Phoria and vergence ranges are often performed behind

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1 Normative data for modified Thorington phorias and prism bar vergences from the Benton-IU study Don W. Lyon, O.D., a David A. Goss, O.D., Ph.D., a Douglas Horner, O.D., Ph.D., a John P. Downey, O.D., a and Bill Rainey, O.D. b a Indiana University School of Optometry, Bloomington, Indiana and b Southern College of Optometry, Memphis, Tennessee Background: The use of a phoropter for measuring phorias and vergences in children is common in the optometric profession. For young children, the use of the phoropter can be confusing, making it difficult to obtain accurate measurements. Free space testing allows for direct observation of the eyes in a natural environment and is easier for children to understand the directions. The normal values for phorias and vergences used with children are derived from testing with a phoropter or free space measurements with mostly adult patients. Methods: The Benton-IU Project was a large multidisciplinary study of factors affecting school performance conducted by the Indiana University School of Optometry and the Indiana University Department of Speech and Hearing with the cooperation of the Benton Community School Corporation (Benton County, Indiana). This project allowed the authors to obtain data on modified Thorington phorias and prism bar vergences from a nonselected group of first and fourth graders as part of an eye/vision examination. Results: In this report, central tendency and variability statistics for modified Thorington and prism bar vergences are reported based on the data from the Benton-IU Study. Conclusion: The data presented in this report can be used by optometrists when deciding if the patient s phorias and vergences are within normal limits for children in the first through fourth grades. Key Words: Modified Thorington, prism bar vergences, free space testing, school-aged children Lyon DW, Goss DA, Horner D, et al. Normative data for modified Thorington phorias and prism bar vergences from the Benton-IU study. Optometry 2005;76: Phoria and vergence ranges are often performed behind a phoropter and then compared with known normative data. Most of the normative data come from studies based mostly or entirely on selected populations of adults. 1-5 Perhaps the most recognized normative data for phorias and vergences come from Morgan. 6,7 Morgan 6,7 performed phorometric testing on 800 prepresbyopic adults. Jackson and Goss 8 performed astudy looking at clinical phoropter tests, including phorias, vergences, and accommodative testing on selected 8 to 16 year olds. Asummary of the results of these studies can be found in Table 1. A few studies have looked at normal values for vergences and phorias outside of the phoropter. Testing vergences and phorias in free space have been shown to be easier for patients, especially children. This allows for direct observation of the patient s eyes during the test and allows for a more natural testing environment. 9 Results further show that free space phoria testing methods are generally more repeatable than von Graefe phoria testing Wesson 9 performed a study on vergences using a prism bar instead of a phoropter. His subjects were selected from clinical patients between the ages of 4 and 70 who had corrected 20/20 visual acuities in each eye and distance phorias between ortho and 4 exophoria and between ortho and 7 exophoria at near; the phorias were measured initially using cover test. Wesson 9 also looked at the role eye dominance may have in the results of prism bar vergences. His study did not find any statistical significant difference when the prism bar was placed over the dominant eye compared with the other, concluding that in clinical testing, it does not matter over which eye the prism bar is held. 593

