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Shyam Sunder Tummanapalli, Haresh Potluri, Pravin K. Vaddavalli, Virender S. Sangwan; Efficacy Of Axial And Tangential Corneal Topography Maps In Diagnosing Keratoconus Suspects. Invest. Ophthalmol. Vis. Sci. 2012;53(14):122.
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© ARVO (1962-2015); The Authors (2016-present)
As tangential radius is sensitive in determining localized corneal curvature changes than the axial radius. We compared the efficacy of axial and tangential corneal topography maps obtained by using the Bausch & Lomb Orbscan IIz in distinguishing keratoconus suspect eyes from normal eyes.
Axial and tangential maps of anterior and posterior surface of cornea were determined and compared retrospectively in one eye of 67 keratoconus suspects (normal fellow eye of a clinically diagnosed keratoconus) and 72 normal subjects (control). The following four measurements were obtained from each map in both groups: i) the location of the corneal apex, relative to the center of the map, ii) the magnitude of corneal apex (maximum curvature), iii) corneal irregularity in the 3mm and 5mm zones and iv) the anterior-posterior ratio of maximum curvature of cornea. One-way ANOVA was performed for comparing variables. ROC curves were generated to determine the cut-off values for the most sensitive parameters. Pearson correlation coefficients were determined to assess the correlation between anterior and posterior corneal apex on axial and tangential maps.
The tangential anterior apex was consistently closer to the center of the map than the axial anterior apex in both groups. The tangential posterior apex curvature was statistically significantly higher than the axial posterior apex curvature in both groups (P<0.001). The correlation between the anterior and posterior curvature on tangential maps in both keratoconus suspect (r=-0.81) and normal groups(r=-0.82) were high. The anterior-posterior maximum curvature ratio on tangential map (cut-off -6.97 or more) had 98.5% of sensitivity and 100% of specificity and posterior irregularity indices of 3mm (cut-off 0.3 or more) had 94% of sensitivity and 68.1% of specificity and 5mm (cut-off 0.4 or more) had 95.5% of sensitivity and 83.3 % of specificity zone in discriminating normal eyes from keratoconus suspects.
Axial and tangential maps differ significantly in posterior apex curvature and anterior-posterior maximum curvature ratio in both groups. Overall, these results suggest that the anterior posterior ratio of maximum curvature of cornea on tangential map and 3mm and 5mm irregularities of posterior corneal surface are sensitive diagnostic parameters that may facilitate the early detection of keratoconus-suspect corneas
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