Mercer, Michele E.*; Drover, James R.*; Penney, Kaitlin J.†; Courage, Mary L.*; Adams, Russell J.*
Because of emerging technologies and heightened clinical attention, we have witnessed the emergence of a variety of experimental and state-run programs designed primarily to diagnose early eye and vision disorders in preschool children.1,2 Despite wide variations in emphasis and methodology, virtually all vision screening programs incorporate some measure of recognition visual acuity. Recognition visual acuity remains the choice of clinicians for diagnosing early spatial vision deficits, notably amblyopia, the most common and most treatable form of pediatric eye disease.3 Not surprisingly, given the prominence of visual acuity testing in pediatric screening regimens, there has been tremendous clinical and commercial interest in developing and modifying new child-oriented tests of early recognition acuity. Our survey of current commercial offerings reveals that there are 27 different optotype-based visual acuity tests available for use with children less than 5 years.
Despite this proliferation of available measures, the interpretation of results from the wide variety of existing pediatric visual acuity tests remains problematic for several reasons. First, most pediatric tests differ from one another in optotype size, range, layout, spacing, and crowding characteristics. They also vary widely in cognitive and attentional requirements, ranging from minimal (e.g., Cardiff optotypes)4 to relatively demanding (e.g., HOTV and Landolt C).3 Not surprisingly then, few direct comparisons of a child’s acuity across different acuity tests exist,5–8 and even these have been limited to measures of agreement between traditional pediatric letter optotypes (e.g., HOTV) and newer symbol optotypes (e.g., Lea Symbols). Second, there is even less comparative data between adults who were evaluated with both a gold standard adult acuity test and with any of the current pediatric optotype tests. The data that do exist, however, show that the existing pediatric letter and symbol tests (e.g., Lea Symbols and HOTV) seem to result in better (i.e., finer) estimates of visual acuity than do adult gold standard visual acuity tests, both in older children and in adults.9–13 A serious problem arises when these apparent overestimates of acuity in preschoolers are then compared with standard letter acuity estimates for older children, or to norms or pediatric standards based on other measures of visual acuity.
Therefore, based on this paucity of definitive evidence, we currently lack an accepted gold standard visual acuity test capable of generating true normative data for preschool children. Moreover, we do not have a pediatric acuity test that is directly comparable to acuity results obtained from measures of older children and adults. Several putative gold standard visual acuity tests for older patients have emerged over the past few decades (i.e., the Bailey-Lovie, ETDRS, and Sloan letters) as they conform to the Committee on Vision14 recommendations for standardized measurement of visual acuity. Such tests all include equal numbers of optotypes per line (usually five), logarithmic progression of optotype size from line to line, and logarithmic proportional spacing of optotypes within a line. Two relatively new pediatric tests, the Lea Symbols (Good-Lite, Elgin, IL) and the Patti Pics (Precision Vision, LaSalle, IL) purport to conform to the Committee on Vision standards, and both have enjoyed recent widespread use by researchers and clinicians alike. Despite their popularity, the Lea and Patti Pics symbols have yet to be compared directly to each other within individual children, and in only one study15 have the Lea Symbols been compared directly to a gold standard within a large sample of adult eyes. In the present study, we use the Mass VAT version of the Lea and Patti Pics tests to measure visual acuity in a broad group of preschool children. The most recent Mass VAT format contains optotypes with proportionally crowded contours, the format recommended for screening amblyopia in young patients.5,6 For comparison, we repeat the process in a group of adults who are also tested with a parallel gold standard test of visual acuity.9 The goal of this study is to determine the level of agreement between the different pediatric acuity tests, but perhaps more importantly, to determine which, if any, of the optotypes better approximates an adult gold standard. This issue is of significant clinical importance because it allows single estimates of a child’s visual acuity to be interpreted in a more definitive and comparable context, thus improving ultimate diagnostic confidence.
Right eyes from 80 3- to 5-year-old preschool children (40 females, 35 males; mean age, 4.1 yr) in their familiar daycare setting were each tested by one of two experienced technicians. In addition, right eyes from 52 adult volunteers (mean age, 31 yr; range, 20–62 yr) were evaluated by one of the same technicians. To obtain a representative sample, no child or adult was excluded for any ophthalmologic, neurologic, or medical condition, or for any social or demographic reason. After a full explanation of the study and its risks and benefits, informed written consent was obtained from each adult subject or the parent of each child participant. The study protocol followed the tenets of the Declaration of Helsinki and was approved by the appropriate institutional ethics review board for human participants.
