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Identification of scanning slit-beam topographic parameters important in distinguishing normal from keratoconic
Baris Sonmez1, Minh-Phuong Doan, D Rex Hamilton
1The Jules Stein Eye Institute, David Geffen School of Medicine at University of California, Los Angeles, Los Angeles, California 90095, USA.
Purpose:
To identify morphologic parameters obtained using scanning slit-beam topography that help distinguish normal from keratoconic corneal morphologic features.
Design:
Observational, retrospective, cross-sectional study.
Methods:
This retrospective review examined 207 normal eyes of patients undergoing an initial consultation for primary refractive surgery and 42 eyes with clinical keratoconus (KCN). The following parameters were examined and compared between the two groups: astigmatism, central corneal power, irregularity indices at 3 mm (II3) and 5 mm (II5), maximal posterior elevation (MPE) magnitude and location, thinnest optical pachymetry (TOP) magnitude and location, anterior elevation best-fit sphere (ABFS), posterior elevation best-fit sphere (PBFS), the ratio of ABFS to PBFS, the difference between average inferior and average superior K values at 3 mm and 5 mm in both keratometric (I-S K3 and I-S K5) and tangential (I-S T3 and I-S T5) topographic maps, and skewed radial axis at 3 mm (SRAX3) and 5 mm (SRAX5) of the keratometric topography map.
Results:
The II3, II5, MPE magnitude, TOP magnitude, ABFS, PBFS, ABFS-to-PBFS ratio, I-S K at both 3 mm and 5 mm, I-S T at both 3 and 5 mm, and SRAX at 3 mm and 5 mm values were significantly different among the two groups (P < .001). The least-correlated parameters were SRAX3, TOP magnitude, and II3 in the KCN group and I-S K3, amount of astigmatism and MPE magnitude in the normal group.
Conclusions:
Parameters obtained using scanning slit-beam topography may allow improved differentiation of keratoconic from normal corneal shapes, especially when the poorly correlated intragroup parameters are used.
