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Published on: November 12, 2015
Variability in Scheimpflug image-derived posterior elevation measurements in keratoconus and collagen-crosslinked
Georgios Labiris1, Athanassios Giarmoukakis, Haris Sideroudi
1Ophthalmology Department, University Hospital of Alexandroupolis, 68100 Dragana, Alexandroupolis, Greece. labiris@usa.net
Purpose:
To determine the variability in Scheimpflug image-derived posterior elevation measurements in eyes with keratoconus or corneal collagen crosslinking (CXL).
Setting:
Eye Institute of Thrace, Alexandroupolis, Greece.
Design:
Prospective observational study.
Methods:
Two operators in 2 sessions obtained posterior elevation measurements using the Pentacam Scheimpflug system. Variability in measurements was assessed using Bland-Altman analysis and intraclass correlation coefficients (ICCs). Measurements were associated with a series of Scheimpflug-derived keratoconus indices using the Pearson correlation and multivariate linear-regression analysis.
Results:
Mean interobserver differences for the central elevation point (e1) and the highest elevation point (HEP) in the keratoconus group were -0.9 μm (95% limits of agreement [LoA], 5.7 to -7.5 μm) and 1.7 μm (95% LoA, 26 to -22.6 μm), respectively. In the CXL group, the mean differences were 0.2 μm (95% LoA, 6.0 to -5.5 μm) and -6.1 μm (95% LoA, 16.8 to -29 μm), respectively. The ICC values in the keratoconus group were satisfactory for most elevation points; the ICC values were unsatisfactory for a series of points in the lower corneal hemisphere. The ICC values for points in both hemispheres were unsatisfactory in the CXL group; HEP had adequate reproducibility. The e1 and HEP parameters correlated significantly with most Scheimpflug-derived indices (P<.05) in the keratoconus group but not in the CXL group. Regression analysis indicated that the central keratoconus index was the primary determinant of the variability in keratoconus eyes.
Conclusions:
Scheimpflug-derived posterior elevation measurements had acceptable reproducibility in keratoconus eyes and post-CXL eyes. Less consistent measurements were obtained in the lower hemisphere.