2 Table 1. Phoria and vergence means (SD) from phoropter testing from various studies Haines Betts and Austin Shepard Morgan Jackson and Goss Distance phoria 0 0 (2) 1 exo (2) 1 exo (2) 1 exo (2) BI break 9 7 (3) 9 (3) 7 (3) 12 (3) BI recovery 5 3 (2) 4 (2) 4 (2) 4 (2) BO blur 9 7 (3) 9 (3.5) 9 (4) 14 (6) BO break (6) 21 (8.5) 19 (8) 23 (8) BO recovery 6 7 (5) 9 (4.5) 10 (4) 6 (5) Near phoria 5 exo 3 exo (2) 5 exo (5) 3 exo (5) 3 exo (4) BI blur (4) 10 (3.5) 13 (4) 15 (6) BI break (4) 20 (5.5) 21 (4) 21 (4) BI recovery 12 (6) 11 (4) 13 (5) 9 (4) BO blur (6) 13 (5.5) 17 (5) 21 (8) BO break (6) 25 (11) 21 (6) 27 (8) BO recovery 6 (7) 13 (7.5) 11 (7) 10 (6) Eso, Esophoria; Exo, exophoria. In 1989, Scheiman et al. 13 reported results on normative data for prism bar vergences for subjects selected after passing a modified clinical technique screening. The ages for their subjects ranged from 6 to 12 years. In the studies mentioned above the subjects were either adults or school-aged children selected from clinical populations. There was a recent study completed in SpainbyJimènezetal. 14 thatreportednormative data for various binocular tests, including prism bar vergences and modified Thorington for school-aged children 6 to 12 years old. In this article we report a portion of the results from the Benton-IU Project. The Benton-IU Project was a large multidisciplinary study of factors affecting school performance conducted by the Indiana University School of Optometry and the Indiana University Department of Speech and Hearing with the cooperation of the Benton Community School Corporation (Benton County, Indiana). The Benton-IU project incorporated assessment of the visual system including visual acuity, stereopsis, cover test, accommodation, phorias, vergences, undilated ocular health assessment and visual information processing, hearing, auditory abilities, reading skills, linguistic abilities, and phonological processing. 15 Inthis report we examine the data obtained for modified Thorington phorias and prism bar vergences. By comparing the results ofstudiesbyjimènezetal. 14 andscheimanetal., 13 and the Benton-IU project, it appears that we now have a significant amount of information on normative data for children 6 to 12 years of age and 594 across a large geographical area for prism bar vergences and the modified Thorington test. Methods Subjects This population was an unselected sample and, therefore, no subject, once informed consent was given, was excluded from the testing and data analysis. The age range of the children in the first grade group was 6 to 8 years at last birthday and in the fourth grade group, 9 to 11 years. Subjects were 453 first graders and 426 fourth graders from Benton County Community School Corporation in northwest Indiana. Parental consent was obtained, and the children were brought from their schools in Benton County to the Indianapolis Eye Care Center by school bus in groups of approximately 16 children per group during the third to eighth weeks of school when they were in first grade and again 3 years later. The fourth grade data include some subjects who were not in the original cohort. The fourth grade data do not include those subjects who moved outside of Benton County between first and fourth grade. Description of tests The tests described here were done as part of an eye/vision examination that also included visual acuities, versions, accommodative evaluation,

3 Table 2. Benton-IU first grade modified Thorington phorias and prism bar vergences Mean Mode Median SD 25th to 75th percentiles Distance phoria to1eso BI break to8 BI recovery to4 BO break to14 BO recovery to8 Near phoria 1 exo exo to 1 eso BI break to 20 BI recovery to 12 BO break to 25 BO recovery to 16 Eso, Esophoria; Exo, exophoria. retinoscopy, refraction, and an ocular health assessment with nondilated fundus examination. All of the optometric tests were performed at the Indiana University School of Optometry s Indianapolis Eye Care Center by fourth-year optometry interns who were instructed on the testing protocol by an optometry faculty member. Modified Thorington phorias. Modified Thorington phorias were performed using a Muscle Imbalance Measure Card from Bernell in the following order: distance lateral phoria, distance vertical phoria, near lateral phoria, and near vertical phoria. The cards were calibrated for distance testing at 10 feet and near testing at 16 inches, with the subjects wearing their habitual prescription. A Maddox rod was placed over the subject s right eye with the grooves oriented in the appropriate direction to measure either lateral or vertical phorias. The subjects were instructed to look at the zero in the center of the card and to keep it clear. They then stated if the line was to the right or left of the zero (subjects could also point to the side of the card that the line fell). The examiner asked the subjects which number was closest to the red line. This number was then recorded. Prism bar vergences. Prism bar vergences were performed outside of the phoropter using a horizontal prism bar with the following powers: 1, 2 to 18 (in 2 prism diopter steps), and 20 to 45 prism diopters (in 5 prism diopter steps). The vergences were measured first at distance, then at near, while the subjects were wearing their habitual prescriptions. For distance testing, the subjects were instructed to look at the 20/100 tumbling E, while at near the target was a drawing of a dog on a near point stick held at 40 cm. For all testing the prism bar was held over the right eye, with base in measured before base out. The examiners were instructed to increase the prism power one step at a time every 2 seconds until a break was noted. A break was defined as when the right eye turned in (for base in) or out (for base out) or there was an alternation of fixation when viewing the target. Once either one of these occurred the examiner was then instructed to decrease the prism power one step every two seconds until the right eye swung back to achieve fusion. Results Spherical equivalent refractive error of the right eye in the first grade group ranged from 3.00 D to 5.63 D, with a mean of 0.42 D and a standard deviation (SD) of 0.83 D. In the fourth grade group, the range of the spherical equivalent was refractive errors 5.25 D to 6.25 D with a mean of 0.0 D and SD of 1.18 D. The means, SDs, and percentile ranges for the first and fourth grade phoria and vergence data are presented in Tables 2and 3, respectively. The mean, median, and mode for distance phoria were 0 prism diopters in the first grade and 0 prism diopters in the fourth grade. The near phoria median and mode for first grade were 0 595

