Methods: This study included three groups of subjects: patients with keratoconus (group 1), patients with keratoconus who underwent uneventful cross-linking (CXL) (group 2), and healthy volunteers (group 3). Each group included 24 eyes from 24 individuals. All subjects underwent corneal topography measurements three times a day (around 09:00 AM, 1:00 PM, and 5:00 PM) by the same experienced operator.

Results: The mean values of morning, noonday, and afternoon measurements for central corneal thickness (CCT) and thinnest corneal thickness (TCT) were 471.0 ± 28.1 µm and 461.0 ± 26.9 µm, respectively, in Group 1. In CCT and TCT, the change over the day was measured as 4.28 ± 1.1 µm and 4.09 ± 1.2 µm, respectively. Significant daily variation was observed in CCT and TCT measurements for Group 1 (p = 0.002 and p = 0.008, respectively). The highest value was observed at 9:00 AM, and the smallest value at 5:00 PM. No statistically significant diurnal variation was observed in any of the parameters of group 2 and group 3 (p > 0.05 for all).

Conclusion: This study found that keratoconic eyes show statistically significant diurnal variations in CCT and TCT measurements. It is appropriate to examine keratoconus patients at the same time during each visit. No diurnal changes in anterior segment parameters were observed in patients who underwent corneal CXL or in healthy individuals. Further studies are needed to clarify the cornea’s exact behavior throughout the day in patients with keratoconus. Keywords : Diurnal variation, Anterior segment, Scheimpflug imaging, Keratoconus, Cross-Linking