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G. Patel, M. A. Qazi, A. M. Mahmoud, P. S. Kollbaum, M. M. Merchea, C. J. Roberts, J. S. Pepose; Application of Multiple Diagnostic Modalities to Differentiate Between Corneal Ectasia Types. Invest. Ophthalmol. Vis. Sci. 2009;50(13):5097.
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To demonstrate how scanning slit videokeratography (Orbscan II, Bausch & Lomb) and wavefront aberrometry (Zywave IIz, Bausch & Lomb) may be used for clinical discrimination of corneal ectasias, such as keratoconus (KCN) and pellucid marginalis (PMD).
A retrospective review identified patients with a clinical diagnosis of KCN or PMD, and a control group of non-ectatic preoperative refractive surgery eyes. Orbscan exams from these eyes (n=72 KCN, 26 PMD, 105 controls) were analyzed to obtain corneal aberrations and topographical and pachymetric characteristics. Shack-Hartmann whole eye aberrations were also evaluated (5 mm).
Minimum optical pachymetry was lower in KCN (473 +/-88 µm) than PMD (548 +/-74) eyes; the thinnest pachymetry point in PMD (-1.27 +/-1.07 mm) was more inferiorly displaced than KCN (-0.71 +/-0.47, p<0.001), with both groups differing from normals (-0.26 +/-0.39, p<0.001). While the 3 mm zone irregularity index was higher in KCN than PMD (p=0.002), there was no difference for the 5 mm zone. There was no difference in the maximum value above the anterior elevation best fit sphere (BFS) between PMD and KCN (p=1.0); the maximum value of the posterior elevation BFS was higher for KCN (0.116 +/-0.05 mm) than for PMD (0.088 +/-0.07, p=0.02). The cone location magnitude index (CLMI), derived from mean curvature maps, was greater for both KCN and PMD relative to controls (p<0.001), with greater radial displacement of the steepest region from the ordinate center in PMD than KCN (p<0.001). Whole eye trefoil was greater in PMD than KCN (p=0.04), while vertical coma was higher in KCN than PMD (p<0.02).
The measured magnitude and location of the ectasia appear to be key differentiators of KCN and PMD. These differences are highlighted by both Orbscan and whole-eye aberrometry measurements. Metrics attained from videokeratography and wavefront aberrometry offer a framework to differentiate between ectasia types, allowing clinicians to provide improved treatment plans.
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