Topographic Indices And Their Diagnostic Ability To Distinguish Keratoconus From High Astigmatism In Children

Keratoconus (KC) is a complex disease characterized by progressive thinning and a conical protrusion of the cornea. Early diagnosis allows closer monitoring of the disease which is crucial to prevent a decline in visual acuity, especially in pediatric population. The Pentacam produces indices of corneal properties that assist in diagnosing KC. However, it might lack representativeness of pediatric patients, especially highly astigmatic ones, in normative databases. The purpose of this study is to assess the accuracy and compare the various indices that can assist ophthalmologists in raising suspicions of KC in children with high astigmatism and to establish thresholds to distinguish those from patients with KC or fruste keratoconus (FK).

A multicentric cross-sectional study was conducted at Instituto Brasileiro de Oftalmologia, Rio de Janeiro, Brazil, Federal University of São Paulo, Brazil and University of Coimbra, Portugal. 

Patients were evaluated at Oculus Pentacam HR by the same investigator and categorized into four groups: KC, FK, astigmatism greater than two diopters (Cyl2D), and control. A subgroup of the Cyl2D group was created with patients with astigmatism greater than 4 diopters. Participants with a history of ocular diseases, ocular surgeries, ocular trauma, corneal scarring, or contact lens use were excluded. The investigated indices included anterior chamber parameters, curvature-based indices, elevation-based indices, pachymetry-based indices, combined indices, and aberrometry values. Receiver-operating characteristics (ROC) curves were employed to differentiate between groups and the optimum cutoff values were obtained based on the Youden index.

A total of 312 eyes from 167 patients (56.3% males) were evaluated. Of these, 90 patients (54%) were from Brazil and 77 (46%) were from Portugal. The mean age was 13.1 ± 3.2 years. The best set of indices to detect KC included anterior and posterior elevation, PPI-avg, Belin/Ambrosio enhanced ectasia, Ambrosio’s relational thickness maximum, IVA, and HOA. In the groups with higher levels of astigmatism, cutoff points to signal KC tended to increase, and AUC values tended to decrease. This was even more noted comparing FK with the control group - only 3 indices with AUC above 0.9 (total aberration RMS, HOA RMS of the entire and anterior cornea), and comparing FK with children with high astigmatism, with no item achieving an AUC above 0.9.

Highly astigmatic pediatric patients represent a particular challenge in KC diagnosis, with fewer valid criteria and higher cut-off values using the Pentacam system. In highly astigmatic patients, we recommend relying on specific indices, namely high-order aberration RMS, Belin/Ambrosio enhanced ectasia total derivation value, Ambrosio’s relational thickness maximum, average pachymetric progression index, and index of vertical asymmetry, in conjunction with clinical assessment to accurately stratify the risk for each patient. Our data also suggests that ophthalmologists should employ different cutoff points than those used in adult populations, especially when distinguishing KC in children with high astigmatism.