Diagnostic Value of Sirius Scheimpflug–Placido Corneal Tomography in Keratoconus and Subclinical Keratoconus

• Sirius+ Scheimpflug–Placido system enables multiparametric corneal photodiagnosis • Posterior symmetry index (SIb) best identifies subclinical keratoconus • SIb >0.11 D achieves 90% sensitivity and 85% specificity for subclinical KCN • Posterior corneal parameters outperform anterior indices in early ectasia • Posterior surface indices improve screening of refractive surgery candidates Purpose: To determine the diagnostic accuracy and optimal threshold values of anterior and posterior corneal parameters obtained with the Sirius+ Scheimpflug–Placido tomography system in detecting clinical keratoconus (KCN) and subclinical keratoconus (sKCN). Methods: In this cross-sectional study, 112 eyes (40 KCN, 32 sKCN, and 40 healthy controls) were classified based on clinical and tomographic findings. Corneal tomography was performed using the Sirius+ system under standardized conditions. The evaluated parameters included anterior and posterior symmetry indices (SIf, SIb), anterior and posterior keratoconus vertex values (KVf, KVb), and minimum corneal thickness (THKmin). Diagnostic performance was assessed using receiver operating characteristic (ROC) curve analysis, including area under the curve (AUC), sensitivity, specificity, and optimal cutoff values. Results: THKmin progressively decreased from controls to sKCN and KCN (528.08 ± 36.26, 479.30 ± 32.55, and 455.75 ± 37.76 µm, respectively; p ≤ 0.001). Conversely, SIf and SIb values increased significantly across groups (SIf: 0.21±0.36, 1.19±0.82, 3.86±3.09 D; SIb: 0.05±0.07, 0.37±0.39, 1.01±0.65 D; all p ≤ 0.001). Similarly, KVf and KVb values showed a progressive increase, with KVb demonstrating marked elevation in KCN (p < 0.001). For differentiating sKCN from controls, SIb showed the highest diagnostic performance (AUC = 0.934), with 90.0% sensitivity and 85.0% specificity at a cutoff value of >0.11 D. Conclusion: Posterior corneal parameters derived from Sirius+ tomography, particularly SIb and KVb, demonstrated superior diagnostic performance in detecting both clinical and subclinical keratoconus. Posterior surface–based multiparametric analysis may improve early detection and should be incorporated into routine tomographic evaluation.