Each child was tested in a single session with both the Mass VAT distance (3 m) versions of the Lea Symbols (model #259101; Good-Lite) and Patti Pics (model #2591; Precision Vision, LaSalle, IL) visual acuity charts designed for pediatric vision screening. As shown in Fig. 1, both tests contained rows of five optotypes per line (the top row of the Lea contains only 4 optotypes), with each line surrounded by crowding bars, the format best recommended to detect amblyopia in young children.5,6 Optotypes in both pediatric tests ranged from 0.6 to −0.1 logMAR (20/80–20/16) and were mounted in a light box and back illuminated at 170 cd/m2, as verified with a Cal-Spot 400 VF Photometer (Cooke Corporation, London, Ontario, Canada). Adults were also tested with the gold standard visual acuity letter test (Sloan Mass VAT model #2189; Precision Vision) that matched exactly the pediatric tests on all chart and optotype specifications (i.e., chart size and layout, optotypes per line, optotype range, crowding characteristics, and illumination). To assess the possible effect of refractive error on the interpretation of the data, all subjects’ refractive status (with and without optical correction in place if applicable) was measured with the Welch-Allyn SureSight noncycloplegic autorefractor (Skaneateles, NY).
With the left eye occluded, both children and adults were seated 3 m from the illuminator box in which each chart was placed. Adult subjects wore their prescribed right eye optical correction during this assessment. For each test, subjects began with the largest optotypes (0.60 logMAR) and then progressed to lines of progressively smaller or finer optotypes, reading each line from left to right. Testing continued until at least two errors were made on a line. Testing of that line was repeated, and if the subject again failed at least two optotypes, his or her visual acuity was determined as the value representing the larger optotype size on the previous line. If instead, he or she made one error or less, testing then continued with the subsequent line on the chart. Chart order was counterbalanced across subjects, with autorefractor measurements always taken at the end of each session.
To assess the effects of uncorrected refractive error, right eyes from 25 adults were also tested on all three charts without optical correction in place. Procedural details were the same as the preceding tests, except that, if an adult could not see the largest optotypes (0.6 logMAR, 20/80) at 3 m, he or she was tested at half the distance (1.5 m) and again at 0.75 m if the largest optotypes at 1.5 m were still undetectable. Acuity values were adjusted accordingly to suit any change in viewing distance.
All adults completed testing, but three 3-year olds did not complete both tests and were excluded from the sample. Results showed that preschoolers’ visual acuity was better on the Lea chart (mean logMAR, 0.19; 20/31 Snellen) compared with the Patti Pics chart (mean logMAR, 0.26; 20/36 Snellen). The mean for the Lea Symbols was consistent with previous data from large samples of preschoolers,5,6 and the mean for the Patti Pics was similar to the limited data that have been reported for this test.2 Functionally, this disparity between tests (0.07 logMAR units) represents more than a half-line difference between the two charts, with the distribution of Lea values shifted toward relatively better and finer estimates of visual acuity. A paired t test revealed that the difference in visual acuity between tests was significant (t = 3.33, P < 0.001, two-tailed).
Further analysis of the individual data contrasting each child’s results on the two tests directly indicated that 38 of 77 eyes (49%) showed better acuity with the Lea Symbols compared with the Patti Pics. Conversely, only 5 of 77 eyes (7%), showed better acuity with the Patti Pics, and the remaining 34 eyes (44%) showed the same level of visual acuity across both tests. To best illustrate these data, a Bland and Altman16 plot was generated (Fig. 2) to show the degree of agreement and its variability across the entire range of preschoolers’ performance, that is, from those with relatively poor mean levels (between tests) of visual acuity to those with better levels of mean acuity. The figure shows that the individual differences (Patti Pics minus Lea) are skewed to values more than 0.00 logMAR, which again indicates relatively better visual acuity with the Lea test. The two SD limits within this distribution of differences was ±0.16 logMAR, a total range of approximately three chart lines. It is important to note, however, that variability appears relatively constant across the entire x-axis range of individual average acuity performance, that is, from −0.1 logMAR (20/16) to 0.5 logMAR (20/63).
To assess the possible interactive effects of refractive error on performance with the different tests, preschoolers were divided into two equal groups (n = 36) based on spherical equivalent refractive (SER) error (sphere+ 1/2 cylinder). Analyses of these subgroups mirrored the results found previously: Visual acuity obtained with Lea Symbols was significantly better than that obtained with the Patti Pics, both among preschoolers who have higher levels of refractive error (SER error, ≥2.25 D) and those who have lower levels (SER error, ≤2.00 D; both, P < 0.01). Again, acuity measured with Lea Symbols was better than Patti Pics acuity by 0.07 to 0.09 logMAR across both subgroups. Overall, spherical refractive error ranged from −0.25 to +3.25 D (mean sphere, +1.75 D), cylindrical refractive error ranged from 0.00 to 2.75 D (mean cylinder, 1.25 D), and SER ranged from −0.25 to +4.25 D (mean SER error, +2.50 D), values consistent with those reported previously2 for preschoolers tested with the Welch-Allyn SureSight autorefractor.
