Visual acuity tests

Visual acuity tests are used more than any other tests in ophthalmology, optometry and in vision screening in preschool, school and occupational health care. Therefore it is important that the tests would have standardized structure and that they would be used correctly. Unfortunately there are numerous visual acuity tests that do not follow the internationally accepted recommendations of ophthalmologic and optometric associations and World Health Organisation (Committee on Vision 1980, WHO 2003). Most users of the tests do not have the basic training in visual psychophysics to know the difference in the quality of different tests and therefore many poor quality tests can remain in the market.

The most widely followed guidelines in the US and many other Western countries were set in 1980 by the Committee on Vision: The optotypes should be equally discriminable and thus each results in the same visual acuity value at threshold. This recommendation was followed in designing the LEA optotypes (in the 1970s called LH Symbols). The repeated testing and minute changes are described in the history of the tests.

The LEA SYMBOLS® have been compared with other optotype sets used in assessment of vision in children, most often with the Early Treatment of Diabetic Retinopathy Study (ETDRS) acuity charts, which in the US have been called ”the golden standard test for visual acuity measurement” because the authors have not been aware of that no letter optotype sets can fulfill the requirement of being equally discriminable. In the EDTRS charts the optotypes of each line have to be chosen so that the average difficulty on each line is closely the same.

THE BASIC RULES IN THE DESIGN OF VISUAL ACUITY TESTS

The international reference test for calibration of other sets of optotypes is the Landolt C test, earlier the Snellen E (Snellen, Herman. 1862. Probebuchstaben zur Bestimmung der Sehschärfe Utrecht: P. W. van de Weijer) . The optotypes (Roman letters, other characters, numbers, paediatric symbols) should be calibrated with Landolt C so that each of the optotypes measures visual acuity like the Landolt C. Roman letters are difficult to design so that all the letters of the set would have the same legibility. In the best set of Roman letter optotypes, the Sloan letters, the legibility varies slightly and therefore on each line there needs to be certain number of difficult and easy letters (Ferris et al 1993, Sloan 1951, 1959). Some computer based visual acuity charts allow random mixture of Sloan letters mixed with other letters like E und T. Such a set of letters has not been calibrated and should not be used.

The paediatric optotypes should be culturally neutral and not require brain functions that are unrelated to the recognition tasks. Answering should be possible without speaking, i.e by matching. Concepts like left, right, up, down (needed in answering in the E-game) develop at varying ages in children and thus prevent answering in young children. E-game is not a recognition task but measures resolving of directions of lines.

Allen pictures are drawn with short lines so the child must be able to imagine the continuous lines that define the forms, which is called visual closure. This function develops in children at different ages and thus affects the visual acuity values. Dr. Allen himself did not consider his optotype to be good optotypes. Also the “Sailboat” chart commonly used by paediatricians in the USA does not meet the rules on visual chart design.

Visual acuity can be measured with several different tests that depict different qualities of vision. The basic test is the line test, which has in the visual acuity levels used in testing at least five optotypes (test symbols) on the test lines with spaces between the optotypes that are equal to the width of the optotypes of that line. The distance between the test lines is equal to the height of the lower line (Visual Acuity Measurement Standard (1984) Consilium Ophthalmologicum Universale, Visual Function Committee).

Even though the international recommendation was published more than twenty years ago and was well known already in the 70's (originally described by Green in 1868) it has not been complied with in the design of visual acuity tests in all countries. The name 'line test' is also used to describe tests that do not follow the recommended design. A near vision test may be called “Reihentest”, a line test, but the space between the symbols varies from circa 80% of the symbols' width on the smallest lines to 5-6% between the largest symbols. This leads to a varying crowding effect from line to line.

In the LEA test series there are line tests, tests with more crowded symbols and single symbol tests to allow assessment of function of the visual system in these three functionally different situations. The number of tests has grown over the years because the needs in screening and assessment of children and adults vary at different ages and different functional levels.

