes on a line. Identifying the majority on a line of 2 or 3 optotypes is different, and less difficult, than identifying the majority on lines of 6, 7, 8, or 9 optotypes.
- Horizontal between-letter spacing should be equal to the width of the optotypes on that line. If you were to cut out a letter on the line, you should be able to paste the cut out letter between 2 optotypes. This is not always the case on a Snellen chart because the chart design is not standardized and may vary across manufacturers. This proportional spacing helps to ensure proper crowding. When optotypes are improperly crowded, individuals, especially those with amblyopia, may find it easier to identify the optotypes.
- Vertical between-line spacing should be equal to the height of the optotypes on the next line down the chart. This is not always the case on a Snellen chart.
- Optotype size progresses geometrically up or down the chart by 0.1 log units. This does not occur on a Snellen chart, which means visual acuity may be over- or under-estimated.
Additionally, the term "Snellen" chart is non-standardized. Thus, "Snellen" charts may differ among manufacturers for font, letters, and spacing ratios.
Sloan letter charts meet the 5 international and national guidelines.
If a school nurse screens a child with a non-standardized chart and the child does not pass, but passes on an eye care professional's standardized eye chart, 3 problems may occur:
- The school nurse's over-referral rate increases.
- The eye care professional may question the appropriateness of referrals from the school nurse.
- The child's parent(s) unnecessarily take time away from work to attend an unnecessary eye examination appointment.
If you want to use a standardized chart that meets international and national eye chart design recommendations, you will want to use a Sloan letter chart.
For younger children, LEA Symbols would be the recommended choice. Both LEA Symbols and HOTV letters are evidence-based, but LEA Symbols work better for younger children, is the only pediatric eye chart with optotypes that blur equally to prevent guessing, and are culturally neutral.
I am new at screening 3 year olds in my school. What type of charts and guidelines are there for this age group?
For this age group, you want to use LEA Symbols. Depending on your setting, you may also require other tests. For example, will you also do color vision deficiency screening and stereoacuity to determine how well both eyes work together at the brain level to achieve binocular vision? Or, perhaps you want a vision screening device, such as Plusoptix or Spot. Contact us for a vision screening consultation and we can help you determine the right equipment to meet your needs (link to consultation)
We purchased the LEA vision screening charts to use for our preschool program ages 3 to 5. I understand how to use the charts but at what results for acuity does a referral need to be made?
The current guidelines from the American Academy of Pediatrics, American Association for Pediatric Ophthalmology and Strabismus, American Academy of Ophthalmology, and the American Association of Certified Orthoptists is:
- 20/40 for children ages 3, 4, and 5 (majority of optotypes per line – 3 of 5 must be identified correctly)
- 20/32 for children ages 6 and older.