New optotypes: are they better than Allen cards?

Kenneth W. Wright,1 Mehmet Cem Mocan,1 Mario Najera-Covarrubias1 and Loren Ong2

1Pediatric Ophthalmology and Strabismus, Los Angeles, CA, USA; 2Cebu City,

Philippines

Introduction

A considerable amount of time and resources has been allocated to research involving the pathophysiology, treatment and amblyopia worldwide. However, early diagnosis of amblyopia still remains the single most important factor influencing the outcome of treatment of amblyopic eyes.1 Early diagnosis of amblyopia, whether it is amblyopic, strabismic, or deprivation-related, can only be appreciated by determination of the visual acuity of both eyes, or estimated by the fixation pattern.2,3

Ordinary visual acuity testing actually evaluates the minimum angle of resolution or recognition. The limit of the minimum angle of resolution is the point at which a pair of points or lines seperated by a certain amount of gap starts appearing as two instead of one. The amount of physical gap determines the threshold for the resolvable visual acuity. Visual charts, which test resolvable acuity, present optotypes of different sizes and determine the threshold of an eye for a certain level of resolution. The accepted threshold for a certain level of resolution is generally accepted as a 75% correct response to optotypes that correspond with a certain visual acuity line on that chart. The minimum angle of resolution for a normal adult is between 30 sec/arc and 1 min/arc. In contrast, the resolvable threshold in amblyopic eyes usually ranges between 1.5 and 20 min/arc.4

In addition to resolvable visual acuity, there are two other forms of visual acuity with lower thresholds. The minimum visible acuity is involved solely in the detection (presence or absence) of any visible stimulus and has a threshold of 1 sec/arc in normal adult subjects. The minimum discernible visual acuity is involved in correct localization of a visual stimulus relative to another one and has a threshold of 2-10 sec/arc in normal eyes.4

In adults, resolvable visual acuity is universally tested using the Snellen chart. However, in the field of pediatric ophthalmology, the Snellen chart has a very limited role. In particular, since the initial evaluation of visual acuity in strabismic children is performed at a very early age, usually before the age of four years, the

Address for correspondence: Kenneth W. Wright, MD, Pediatric Ophthalmology and Strabismus, 520 S. San Vicente Blvd, Los Angeles, CA 90048, USA

At the Crossings: Pediatric Ophthalmology and Strabismus, pp. 213–217

Proceedings of the 52nd Annual Symposium of the New Orleans Academy of Ophthalmology, New Orleans, LA, USA, February 14-16, 2003

edited by Robert J. Balkan, George S. Ellis Jr. and H. Sprague Eustis © 2004 Kugler Publications, The Hague, The Netherlands

Snellen chart cannot be utilized at all. Consequently, in the determination of resolvable visual acuity in preschool children, several visual charts have been developed as substitutes for the Snellen chart. Allen optotypes, LEA figures, Landolt C chart, and the HOTV chart are the most popular visual acuity testing methods in children.1.5-7

Ideally, the charts developed to check visual acuity in small children should have two important attributes. First of all, they should include optotypes that children are familiar with. These figures should be interesting enough to be able to maintain the fixation of the child long enough to be recognized, as children under four years of age have a very limited attention span. Moreover, the figures should be able to correctly test resolvable visual acuity.

Testing resolvable visual acuity necessitates the presence of pairs of points or lines separated at certain distances that, in turn, determine the minimum angle of resolution of the visual system. Therefore, any optotypes used in measuring the resolvable visual acuity should be designed to have a configuration made up of points or lines separated by uniform distance throughout its internal configuration. In addition, to only measure the resolvable acuity and not the hyperacuity, the optotypes must be void of any pattern-related clues. When current individual pediatric charts are evaluated based on these two features, it can be seen that none of them completely fulfills both criteria. Allen optotypes include figures, some of which children are very familiar with and can recognize easily (tree, cake, horse), that do not correctly measure the resolvable acuity because they have shape-re- lated clues and are not made of uniform internal configuration (Fig. 1). LEA figures, which include geometric shapes such as circles, also lack uniform internal configuration. The Landolt C and the illiterate E are excellent charts in that they actually measure resolvable visual acuity, but the ‘C’ and ‘E’ optotypes are too abstract in the minds of most three-year-olds and are not worth fixating at. Therefore, these charts do not fulfill both criteria either.