A common practise causing errors in the results is pointing at the optotype and holding the pointing device at the optotype to be read. Therefore the following paragraph is repeated here:

Even when the tests are standardized there are still other factors that affect the results and that are difficult to control. Luminance level is one of the important variables. If regular room illumination is used, luminance on the vertical test surfaces is lower than recommended 85 candelas per square meter or more. In the 70s this was solved in the large multicentre studies by using back illuminated tests in standardized lightboxes. This ETDRS-lightbox (Good-Lite) has become a standard worldwide. It is big and heavy, even if it has become much lighter because of its new frame and illumination based on diodes. It is difficult to move and therefore smaller lightboxes have been manufactured in several countries. With the small ESV1500 lightbox also a low luminance level, 3 cd/m2, which is just below the photopic luminance level, is possible.

Even when the tests, test situations and luminance level are standardized, there is still one important variable that resists standardization, the tester. Every tester has his/her personal way of talking to the child, waiting for the response and supporting the child. Therefore visual acuity values measured by two people, well-trained and motivated to test in the same way, still vary. This was clearly seen in a study that compared VA values measured by experienced school health nurses to those measured by an optometrist (with a background in training of kindergarten teacher) in the same rooms, using the same instruments and closely the same expressions during testing. Of the 506 seven-year-old children (all children on the first grade of the regular schools) in the town of Rauma, 449 children who had not been examined by an ophthalmologist before school age and were examined by the nurses, were included in the investigation.

Figure 1. Visual acuity values measured during the two screening situations were closely the same; the mean value was 1.37 (20/14.6) for both the nurses and the optometrist. The results of the VA measurements were to be written as the exact number of symbols read correctly at threshold [for example 1.25 (-2)]. The pass/fail level of binocular and monocular visual acuity at distance was <1.0 (–2) [20/20 (-2)]. In a few individual cases the values of the second measurements were up to two lines better or worse than during the first measurement.

The vision screening technique based on threshold measurement functioned well, as used by the school nurses according to the WHO Publication 03.91¹. In a small number of cases, however, there is variation in the measured values. although exactly the same measurement techniques were used in the two measurements. This variation does not show any obvious regularity and is likely to mostly depict variation in the functioning of the children but also there are differences between the nurses (compare  e.g. nurse 1, 2, 3, 5, and nurse 11.) and between results of a nurse in different schools (marked as a,b ..f. See for example nurse 1.). The number of students screened in each school is in the column “stud”.

The Figure 1 shows that the number of exactly the same values of the binocular measurements was the highest and that there is a range of plus and minus 10 optotypes between the two measurements that have been made in the same room and with the same instruments by the school nurse and the optometrist. The mean difference in all cases is less than two optotypes. For the nurses, number 4, 7, 8 and 12 who screened children in very small schools, the difference is calculated based on statistically insufficient number of measurements. The monocular values of the right eyes showed similar variation. In five children the value of the right eye decreased two whole lines and in three children the value of the right eye improved two whole lines and in one child the improvement was 11 optotypes. There were no similar large changes in the values of the left eyes. The right eye was always tested first, which might explain the larger variations.

The testers should follow the instructions to the letter. Such 'details' as the distance of the card covering the line above or pointing or not pointing at the optotypes may cause a surprising difference in the visual acuity values in some children. Test instructions in this manual are written in a very detailed way with the hope that the test situations will become standardized.

Vision screening started in Finland in 1962 and was included in the Public Health Act of 1972². At school age, it is a part of the school health nurses’ work. The vision and the vision screening of preschool and school age children had been thoroughly investigated in Finland ^{3, 4, 5, 6, 7}. One aspect of screening that had not been studied was the reliability of the measurements used in vision screening in terms of variation of results when the tests are repeated in the school nurses’ offices by an optometrist, whose measured values are likely to contain similar variations as those by the nurses. We could conclude that the use of *measurements until threshold* (used always in the Nordic countries) functions well, but values that differ from the previously measured values should be repeated before a referral is considered.

The power of our screening is that the screening is performed at schools by the same nurse who examined the children in preschool and may know the families in three generations. The movement of families to other areas in Finland is increasing but the documents follow the family. Several publications have analyzed the quality of referral and found it good. Treatment follows the usual European/Scandinavian tradition, the number of noncompliant families is low.