4 Table 3. Benton-IU fourth grade modified Thorington phorias and prism bar vergences Mean Mode Median SD 25th to 75th percentiles Distance phoria to1eso BI break to8 BI recovery to6 BO break to16 BO recovery to10 Near phoria 1 exo exo to 2 eso BI break to 16 BI recovery to 12 BO break to 30 BO recovery to 20 Eso, Esophoria; exo, exophoria. prism diopters and 0 prism diopters for the fourth grade data. Near point phoria mean was 1 prism diopter exophoria for first grade and 1 prism diopter exophoria for fourth grade. The SDs for phorias were similar between the first and fourth grade data sets. The total range for distance-modified Thorington phorias were 4 exophoria to 4 esophoria for first grade and 4 exophoria to 4 esophoria for fourth grade. The 25th to 75th percentiles for the distance phorias were 0 to 1 esophoria for both first and fourth grades. Ranges for near modified Thorington phorias for first grade were 22 exophoria to 16 esophoria and 18 exophoria to 25 esophoria in the fourth grade. The 25th and 75th percentiles for near phorias were very similar, 2 exophoria to 2 esophoria for fourth grade and 2 exophoria to 1 esophoria for the first grade. Cumulative frequency distributions of phorias are shown in Figures 1and 2. Comparisons between first and fourth grade vergence range means were made using the paired t test for the students that were tested in both first and fourth grades (n 335). Because multiple t tests were performed, the Bonferroni correction was used to set the value of P for statistical significance at rather than Means for the vergence ranges were similar between grades at both distance and near (see Tables 2 and 3). Only the distance base in recovery and the near base in break showed statistically significant differences between first and fourth grades (P 0.005). For both grades the means of the base in fusional ranges were smaller than the 596 Figure 1 Cumulative frequency distribution of first grade near phoria (upper left), distance phoria (upper right), fourth grade near phoria (lower left), and distance phoria (lower right). means of the base out ranges, both at distance and near. Cumulative frequency distributions of the prism bar vergences are shown in Figures 2 and 3. The distributions were very similar for both first and fourth grades, so only the plots for the fourth grade are shown. Lower SDs, narrower 25th to 75th percentile ranges, and steeper ascent on the cumulative frequency distributions indicate less variability on base in vergences than base out vergences and less variability on recovery findings than on break findings.

5 Figure 2 Cumulative frequency distribution of fourth grade near prism bar vergences, BI break (upper left), BI recovery (upper right), BO break (lower left), and BO recovery (lower right). mean. The suggested normal ranges in Table 4 for modified Thorington and prism bar vergences were derived in the same way using the data from the current study. As with Morgan s norms, half the SD was rounded up to the nearest whole number. The suggested normal values for modified Thorington would thus be the same for both first and fourth grade. The prism bar vergence test results show statistically significant differences between the first grade and fourth grade cohorts in the areas of distance base in recovery and near base in break. The differences between the means were 1.8 and 2.9, respectively, differences that might not be considered clinically significant. For instance, under ideal circumstances, 2 may be considered the smallest noticeable eye movement, 16 but, under normal conditions, the threshold limit for noticeable movement on cover test is about 3to Thus, it was not thought to be necessary to suggest separate norms for first and fourth grades for the 2 tests in question. For the vergence range data, when the means differed in the 2 grades, the range went from the lower mean minus one half SD to the higher mean plus one half SD. Figure 3 Cumulative frequency distribution of fourth grade distance prism bar vergences, BI break (upper left), BI recovery (upper right), BO break (lower left), and BO recovery (lower right). Discussion The Benton-IU project provides an opportunity to suggest normative data for the modified Thorington test at distance and near for school-aged children ages 6 to 11 (see Table 4). The oftenused Morgan s normal ranges were derived based on plus or minus one half SD from the The Benton-IU project prism bar vergence test results do differ somewhat from the results from Scheimanetal. 13 andtheresultsfromjiménezet al. 14 (see Table 5). One possible explanation for the differences between the results of Jiménez et al. 14 and ours could be the way in which the tests were performed. For near prism bar vergence testing, Jiménez et al. had the subjects place their heads in a chin restraint while looking at the target. We performed prism bar vergence testing in a more traditional clinical manner without restraining movement of the subject s head while performing near testing. The minor differences that we had with the Scheiman et al. results may be owing to their addition of a suppression target. Our target was a black and white target without suppression clues. To better understand why there are differences between the 3 studies on step bar vergences, future studies could be performed looking at the interexaminer repeatability and the effect that suppression targets may have on prism bar vergence results. The modified Thorington test is easy to administer as shown by the fact that data were ob- 597

