Evaluation of indices for the assessment and classification of keratoconus based on optical coherence tomography and Scheimpflug technology

Abstract

Purpose: To compare the parameters and indices of a novel swept-source optical coherence tomography device (SS-OCT, ANTERION) with those of a rotating Scheimpflug camera (RSC)-based tomograph (Pentacam) in normal and keratoconic (KC) eyes.

Methods: This prospective, monocentric, cross-sectional study included individuals with unoperated normal and KC eyes, selecting one eye per subject. Ectasia-specific parameters analysed with the SS-OCT were difference in mean keratometry (K_mean) in the inferior and superior meridians, maximum keratometry value (K_max), elevation of the posterior surface at the thinnest point, screening corneal objective risk of ectasia (SCORE) and thinnest point thickness. With the RSC, parameters determined were Belin/Ambrosio total deviation value (BAD-D), index of height decentration and index of vertical asymmetry. KC classification with the SS-OCT was based on the anterior and posterior radii of curvature and thinnest point thickness according to the ABCD classification of the RSC system.

Results: This study included 117 individuals with healthy eyes and 335 eyes with KC. The indices with the highest diagnostic discriminatory ability between the two cohorts were SCORE, difference of K_mean in the inferior and superior meridians and posterior elevation of the thinnest point (SS-OCT), as well as the index of height decentration, index of vertical asymmetry and BAD-D (RSC). The classifications using SS-OCT defined mild-stage KC as K_max, posterior elevation of the thinnest point and thinnest point thickness as ≤50.9 D, ≤30 and ≥472 μm, respectively. Moderate stage values were 51-55.9 D, 31-69 and 471-438 μm, respectively, while respective advanced stage were ≥56 D, ≥70 and ≤437 μm.

Conclusion: The diagnostic capabilities for both devices were found to be comparable. KC classification using SS-OCT can be independently based on the anterior surface, posterior surface and corneal thickness.

Keywords: ANTERION; Pentacam; corneal tomography; epithelium thickness; keratoconus; swept‐source optical coherence tomography.

FIGURE 1

‘Ectasia’ view of the swept‐source optical coherence tomography (SS‐OCT) device with axial curvature map of the anterior corneal surface, corneal thickness map, elevation map of the anterior and posterior surfaces (left), Screening Corneal Objective Risk of Ectasia (SCORE) value (top right), radar map (centre right) and corneal thickness distribution (bottom right).

FIGURE 2

Overview of the radar map parameters. (a) K _max: maximum keratometry value of the anterior surface; (b) K _max—opposite K: difference of the maximum keratometry value and the opposite keratometry value; (c) Inf–sup K mean: difference of the mean keratometry value below (inferior) and above (superior) the merdian; (d) anterior irregularity (3 mm): irregularity of the axial curvature in the central 3 mm ring; (e) posterior elev. of the thinnest point: posterior elevation at the thinnest point of the cornea; (f) thinnest point: thinnest point of the cornea.

FIGURE 3

(a) Box‐plot of the single parameters of the swept‐source optical coherence tomography (SS‐OCT) depending on the ABC classification. Ant, anterior; A parameter, staging of the anterior corneal surface from 0 to 4; B‐parameter, staging of the posterior corneal surface from 0 to 4; C parameter, staging of the corneal thickness from 0 to 4; inf, inferior; posterior elevation of thinnest point, elevation data of the posterior corneal surface at the thinnest point of the cornea; K, keratometry; K _max, maximum keratometry value; K _mean, mean keratometry value; sup, superior; Thinnest point thickness, minimum corneal thickness. (b) Box‐plot of the Screening Corneal Objective Risk of Ectasia (SCORE), SS‐OCT parameter as a function of the ABC classification.

FIGURE 4

Receiver operating characteristic curves of the parameters with the highest discriminatory power between healthy and keratoconic eyes. Ant, anterior; BAD‐D, Belin/Ambrosio total value; index of height decentration, Index of Height Decentration; Inf, inferior; IHD, Index of height decentration; IVA, Index of Vertical Asymmetry; K _mean, mean keratometry value; Posterior (Post) elevation (Elev) of thinnest point, elevation data of the posterior corneal surface at the thinnest point of the cornea; SCORE, Screening Corneal Objective Risk of Ectasia; SS‐OCT, swept‐source optical coherence tomography; sup, superior.