As a byproduct of this study we found only one child who had untreated amblyopia at the age of 7 years. This child had been referred from the 4-year screening to an ophthalmologist who had not properly assessed the child and had not started treatment. The nurses’ work had been correct.

Variation of the testers’ functioning needs to be taken into account when values from different test series are compared. There is also variation in the child cohorts' cultural background, motivation and intellectual capabilities. When different visual acuity tests are compared the values should preferably be measured at least twice to balance for the different uncontrollable variables.

During testing it has been noticed that line tests contain a poorly known source of error. Children/ persons see during testing that there are never two same optotypes next to each other. If a person wrongly recognises an optotype as the one, which is the next on that row, also the next symbol is recognised wrongly. This can lead to a situation that a whole row is given wrong responses at a general level of 80% correct.

For example, if the child perceives the “house” as the “window”, he will not call the next optotype “window” because he has just used that name but may say “boll”, which then leads also the next symbol to given a wrong name.

Therefore whenever an error like the one described above, it is correct to go to the end of the line and say to the child “Let's start from this end now.”

All LEA visual acuity test optotypes have been calibrated with the Landolt Ring. For details see the history of the LEA SYMBOLS® . Among the paediatric visual acuity tests the LEA SYMBOLS® are the only set of optotypes that have been calibrated with the Landolt C and blur equally. The LEA SYMBOLS® tests are used in assessment of children’s vision and also in examination of soldiers in many developing countries where Western military health services cover all actively involved soldiers.

Among the number sets of optotypes the LEA NUMBERS® are the only set that has been calibrated with the Landolt C test and has even legibility of the numbers, they blur evenly. They are used in the vision examinations and assessments of older children and especially in occupational medicine where letter charts do not work in countries that use other characters.

References:

Committee on Vision. Recommended standard procedures for the clinical measurement and specification of visual acuity. Report of working group 39. Assembly of Behavioral and Social Sciences, National Research Council, National Academy of Sciences, Washington, D.C. Adv Ophthalmol 1980;41:103‐148.

Sloan L, Rowland WM, Altman A. Comparison of three types of test target for the measurement of visual acuity. Quarterly Review of Ophthalmology 1952;8:4‐16.

Louise L. Sloan, Ph.D. Measurement of Visual Acuity, A Critical Review. Reprinted from the A, M. A. Archives of Ophthalmology, June 1951, Vof. 45, pp. 704-725 Copyright, 1951, American Medical Association

Louise L. Sloan, PH.D. New Test Charts for The Measurement of Visual Acuity at Far And Near Distances. American journal of ophthalmology, 1959; 48: 807-13.

Ferris FL, 3rd, Kassoff A, Bresnick GH, Bailey I. New visual acuity charts for clinical research. Am J Ophthalmol 1982;94:91‐96.

Frederick L. Ferris III, M.D., Valeria Freidlin, Ph.D., Aaron Kassoff, M.D., Sylvan B. Green, M.D., and Roy C. Milton, Ph.D. Relative Letter and Position Difficulty on Visual Acuity Charts From the Early Treatment Diabetic Retinopathy Study.American Journal of Ophthalmology Vol. 116, No.6. December 15, 1993, pp. 735 – 740.

TESTS

When testing adults it is customary to test distance vision first, followed by near vision. It is also customary to first test each eye separately, then binocularly. When testing children, better results are obtained by starting with near vision testing before proceeding to greater distances. This allows the child to learn the testing procedures and symbols. The examiner learns what to expect from the child under the most favorable conditions. Also, when testing children, it is important to create a pleasant play situation before testing near and distance vision. Test binocularly; both eyes open first, then each eye separately.

I have been repeatedly asked why there are no published norms for visual acuity values in young children. The cause lies in the huge variation in the populations and the testers. What could be the norm in a cohort of children in Nordic countries is not the same as the mean or median values in a cohort of inner city children in the US. In the US it is still common to point at the optotypes because the time allowed for vision screening is very short. The differences in the measurement techniques mean that each screening program should create their own norms and also be aware of the weaknesses and strengths of their strategies.