Is it so hard to develop a visual acuity chart containing optotypes that incorporate the fundamental principles for measuring resolvable visual acuity and at the same time be interesting enough for children to look at? The goal in designing the Wright figures was to come up with better pediatric optotypes for measuring the minimum angle of resolution/recognition (Fig. 2). The Wright figures have undergone three trials in order to compare their visual performances with Allen cards, which have enjoyed worldwide recognition and popularity by pediatric ophthalmologists and pediatricians. The Wright cards have also been evaluated for their value as a potential method of measuring visual acuity in children as accurately as the Snellen chart.

Wright figures consist of five figures that are made up of black lines on a white background with fairly consistent interlinear gaps (Fig. 2). The overall size of the figures is 2.7 times larger than visual acuity level-adjusted Snellen letters, but still confers the same resolution limit due to their internal configuration and gap width. The Wright figures are copyrighted.

The study design

Phase I

The purpose of the initial phase was to compare the ability of normal children to identify Wright figures versus Allen optotypes. More than 70 patients between two

Fig. 1. Allen cards.

Fig. 2. Wright figures©.

and six years of age were included in the first phase. These subjects were completely healthy and had no accompanying ocular problems. The patients were asked to identify each optotype presented through an Imac PC monitor (Accommodata software version 1.8). Figures were presented at super-threshold at approximately 20/200, so they could easily be seen by normal children. It was interesting to note that 23% of the patients were unable to identify either one or more of the five Allen optotypes. The most commonly unrecognizable optotypes were determined to be the ‘phone’ (28%), the ‘jeep’ (28%) and the ‘horse’. However, only 8% of the subjects were unable to identify one or more of the Wright figures presented. The least recognizable optotypes were the house (2%), the cow (2%), and the chick (2%). There was a statistically significant difference in the recognition of the two different sets of optotypes, favoring the Wright figures in normal children (p < 0.005).

The results of Phase I suggested that Allen optotypes were not uniform, regarding their ease of identification by normal children, compared to Wright figures that had more consistency in their ease of identification.

Phase II

Any visual acuity chart intended for measuring resolvable visual acuity should be compared to the Snellen chart, which is currently the accepted gold standard for testing this type of vision. Therefore, the purpose of the second phase was to evaluate how well the visual acuity scores obtained through Wright figures correlated with Snellen letter acuity in normal subjects. The patients investigated were chosen from those who came in for a routine ophthalmic examination and had no accompanying ocular problems. The patient population investigated had a higher mean age than those investigated in Phase I due to the fact that they had to be able to cooperate well with the Snellen letters. Therefore, patients with an age of seven years and older were evaluated. The visual acuity was measured in random order; either the Wright figures or the Snellen letters could be presented first.

The results obtained with Wright figures and Snellen letters had an agreement of over 80%. The results showed that Wright figures correlated with Snellen letters in normal subjects.

Phase III

The final phase of the study was designed to test how well the visual acuity measurements obtained through the Wright figures could be compared with those obtained through the Snellen chart and the Allen optotypes in amblyopic pediatric subjects. The visual performance of any visual acuity chart can differ between testing in normal and amblyopic subjects, due to how the optotypes are constructed and how they are presented. Amblyopic eyes demonstrate abnormalities in several other psychophysical visual tasks, such as contrast sensitivity, crowding phenomenon, spatial summation, and Vernier acuity.8 With a complex spectrum of abnormalities in visual recognition and processing functions, amblyopic eyes differ significantly in their response to visual acuity charts when compared to normal eyes. As such, any inherent flaw or variability in optotype construction might reveal significantly different resolvable visual acuity performances when tested in amblyopic eyes, even if they may be able to correlate perfectly in normal subjects.

Phase III evaluated subjects who had amblyopia and who were old enough to cooperate with the Snellen chart. All the subjects were otherwise in normal health. Both anisometropic and strabismic amblyopic eyes were evaluated in the study. The results of the Phase III study revealed that the visual acuity levels obtained with the Wright figures were not significantly different from those obtained with the Snellen chart. Moreover, Wright cards were correctly able to identify all the amblyopic subjects included in the study. In addition, the visual acuity levels obtained with the Wright figures correlated very strongly with those obtained with the Snellen chart. On the other hand, the visual acuity levels obtained with Allen optotypes were significantly higher than those obtained with the Snellen chart. More importantly, Allen cards falsely identified more than one-third of the amblyopic eyes as normal. Allen optotypes also correlated strongly with the Snellen chart; however, the correlation was not as strong as with the Wright figures.

