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A.A. Khanifar, M. Lai, R. Varma, LALES Group; Effect of Magnification Correction Factors on Optic Disc Measurements . Invest. Ophthalmol. Vis. Sci. 2003;44(13):3655.
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© ARVO (1962-2015); The Authors (2016-present)
Purpose: To compare optic disc measurements when using different methods of magnification correction. Methods: A sample of participants from the Los Angeles Latino Eye Study aged 40 years and older were included in this study. Simultaneous stereoscopic optic disc images were obtained on all participants. The optic disc margin was identified by the operator. The Topcon Image analyzer was used to measure optic disc parameters. The cup margin was defined as 150µm below the surface of the disc margin. The following optic disc parameters were measured: disc area, neural rim area, neural rim area-to-disc area ratio, vertical cup-to-disc ratio, and horizontal cup-to-disc-ratio. Four methods of magnification correction were studied on all participants. Method #1 (M1) using refractive error alone. Method #2 (M2) using both refractive error and axial length. Method #3 (M3) using both refractive error and corneal curvature. Method #4 (M4) using refractive error, axial length, and corneal curvature (Littmann method). The means for all parameters were compared using t-tests. Absolute differences between different methods were calculated by individual for each of the 3 combination of magnification correction factors (|M1-M4|, |M2-M4|, and |M3-M4|). All of the images were of good quality (% bad points < 25%). Results: Data from the right eyes of 96 individuals included in this analysis. The mean values ± SD for the measurements were: disc area 2.32 ± 0.46 mm² (M1), 2.18 ± 0.44 mm² (M2), 2.32 ± 0.46 mm² (M3), 2.21 ± 0.44 mm² (M4); neural rim area 1.74 ± 0.33 mm² (M1), 1.65 ± 0.34 mm² (M2), 1.73 ± 0.34 mm² (M3), 1.70 ± 0.34 mm² (M4); neural rim area-to-disc area ratio 0.76 ± 0.10 (M1), 0.77 ± 0.10 (M2), 0.75 ± 0.10 (M3), 0.78 ± 0.10 (M4); vertical cup-to-disc ratio 0.49 ± 0.12 (M1), 0.50 ± 0.12 (M2), 0.51 ± 0.12 (M3), 0.47 ± 0.13 (M4); and horizontal cup-to-disc ratio 0.41 ± 0.13 (M1), 0.40 ± 0.12 (M2), 0.41 ± 0.13 (M3), 0.39 ± 0.12 (M4). There were no statistically significant differences observed among the means of the 4 methods. There were no statistically significant relationship between refractive error and the absolute difference between any two methods for any parameter. Conclusions: No significant differences were seen among the 4 magnification correction methods in this sample. While there was a suggestion of difference in the means of various parameters measured using different methods of magnification correction studies using larger sample sizes should be conducted to confirm these results.
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