Abstract
Purpose: :
To compare differential pachymetry (DP), posterior elevation (PE), and anterior elevation (AE), using Scheimpflug system, in normal, subclinically keratoconic, and keratoconic corneas.
Methods: :
This prospective, comparative study included 235 eyes (79 normal, 9 subclinical keratoconus, 147 keratoconus) between April 2007 and September 2009. Keratoconic eyes were divided into four groups according to Amsler-Krumeich classification. Differential pachymetry (defined as central corneal thickness minus the pachymetry at the thinnest point of the cornea), PE, and AE were measured using the Pentacam system (Oculus Optikgeräte GmbH). Receivers operating characteristic curves were used to determine the test’s overall predictive accuracy and to identify optimal cutoff points to maximize sensitivity and specificity in discriminating keratoconus and subclinically keratoconus from normal corneas.
Results: :
The most effective parameter for discriminating keratoconus and subclinically keratoconus from normal corneas was PE, followed by DP and AE. All parameters had the smallest values in normal eyes and increased in subclinical keratoconus and each progressive stage of keratoconus. Mean PE and DP values were, respectively, -1.75 ± 4.03 µm and 2.68 ± 3.13 µm in normal corneas, 2.78 ± 3.83 µm and 7.44 ± 6.87 µm in subclinically keratoconus, and 20.80 ± 24.64 µm and 13.73 ± 20.76 µm in keratoconus. PE and DP cutoff points of 1.5 µm and 5.5 µm for keratoconus had sensitivities of 81.6% and 65.3% and specificities of 86.2% and 92.3%, respectively. There was a statistically significant difference between the normal eyes and the keratoconus group, for PE, DP and AE (p <. 001 for all of them).
Conclusions: :
PE and DP are efficacious in discriminating keratoconic from normal corneas and are less efficacious in discriminating subclinical keratoconus. PE and DP values are lowest in normal corneas and increase with disease progression.
Keywords: keratoconus • imaging/image analysis: clinical