To determine which of the pediatric acuity tests best predicted adults’ visual acuity, it was important to compare children’s and adults’ data in several ways. First, like preschoolers, adults also had better estimates of visual acuity with Lea Symbols (mean, −0.05 logMAR; 20/18 Snellen) compared with the Patti Pics test (mean, 0.06 logMAR; 20/23 Snellen). Second, as with children, this pattern for adults was also apparent within the individual data, with only 1 of 52 adult eyes (2%) showing the better estimate of acuity with the Patti Pics test. Conversely, 22 of 52 eyes (42%) showed better acuity with the Lea Symbols, and 29 of 52 eyes (56%) revealed identical values on both tests. A paired t test revealed that this difference between the Lea and Patti Pics tests was significant (t = 2.61, P < 0.01, two-tailed). Third, comparisons among adults’ performance on the Patti Pics, Lea, and Sloan tests revealed that the Patti Pics estimates appeared to be a more similar approximation of adult Sloan letter acuity. Among adults, mean acuity on the Sloan letters was virtually identical to their performance with the Patti Pics optotypes (a mean of 0.04 logMAR and Snellen of 20/22 for Sloan compared to a mean of 0.06 logMAR and Snellen of 20/23 for the Patti Pics). A paired t test revealed no statistical difference between the Sloan and Patti Pics tests. In contrast, mean acuity for the Sloan letters was approximately 0.1 logMAR or approximately one chart line worse than adults’ mean visual acuity measured with the Lea Symbols (−0.05 logMAR, 20/18 Snellen), and this difference between the Lea and Sloan tests was statistically significant (t = 3.03, P < 0.004, two-tailed).
A related comparison was to examine each adult’s individual performance on the gold standard Sloan letters versus his or her performance on each of the two pediatric optotype tests. First, comparing the Lea Symbols to Sloan letters, 26 of the 52 adult eyes (50%) performed better with the Lea Symbols, only 4 of 52 eyes (8%) performed better with the Sloan letters, and the remaining 22 eyes (42%) showed equal levels of acuity across both tests. However, in comparing the Patti Pics to Sloan letters, performance was much more symmetrical with 30 (58%) of the 52 eyes showing equal levels of visual acuity across both tests and with 9 (17%) and 13 (25%) of the 52 eyes showing better performance on the Patti Pics and Sloan tests, respectively. These individual data are illustrated in Figs. 3 and 4, which show the Bland and Altman16 plots that compare directly each individual adult’s acuity on the Sloan and Lea tests (Fig. 3), and on the Sloan and Patti Pics tests (Fig. 4) across the entire range of mean visual acuity scores. First, consistent with the previous group data, Fig. 3 reveals that adults’ individual acuity estimates for the Lea Symbols were approximately 0.10 logMAR better than with Sloan letters, and the two SD limits of agreement (±0.20 logMAR) were also broad, a range that spans four chart lines. In contrast, Fig. 4 reveals not only that the Patti Pics and Sloan letters yielded a very symmetrical distribution of individual differences and thus a similar mean difference between tests (−0.01 logMAR) but also that the two SD limits of agreement were relatively small (±0.11 logMAR), a total range just over two chart lines. Note also that the variability of the differences across the entire range of mean acuity scores is more constant for the Patti Pics versus Sloan comparison (Fig. 4) than for the Lea versus Sloan comparison (Fig. 3), at least for this range of adult visual acuity. Finally, in comparing adults’ with children’s individual performance across the two pediatric acuity tests, it was noteworthy that, although adults’ acuity is still approximately 0.10 logMAR better with the Lea test compared with that with the Patti Pics, the two SD range of agreement reduces across with age from 0 .32 to 0.24 logMAR, a difference of approximately one chart line.
It was also important to examine the data obtained from adults with uncorrected refractive errors to increase the range of possible visual acuities for which the different tests could be compared. Results for the 25 adult right eyes tested without optical correction (all myopes or astigmats) revealed the same differences between tests. Mean visual acuity with the Lea Symbols was better (mean, 0.52 logMAR; 20/66 Snellen) than with both the Patti Pics symbols (mean, 0.70 logMAR; 20/100 Snellen) and the Sloan letters (mean, 0.67 log MAR; 20/94 Snellen). Paired t tests revealed that the difference between the Lea Symbols and both the Sloan and Patti Pics tests was significant (both, P < 0.05). The average level of noncycloplegic refractive error among the 25 uncorrected adult eyes was −1.54 D for spherical error (range, −0.25 to −3.75 D), 0.59 D (range, 0.25–3.00 D) for cylindrical error, and −1.29 D (range, −0.50 to −4.25 D) for SER error. As with preschoolers, adults with higher levels of refractive error (SER error, ≥−2.00 D; n = 12) did not appear to differ from those with lower levels (SER error, <−2.00 D; n = 13) across all of the different visual acuity tests.
