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M.A. Ciuffreda, K.J. Ciuffreda, B. Wang; Variability and Repeatability of Near Fusional Convergence Endpoint . Invest. Ophthalmol. Vis. Sci. 2005;46(13):2927.
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Purpose: A primary measure used in the daignosis and treatment evaluation of patients with convergence insufficiency (CI) is the near fusional endpoint. This refers to the maximum amount of base–out prism tolerated before fusion is disrupted. Despite the clinical importance of this parameter, there has been considerable debate regarding both its variability and repeatability. The present study was conducted to address this issue. Methods: Three experienced adult subjects, ages 26, 31, and 57 years, were tested in both free–space(FS) and the phoroptor(P). All testing was done with habitual near prescription in place using a martix of 20/30 Snellen letters at 40 cm. Vergence ranges were obtained once a week for 10 consecutive weeks taken at the same time of day. Each week, the base–out blur, break, and recovery points (in prism dioptors) were measured 3 times and averaged, as sequentially performed clinically. The same examiner was used for each subject during all 10 weeks. All measurements were counterbalanced within and across test sessions. The maximum prism dioptor power was 40. Results: Analysis of the base–out fusional endpoints in both FS and P revealed relatively consistent and repeatable values over time. The mean endpoint and S.D. over the 10 week period in FS for each subject were: 40.0 and 0.0, 39.5 and 0.50, 37.7 and 2.71, respectively. In the P, mean values were lower: 32.4 and 1.31, 35.5 and 0.95, 29.0 and 1.81, respectively. The ratio of the group mean standard deviation to the mean endpoint over time ranged in percentage from 0 to 7.19% in FS and from 2.68 to 6.24% in P. The group S.D. ratio was 2.82% for FS and 4.32% in P. The range of P and FS measurements over time was 10 prism dioptors or less for each subject. Conclusions: Variability both within and across test sessions was less than previously reported. In this experienced group, these values may approximate optimal performance that can then be used for clinical comparison. Understanding the inherent variability and repeatability of the base–out fusional endpoint is essential for the proper diagnosis of CI and other binocular vision disorders, and allows for smaller treatment–induced changes over time to be deemed as real improvment in convergence ability.
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