Fusional stamina: An alternative to Sheardâ€™s criterion
Sheard postulated that symptoms from heterophoria can be avoided if the fusional reserves in the opposite direction are at least twice the size of the phoria itself; this is known as â€œSheardâ€™s criterionâ€. Although later studies have provided supporting evidence for this postulate, dichotomous criteria of this type are subject to a lack of sensitivity since small differences in measurements can result in a change in classification. The main purpose of this study was to provide a reliable continuous alternative measure to Sheardâ€™s criterion in children; we have called this â€œFusional Staminaâ€. Heterophorias and positive fusional vergence (PFV) were measured in a group of 82 typical children aged 5â€“11 years. The suggested new variable, fusional stamina, was calculated by dividing PFV by two and then subtracting the (hetero-) phoria. Repeatability at near was checked after one year for a subgroup of 40 children assumed to be at greater risk for binocular vision deficits. Mean fusional stamina was 8.1 PD (Prism Dioptres base out) (SD = Â±5.9 PD) for distance and 12.3 PD (SD = Â±8.2 PD) for near. Repeatability at near was r = 0.62, p < 0.001, compared to r = 0.38, p = 0.02 for PFV break-value. Five children passed Sheardâ€™s criterion on only one of two repeated tests, but showed low fusional stamina on both tests. Normative values of fusional stamina for a group of typical children are presented. Repeatability for near is high and better than for PFV. Results demonstrate that there is a risk of missing binocular problems based only on passing Sheardâ€™s criterion. The continuous alternative of fusional stamina can be useful for researchers as well as clinicians in quantifying binocular vision problems and to monitor the effects of treatment.
Atzmon, D., Nemet, P., Ishay, A., & Karni, E. (1993). A randomized prospective masked and matched comparative study of orthoptic treatment versus conventional reading tutoring treatment for reading disabilities in 62 children. Bin Vis Eye Muscle Surg Q, 8(2), 91-106.
Berard, P. V., & Reydy, R. (1984). Treatment of heterophorias. Documenta Ophthalmologica, 58(1), 35-43. doi:10.1007/BF00140896
Ciuffreda, K. J., Rutner, D., Kapoor, N., Suchoff, I. B., Craig, S., & Han, M. (2008). Vision therapy for oculomotor dysfunctions in acquired brain injury: a retrospective analysis. Optometry-Journal of the American Optometric Association, 79(1), 18-22. doi:10.1016/j.optm.2007.10.004
Conway, M. L., Thomas, J., & Subramanian, A. (2012). Is the aligning prism measured with the Mallett unit correlated with fusional vergence reserves? PloS one, 7(8), e42832. doi:10.1371/journal.pone.0042832
Dalziel, C. C. (1981). Effect of vision training on patients who fail Sheard's criterion. American journal of optometry and physiological optics, 58(1), 21-23. doi:10.1097/00006324-198101000-00006
Daum, K. (1986). Double-blind placebo-controlled examination of timing effects in the training of positive vergences. Am J Optom Physiol Opt, 63(10), 807-812. doi: 10.1097/00006324-198610000-00003
Daum, K., Rutstein, R. P., Houston, G. I., Clore, K. A., & Corliss, D. A. (1989). Evaluation of a New Criterion of Binocularity. Optometry & Vision Science, 66(4), 218-228. doi:10.1097/00006324-198904000-00008
Evans, B. (2000). An open trial of the Institute Free-space Stereogram (IFS) exercises. British Journal of Optometry and Dispensing, 8(1), 5-14.
Evans, B. (2008). Optometric Prescribing for Decompensated Heterophoria, OiP, Volume 9, Issue 2, 2008, Bruce JW Evans. Optometry in Practice, 9(2), 63-78.
Evans, B., Busby, A., Jeanes, R., & Wilkins, A. J. (1995). Optometric correlates of Mearesâ€“Irlen syndrome: a matched group study. Ophthalmic and Physiological Optics, 15(5), 481-487. doi:10.1016/0275-5408(95)00063-J
Field, A. P. (2009). Discovering statistics using SPSS : (and sex and drugs and rock 'n' roll) (3rd ed.). Los Angeles ; London: SAGE.
Goss, D. A., & Becker, E. (2011). Comparison of near fusional vergence ranges with rotary prisms and with prism bars. Optometry, 82(2), 104-107. doi:10.1016/j.optm.2010.09.011
Goss, D. A., Reynolds, J. L., & Todd, R. E. (2010). Comparison of four dissociated phoria tests: reliability & correlation with symptom survey scores. J. Behavioral Optom, 21, 99-104.
Griffin, J. R., & Grisham, J. D. (2002). Binocular anomalies : diagnosis and vision therapy (4th ed.). Amsterdam ; London: Butterworth-Heinemann.
Hofstetter, H. W. (1945). The zone of clear single binocular vision: American Academy of Optometry.
Jackson, T. W., & Goss, D. A. (1991). Variation and correlation of standard clinical phoropter tests of phorias, vergence ranges, and relative accommodation in a sample of school-age children. J Am Optom Assoc, 62(7), 540-547.
Karania, R., & Evans, B. J. W. (2006). The Mallett Fixation Disparity Test: influence of test instructions and relationship with symptoms. Ophthalmic and Physiological Optics, 26(5), 507-522. doi:10.1111/j.1475-1313.2006.00385.x
Landolt, E. (1886). The refraction and accommodation of the eye and their anomalies. Edinburgh: Young J. Pentland.
Lie, I., & Opheim, A. (1985). Long-term acceptance of prisms by heterophorics. J Am Optom Assoc, 56(4), 272-278.
Percival, A. S. (1928). The prescribing of spectacles. Bristol, England: J. Wright and Sons.
Pestalozzi, D. (1975). Erfahrungen mit der Kombination von Polatest und Prismenvollkorrektion in der Behandlung von StÃ¶rungen des Binokularsehens. Ophthalmologica, 170(2-3), 274-279. doi:10.1159/000307219
Rainey, B. B., Schroeder, T. L., Goss, D. A., & Grosvenor, T. P. (1998). Inter-Examiner Repeatability of Heterophoria Tests. Optometry & Vision Science, 75(10), 719-726. doi:10.1097/00006324-199810000-00016
Rouse, M. W., Borsting, E., & Deland, P. N. (2002). Reliability of binocular vision measurements used in the classification of convergence insufficiency. Optom Vis Sci, 79(4), 254-264. doi:10.1097/00006324-200204000-00012
Scheiman, M., & Wick, B. (2008). Clinical management of binocular vision : heterophoric, accommodative, and eye movement disorders. Philadelphia: Wolters Kluwer Health/Lippincott Williams & Wilkins.
Sheard, C. (1930). Zones of ocular comfort. American Journal of Optometry(7), 9-25. doi:10.1097/00006324-193001000-00001
Sheedy, J. (1983). Validity of diagnostic criteria and case analysis in binocular vision disorders. Am J Optom 1980a, 57(9), 618-631.
Sheedy, J., & Saladin, J. (1978). Association of symptoms with measures of oculomotor deficiencies. American journal of optometry and physiological optics, 55(10), 670-676. doi:10.1097/00006324-197810000-00002