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First Author: S.Ortiz SPAIN
Co Author(s): V. de Juan G. Rodriguez D. Galarreta R. Martin
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Keratoconus is a progressive corneal disorder characterized by thinning and steepening of the central and paracentral cornea which leads to protrusion. Corneal protrusion causes high myopia and irregular astigmatism, affecting between 50 and 230 cases per 100,000 habitants. Placido disk-based videokeratography is the most extensively used type of corneal topographic assessment of corneal curvature in keratoconus, grading the disease severity and monitoring progression. Corneal topography analysis permits keratoconus patient management, with different approaches like specific designed contact lenses. Moreover, this analysis is of paramount importance in early diagnosis of keratoconus, especially in preoperative assessment of patients for corneal refractive surgery avoiding potentially undesirable side effects. Allegro-Topolyzer (WaveLight Technologie AG, Alcon Laboratories, Erlangen, Germany) is a computerized Placido-based videokeratography developed especially for the clinical management of refractive surgical patients (preoperative assessment, surgery and follow up). However, as far as our knowledge, the Allegro-Topolyzer corneal indices used in keratoconus assessment have not been evaluated previously comparing differences in healthy or keratoconus eyes and repeatability have been not described. The aim of this study is to determine and compare the indices and repeatability of the Allegro-Topolyzer in a sample of healthy and keratoconus eyes to improve the preoperative assessment of refractive patients.
IOBA-Eye Institute, University of Valladolid (Valladolid, Spain).
Fifty eyes were enrolled in the study and corneal topography was assessed with Allegro-Topolyzer. This device uses 22 rings to generate a high-resolution image of the corneal surface with 22,000 data points analyzed in each exploration. The corneal topography is portable and could be imported into the WaveLight Excimer Laser systems, allowing customized topography-guided treatments. Eyes were divided into two study groups: healthy eyes (n=25 eyes) and keratoconus eyes (n=25 eyes). Corneal topography was assessed with three consecutive measurements on each cornea determining the simulated keratometry (diopters) in the main corneal meridians [maximum (MaxP) and minimum (MinP) power] at the 3.0 mm zone, eccentricity, maximum corneal power point (MCPP) (diopters), index of surface variance (ISV), index of vertical asymmetry (IVA), keratoconus index (KI), smallest sagittal curvature radius (RMin), coma value and aberration coefficient (AbCoeff). Coefficient of variation (CV) and Bland and Altmans limits of agreement (LoA) were calculated for each topographic values collected in each study group. The mean value of the topographic variables and the CV differences between healthy and keratoconus eyes were compared with non-parametric Kruskal-Wallis ANOVA (P-value <0.05 was considered statistically significant).
Healthy eyes showed a low value (p<0.05) than keratoconus: [MaxP (43.98ḟ1.45D versus 48.80ḟ2.92D), MinP (43.20ḟ1.32D versus 45.98ḟ3.10D), MCPP (44.68ḟ1.31D versus 58.24ḟ5.90D), ISV (17.17ḟ4.48 versus 81.41ḟ29.73), IVA (0.12ḟ0.05 versus 0.93ḟ0.38), KI (1.02ḟ0.02 versus 1.22ḟ0.01), coma ([4.28ḟ1.83]x0.0001 versus [37.85ḟ18.18]x0.0001) and AbCoeff (0.00ḟ0.00 versus 2.23ḟ0.54)] except RMin (7.59ḟ0.23mm versus 6.23ḟ0.53mm). Eccentricity was similar (p=0.90) in healthy (0.49ḟ0.11) than keratoconus eyes (0.55ḟ0.34). All variables showed good CV in healthy and keratoconus eyes [MaxP (0.21% LoA -0.20 to 0.20 and 0.63% LoA -0.64 to 0.64 respectively), MinP (0.19% LoA -0.20 to 0.20 and 0.58% LoA -0.56 to 0.56), eccentricity (5.79% LoA -0.6 to 0.6 and 12.40% LoA -0.15 to 0.15), MCPP (0.59% LoA -0.69 to 0.69 and 1.19% LoA -1.90 to 1.90), ISV (4.82% LoA -1.75 to 1.75 and 3.58% LoA -6.68 to 6.68), IVA (7.05% LoA -0.02 to 0.02 and 3.95% LoA -0.07 to 0.07),KI (0.29% LoA -0.01 to 0.01 and 0.86% LoA -0.02 to 0.02), Rmin (0.53% LoA -0.08 to 0.08 and 0.93% LoA -0.14 to 0.14) coma (12.62% LoA -1.2x0.0001 to 1.2x0.0001 and 4.94% LoA -4x0.0001 to 4x0.0001) and AbCoeff (0% and 4.58% LoA -0.22 to 0.22)]. CV differences between groups were significant (p<0.05) for all variables, except in the ISV (p=0.15).
The Allegro-Topolyzer provides repeatable measurements of the principal corneal indices (simulated keratometry, eccentricity, MCPP, ISV, IVA, KI, RMin, coma and aberration coefficient) in healthy and keratoconus eyes, with differences between healthy and keratoconus eyes, that could improve topographical diagnosis of the keratoconus and helping the patient management. This Placido-based corneal topography is a repeatable and valuable tool for preoperative assessment of patients for refractive surgery and keratoconus patient management.