6 Table 4. Normal ranges for modified Thorington and prism bar vergences based on the means plus minus one half SD from the Benton-IU study Phoria Base In break Base in recovery Base out break Base out recovery Distance 1 exo to 1 eso 5 to 10 2 to 7 8 to 16 4 to 10 Near 3 exo to 1 eso 10 to 20 7 to to 27 9 to 18 Eso, Esophoria; exo, exophoria. Table 5. Comparison of means (SD) from Benton, Scheiman et al., 13 and Jiménez et al. 14 Benton first grade Benton fourth grade Scheiman et al 13 Jiménez et al 14 Distance Phoria 0 (2) 0 (1) 0.6 (2) BI break 7 (4) 8 (4) 6 (2) BI recovery 4 (3) 5 (3) 4 (2) BO break 12 (7) 12 (7) 17 (7) BO recovery 6 (4) 7 (5) 11 (6) Near Phoria 1 (4) 1 (4) 0.4 (3) BI break 16 (7) 13 (6) 12 (5) 11 (3) BI recovery 10 (5) 9 (4) 7 (4) 7 (3) BO break 21 (11) 20 (11) 23 (8) 18 (8) BO recovery 13 (8) 14 (8) 16 (6) 13 (6) Eso, Esophoria; exo, exophoria. Note: For phorias, exo is negative in sign and eso is positive in sign. tained for 96% of the subjects for distance and 98% for near. Prism bar vergence data were obtained for more than 92% of the subjects. Given the ease of modified Thorington and prism bar vergence testing, the normal values suggested by this study and similar studies could be applied to children in first through fourth grades. Acknowledgment We wish to thank the administrators, parents, and students of Benton County Community School Corporation. Without their help this project would not have existed. We also wish to thank Richard Meetz, O.D., M.S., for his assistance with statistical analysis. References 1. Haines HF. Normal values of visual function and their application in case analysis. Part IV: the analysis of findings and determination of normals. Am J Optom Arch Am Acad Optom 1941;18: Haines HF. Normal values of visual functions and their application in case analysis. Part V: presenting a table of normal values of visual function. Am J Optom Arch Am Acad Optom 1941;18: Haines HF. Normal values of visual functions and their application in case analysis. Part VI: case typing and corrective procedure. Am J Optom Arch Am Acad Optom 1941;18: Betts EA, Austin AS. Seeing problems of school children. Optom Weekly 1940;31: Shepard CF. The most probable expecteds. Optom Weekly 1941;32: Morgan MW. The clinical aspects of accommodation and convergence. Am J Optom Arch Am Acad Optom 1944;21: Morgan MW. Analysis of clinical data. Am J Optom Arch Am Acad Optom 1944;21: Jackson TW, Goss DA. Variation and correlation of clinical tests of accommodative function in a sample of schoolage children. J Am Optom Assoc 1991;62(11): Wesson MD. Normalization of prism bar vergences. Am J Optom Physiol Opt 1982;59(8): Morris FM. The influence of kinesthesis upon near heterophoria measurements. Am J Ophthalmol 1960;37: Rainey BB, Schroeder TL, Goss DA, et al. Reliability of and comparisons among three variations of the alternating cover test. Ophthalmic Physiol Opt 1998;18(5): Hirsch MJ. The effect of testing method on values obtained for phoria at forty centimeters. Am J Optom Arch Am Acad Optom 1948;25:

7 13. Scheiman M, Herzberg H, Frantz K, et al. A normative study of step vergence in elementary schoolchildren. J Am Optom Assoc 1989;60(4): Jiménez R, Pérez MA, García JA, et al. Statistical normal values of visual parameters that characterize binocular function in children. Ophthalmic Physiol Optics 2004;24(6): Watson CS, Kidd GR, Horner DG, et al. Sensory, Cognitive, and Linguistic Factors in the Early Academic Performance of Elementary School Children: The Benton-IU Project. J Learning Disabilities 2003;36(2): Romano PE, von Noorden GK. Limitations of cover test in detecting strabismus. Am J Ophthalmol 1971;72(1): von Noorden GK. Burian-von Noorden s binocular vision and ocular motility: theory and management of strabismus. St. Louis: Mosby; 1980: Corresponding author: Don W. Lyon, O.D. Indiana University School of Optometry 800 East Atwater Avenue Bloomington, Indiana dwlyon@indiana.edu 599