Communication

Before testing starts, a method of communication must be established such as naming (signing) or matching. If the child does not spontaneously name the symbols ask "What should we call this? Should we call it 'apple' or 'heart', 'house' or 'garage', 'window' or 'box', 'ball' or 'ring'?"

The child can decide the names of the symbols by playing with the LEA 3-D Puzzle (#251600), Flash Cards (#251800), or the Response Key Card (#251700). Let the child choose which names to use. Note that the child may change the names during the test. For example, if the larger symbol was called "house" the smaller one may be called "dog house" and "apple" may become "berry", etc. If a multihandicapped child cannot point at the symbol or make a selection with his or her hand or foot, arrange the Flash Cards or the LEA 3-D Puzzle pieces farther apart so the child can point with eye gaze, if voluntary eye movements are accurate enough, or with head movements. See Playing cards.

Luminance level

Luminance level should be kept at or above the standard level of 85 candelas per square meter. This is difficult to achieve in a regular room because the test is vertical and thus does not reflect much of the light from ceiling lamps. The small lightbox for the 9"x14" tests has a luminance level of 120 cd/m2. During measurement of near vision luminance level is usually at acceptable levels because the test is held tilted to be perpendicular to the line of sight of the child.

Definition of Visual Acuity Threshold

According to the Visual Acuity Measurement Standard, "A line of optotypes is generally considered to have been read correctly when more than 50% (e.g., 3 of 5, 4 of 6) of the optotypes presented have been read correctly."

Details about Testing

Start testing with binocular testing at near. Distance testing and monocular testing with occlusion of one eye follows naturally once confidence with the child is established. When testing monocularly, test the right eye (O.D.) first followed by the left eye (O.S.), unless there is an obvious negative response to occlusion of the left eye.

In follow up testing and in amblyopia training the -1, -2, and +1, +2 system should be used to give credit for minimal changes. For example, "20/32 (+1)" "( 6/9.5 (+1) or 0.63 (+1)" indicates the child met the 20/32 line criteria and also correctly named one symbol on the next smaller size, 20/25 ( 6/7.5 or 0.8).

Since these tests are mainly used in amblyopia screening it is important to keep in mind that skipping symbols is a feature typical to amblyopic eyes. Even if the visual acuity difference is less than two lines between the eyes, it is an important finding. If one notices that the child has motor difficulties, like skipping symbols, the tester should be alerted to the possibility that the child might have beginning mild amblyopia developing in an eye or has an undiagnosed brain damage.

Visual acuity charts are among the least expensive clinical tests that we use. However, in a number of hospitals and screening places visual acuity tests have not been replaced in 10-15 years. The tests are old, brown and smudged, no more proper high contrast tests. Take good care of your tests, do not leave them in sunshine and clean them with a nonabrasive detergent. Watch that children do not have an opportunity to draw on your tests with pen. Never use a pen to point on optotypes when you are testing a child who cannot fixate at the symbols without pointing. Use a pointed wooden stick, if you are examining the effect of a visual reference (stick) on the visual acuity value of a child. The low contrast visual acuity charts need special attention, keep them in the plastic cover.

Visual acuity values

Visual acuity is recorded as the last line on which at least 3 of the 5 symbols are identified correctly.

When tested at 3 meters (10 feet) the visual acuity value is found in the margin adjacent to that line.
 
After obtaining good responses with binocular testing, proceed by testing each eye separately.

When testing monocularly, use the first symbol of each line or every second line for one eye and the last symbol of each line for the other eye to determine on which line to start testing.

Testing at Different Distances

If the chart is used at a distance other than the usual 3 meters (10 feet), measure and record the viewing distance and the symbol size (the M value) or the visual acuity value printed at the threshold line.

To determine the visual acuity use one of the following formulas:

Note that it is incorrect to report 'V.A. 0.8 (6/7.5, 20/25) at 20cm, if the near vision card with standard testing distance of 40cm was used and the child could read the line with the marking 0.8. If the testing distance is one half of the standard testing distance, the visual acuity value is half of the value written next to the threshold line. In this case thus 0.4 (6/15, 20/50).

