Differences in central and non-central keratoconus, and their effect on the objective screening thresholds for keratoconus

Purpose: To evaluate the differences in central and non-central keratoconus (based on cone location), and their effect on the objective screening thresholds for keratoconus. Methods: This comparative case series was performed at tertiary care cornea and refractive surgery service. Three groups were made: KC apex within central 2 mm (central keratoconus, n = 50), apex outside central 2mm (non-central keratoconus, n = 50) and normal controls (n = 100, with 50 cases each with apex within and outside central 2 mm). All cases underwent clinical evaluation and corneal topography (CSO, Sirius, Italy). Apex keratometry (ApexK), simulated keratometry at 3 mm (SimK), central corneal thickness (CCT) and minimum corneal thickness (MCT), anterior corneal higher-order aberrations root mean square (HOARMS), and Zernike's coefficients up to fourth order at different zones were measured. Results: In spite of the keratoconic groups having comparable ApexK (p > 0.05), central keratoconus had higher SimK and thinner CCT and MCT (p < 0.001). HOARMS was significantly more for central keratoconus at 3 mm zones. These findings had moderate to large effect size (Cohen's d). Receiver operating curve analysis was carried out to compare central keratoconus and non-central keratoconus with control group. ApexK and HOARMS had best discriminative parameters. Using single parametric suspicion cut-offs of 'either SimK steep > 47.2 D or CCT < 491.6 mu' had a good sensitivity (0.98) for central keratoconus, but not for non-central keratoconus (0.80). Changing this cut-off to 'either SimK steep K >= 45.8 D or CCT <= 503 mu' gave a sensitivity and specificity of 0.95 and 0.87 for non-central keratoconus and 0.99 and 0.87 for central keratoconus. Conclusion: Non-central keratoconus has lesser effect on SimK, pachymetry and smaller-aperture HOARMS. Using 'SimK steep > 47.2 D or CCT < 491.6 mu' may miss timely referral for topography in many of these cases. Using more stringent criteria of SimK steep K >= 45.8 D or CCT <= 503 mu to get a corneal topography done to rule out keratoconus is recommended, especially in cohorts with higher risk.