Conclusions

Visual acuity charts intended for the preschool pediatric population have been evolving considerably over the last five decades. The primary purpose these charts serve is the early detection of amblyopia, so that amblyopic eyes can be treated at the earliest possible stage in the critical period of visual development. Therefore, it is crucial that these charts identify amblyopic eyes as accurately as possible. Although amblyopic eyes demonstrate several other abnormalities in psychovisual tests, they are not easy to demonstrate in children from a practical point of view.8 Consequently, testing visual acuity is the single most important tool we have for detecting amblyopia.

The results of all the phases conducted to evaluate the value of the Wright figures show strong evidence that these optotypes can accurately reflect visual loss in the pediatric population. Wright figures have three important distinct characteristics that enable their high correlativity with the Snellen chart: firstly, the figures are constructed with uniform lines with equal angles of resolution; secondly, the figures lack pattern-related clues and have no giveaways. In fact, at the time when the figures were first developed, some of them had slight pattern-related clues. However, over time, these clues have been disguised with additional lines, making

the figures appear more homogeneous. Finally, the figures have complex internal crowding that enables the appreciation of the crowding phenomenon present in amblyopic eyes.

Overall, the Wright figures take into consideration the principles of minimum angle of recognition and the crowding phenomenon, and are able to avoid overestimation. This is a major advantage over Allen optotypes that have been shown to overestimate visual acuity results, not only in our three trials, but also in other earlier studies.9,10 It is interesting to note that modified Allen optotypes with crowding bars have prevented the overestimation of visual acuity, thus supporting our view that Allen cards are not appropriate for judging the visual performance of amblyopic eyes.10

We believe that the Wright figures represent the most advanced form of pediatric vision chart available, and represent a major step in the development of scientifically reared optotypes to exactly measure resolvable visual acuity.

Acknowledgments

Supported by grants from the following: Discovery Fund for Eye Research, Gustavus and Louise Pfeiffer Research Foundation, Henry L. Guenther Foundation, and Wright Foundation for Pediatric Ophthalmology and Strabismus.

References

1.Wright KWW: Visual development and amblyopia. In: Wright KWW, Spiegel P (eds) Pediatric Ophthalmology and Strabismus, 2nd Edn, pp 157-170. New York, NY: Springer-Verlag 2003

2.Wright KW, Edelman P, Walonker F, Yiu S: Reliability of fixation preference testing in diagnosing amblyopia. Arch Ophthalmol 104:549-553, 1986

3.Wright KW, Walonker F, Edelman P: 10-Diopter fixation test for amblyopia. Arch Ophthalmol 99:1242-1246, 1981

4.Westheimer G: Visual acuity. In: Hart WM (ed) Adler’s Physiology of the Eye, 9th Edn, pp 531-547. St Louis, MO: CV Mosby Co 1992

5.Hyvarinen L, Nasanen R, Laurinen P: New visual acuity test for preschool children. Acta Ophthalmol (Kbn) 58:507-511, 1980

6.Allen HF: A new picture series for preschool vision testing. Am J Ophthalmol 44:38-41, 1957

7.Graf MH, Becker R, Kaufmann H: LEA symbols: visual acuity assessment and detection of amblyopia. Graefe’s Arch Clin Exp Ophthalmol 238:53-58, 2000

8.Von Noorden GK, Campos EC: Examination of the patient. IV. Amblyopia. In: Von Noorden GK, Campos EC (eds) Binocular Vision and Ocular Motility, Theory and Management of Strabismus, 6th Edn, pp 269-282. 2001

9.Lueder GT, Garibaldi D: Comparison of visual acuity measured with Allen figures and Snellen letters using the B-VAT II monitor. Ophthalmology 104(11):1758-1761, 1997

10.Mayer DL, Gross D: Modified Allen pictures to assess amblyopia in young children. Ophthalmology 97(6):827-832, 1990