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Vishal Jhanji, Gilda Lai, Marco Yu, Christopher Kai-Shun Leung; Test-retest variability of Corvis measurements in Normal and Keratoconus Eyes. Invest. Ophthalmol. Vis. Sci. 2014;55(13):3712.
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To compare the corneal deformation response (CDR), central corneal thickness (CCT), and intraocular pressure (IOP) measurements and their test-retest variabilities obtained with a high-speed Scheimpflug camera in normal and keratoconus eyes.
Three consecutive measurements were obtained using Corvis ST (Oculus Optikgeräte, Inc., Germany). The following parameters were analyzed: corneal deformation amplitude (maximum deformation of cornea), A1 length (length of flattened cornea at first applanation), A1 time (time from air-puff to first applanation), A2 length (length of flattened cornea at second applanation), A2 time (time from start to second applanation), peak distance (distance of two apices of cornea during highest concavity), and radius of corneal curvature (radius of curvature of a circle that fits to corneal concavity during maximum deformation). Intraclass coefficient (ICC), repeatability (RC) and coefficient of variation (CV) were evaluated.
12 normal and 12 keratoconus patients were included. Data from one eye of each participant was analyzed. Significant differences were found in the mean values of all parameters between keratoconus and normal eyes (IOP 11.92 vs 15.50; CCT 518.11 vs 568.50; A1 length 1.59 vs 1.76; A2 length 1.39 vs 1.67; A1 velocity 0.17 vs 0.15; A2 velocity 0.44 vs -0.34, deformation amplitude 1.25 vs 1.07; peak distance 5.29 vs 5.01; radius 5.61 vs 7.78) (all p≤0.050). The RCs of CCT, IOP and deformation amplitude were 12.84 μm, 3.08 mm Hg and 0.15 mm for normal eyes and 10.54 μm, 2.59 mm Hg and 0.11 mm for keratoconus eyes, respectively (p>0.05). The corresponding CVs were ≤7.85%, suggesting good repeatability for these parameters. Statistically significant differences were noted in the RCs for A1 length (0.51 vs 0.16; p<0.001), A2 length (0.93 vs 0.54; p=0.002), A1 velocity (0.03 vs 0.02; p=0.023), A2 velocity (0.19 vs 0.10; p=0.050), and peak distance (0.84 vs 0.34; p=0.011) between the two groups with smaller RCs found in normal eyes. ICCs were adequate for CCT (≥0.983) and good for IOP (≥0.776) and deformation amplitude (≥0.698). No significant difference in ICC was observed in both groups (p>0.05).
Corvis ST showed adequate repeatability for measuring CCT, IOP and corneal deformation amplitude in normal and keratoconus eyes. It may be employed to further understand the etiology of conditions associated with altered ocular biomechanical properties.
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