Similarly if the measurement at distance was at 1.5m (5') instead of 3m (10’) and the child could read the 0.8 (6/7.5,20/25)line (3.8M line), visual acuity is: half of the value at the threshold line: half of 0.8 is again 0.4.  5'/10' x 20/25 = 1/2 x 20/25 = 20/50. (When using the British notation: 6/7.5 line at 1.5m equals: 1.5m/3m x 6/7.5 = 1/2 x 6/9 = 6/15.

If you do not want to do the calculations, report the result as M-unit value*, i.e. in the previous case 3.8M at 5 feet (1.5m). Visual acuity is easy to calculate based on these values:
VA = 1.5m/3.8M = 0.4 (=20/50).

The calculation is based on metric measurements. The corresponding visual acuity values in the American and in the British notation you most often find also on the visual acuity chart. If the exactly corresponding value is not printed on the chart, calculate it as follows: For example 0.07= 7/100= [7x3/100x3]=21/300 or 20/300; or for the British notation: 0.07=7/100=6/86 (6x100/7=86)

You multiply both the numerator and the denominator with the number that makes the numerator equal or closely equal to 20 or 6.

When the distance is one half (or one third) of the standard distance, the visual acuity value is also one half (one third) of the value printed next to that line.

Notations of visual acuity values
There are several different ways of writing the visual acuity values. In vision rehabilitation and when working at schools where other than the standard distances are used in the assessment of visually impaired children, the most practical way to describe the size of the optotypes that a child has recognized is to write down the M size of the optotype and the distance that was used. For comparison of the visual acuity values at different distances we have to calculate the visual acuity values by dividing the metric distance by the M-value (m/M = VA). This results in decimal visual acuity values.

Even if the decimal notation is dominant in large parts of the Western world, we need to know other ways of expressing visual acuity values. The Snellen fractions (6/6, 3/3, 20/20, 10/10) do not work well in vision rehabilitation because they give a wrong idea of the testing distance, except when it is 3m (10’). VA values measured at the distance of 3m should be written as fractions 3/__ or 10/__, not 6/__ or 20/__.

In the assessment of impaired vision we use other than standard distances. Decimal visual values are then much easier to use than Snellen fractions. Calculation of visual acuity values based on fractions seems to be too difficult in most countries. Therefore it is simplest to calculate the visual acuity as a decimal acuity and then use the Table 1 to find the corresponding Snellen fraction if you want to use the fractions when explaining the result to the parents and your co-workers. You should be aware that if the value is measure at another distance than 3 meters, the fractions give a wrong idea of the visual acuity. This is especially confusing when near vision acuity values are reported.

The list of visual acuity values using different notations (Table1) is good to have easily available. Therefore it is good to have it printed on thick paper or plastic so that you can keep it on the wall or in a drawer near you.

Table 1.

Table 1. Different notations of visual acuity values as decimal values, Snellen fractions, MAR (Minimum Angle of Resolution) and LogMAR.

LogMAR values are used in scientific work that concerns cohorts with greatly varying visual acuity values. In clinical work they have not been popular because lower values mean better visual acuity and best values are negative.

In clinical reports we use decimal values or Snellen fractions.  As an example of visual acuity values of children with complex visual problems we can use the results in Table 2.

Table 2.

Table 2. This table presents the three most commonly used visual acuity notations and also the size of the optotypes as M-size and the distance used.

Table 2 also allows us to compare the results of a child tested with different visual acuity tests including tightly crowded tests. The visual acuity value measured with single LEA SYMBOLS® at 3m was 1.0 (3/3, 10/10). The line test with the same symbols at the distance of 3 m resulted in visual acuity value 0.63 (3/5, 10/17). Visual acuity measured with the standard near line test was 0.20 (3/15, 10/50) and the value for assessment of text size for reading measured with 25% spacing between optotypes was 0.12 (3/25, 10/80), slightly better than one tenth of the value measured with single optotypes at distance. Each of the visual acuity values is correct and depicts visual functioning in different tasks. The large differences between the visual acuity values were related to poor fixation, poor saccadic functions, irregular variations in accommodation, and poor head control.

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* M-unit, metric unit is the distance in meters, at which the reference optotype C is seen at a visual angle of 5'.