The purpose of this study was to compare directly two pediatric optotype charts that adhere most closely to the international standard for visual acuity tests. Our results suggest that, in many children and adults, the Lea Symbol optotypes yield consistently better estimates of visual acuity than the Patti Pics optotypes. Considering first the adult data, performance with Lea Symbols exceeded both the Patti Pics and Sloan tests by approximately a full logMAR chart line, and this difference in adults might have been even greater had the Lea test contained additional lines composed of optotypes with resolution values even finer than −0.1 logMAR (20/16 Snellen). In our sample, approximately 60% of adults detected the line representing this maximal optotype value. This apparent ceiling effect within the data likely results in an underestimate of the true adult mean for the Lea Symbols and, consequently, the magnitude of the difference between tests. Only one previous study9 has reported data on adults’ performance with both the Lea and Patti Pics tests. Unfortunately, however, they did not compare the two tests statistically nor did they use the crowded optotype versions of the tests used here. However, Fig. 1A of that study shows that the visual acuity of every adult tested (n = 8) was better with the Lea optotypes than the Patti Pics, with the magnitude of the mean difference (∼0.1 logMAR) almost identical to that reported here.
Unlike adults, the preschoolers’ results for both pediatric tests are more normally distributed, implying that, at least compared with the skewed adult data, the significant difference between tests (0.07 logMAR) is a reasonable estimate of the true level of discrepancy. The range of agreement between Lea and Patti Pics tests shown in Fig. 2 is moderate (0.32 logMAR) and also symmetrical, a result which implies that conversion from one test to another (e.g., by adding or subtracting a fixed value) is possibly feasible, at least within this range of acuity values. The mean value reported here for the Lea Symbols is consistent with previous work on preschoolers tested with the same crowded version of the test.5,6 Unfortunately, there are no published data available to compare for the Patti Pics test, so the data reported here represent the first pediatric data for that test. Nonetheless, the importance of the present data is that they represent the first systematic comparison of these clinical tests within a sample of typical (i.e., nonreferred) preschool children.
A second topic of importance is determining which of the pediatric optotype tests best predicts an equivalent format adult gold standard letter test. Our results showed that adults’ performance with the Patti Pics optotypes and crowded Sloan letters was virtually identical, both in mean performance and in the overall distribution of acuity scores. Moreover, individual subject agreement as shown in Fig. 4 was excellent, with just more than half of the sample showing identical visual acuity across both tests and the rest of the sample differing by only one line. Approximately half of those individual adults scored one line higher, and the other half, one line lower. In addition, variability was relatively low (a range of about two chart lines) and appeared consistent across the range of individual acuity values.
As with the Patti Pics optotypes, approximately half of the adult eyes showed identical visual acuity on the Lea and Sloan tests. However, unlike the Patti Pics, almost all of the remaining adult eyes showed better levels of visual acuity on the Lea test, in some cases, as much as two lines better. Not surprisingly, individual variability between tests was also relatively high (a range of four chart lines) and was also uneven across the range of acuities because of the ceiling effect found with adults tested with the Lea chart. Previous studies, in which different versions of the Lea Symbols have been tested against gold standards (Landolt C, ETDRS, Sloan, and Bailey Lovie), have also found that the Lea test overestimates gold standard visual acuity in both adults9–11 and children,10,13,15 typically by a little less than one chart line. Our results with the latest Mass VAT version of the Lea optotypes are highly consistent with these previous findings. Moreover, we also find this apparent overestimate of visual acuity among adults with uncorrected refractive errors.
In conclusion, because these pediatric tests are used widely and are at times used interchangeably, the small but consistent difference between tests is important to consider for several clinical reasons, namely (1) for setting and interpreting fixed visual acuity screening referral criteria, (2) for comparing preschool estimates of visual acuity to those obtained in later childhood, and (3) for diagnosing spatial vision deficits and clinical decision making in individual patients. However, note that given that our sample was composed of typical unscreened preschoolers, our conclusions are limited to a nonclinical (i.e., an unreferred) pediatric population. Nonetheless, although both tests appear equal in both test time and patient compliance and the Lea Symbols have shown excellent testability in previous large-scale screening studies,5,6 the Patti Pics seems to have the distinct advantage of greater consistency with an adult acuity test that possesses gold standard optotype design. This similarity is a benefit in the interpretation of visual acuity results from individual children because it may result in a more accurate prediction of early visual pathology and also better continuity with the results of other standard tests of visual acuity taken at later ages. This has particular implications for those children whose acuity estimates fall near clinical referral cut-off values because the proper clinical decisions may depend critically on which specific pediatric optotype test a child has completed.
Michele E. Mercer
Department of Psychology
St John’s Newfoundland A1B 3X9
Received March 30, 2012; accepted November 19, 2012.
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