Evaluation of Circadian Variations of Central Corneal Thickness and Its Correlation With Intraocular Pressures in Healthy Horses

JUSTIFICATION: Circadian variations in central corneal thickness (CCT) and their correlation with intraocular pressure (IOP) have not been reported in horses. This information may be clinically relevant when interpreting IOP readings. PROCEDURES: Twenty-four eyes of 6 healthy adult gelded males and 6 healthy adult intact mares were evaluated. Rebound tonometry (TonoVet, Jorgensen Laboratories) followed by ultrasonic pachymetry (iPac Pachymeter, Reichert technologies) was performed in both eyes every 3 h over 24 h during the natural 14-h daylight summer photoperiod. RESULTS: Central corneal thickness varied significantly over time (F = 5.296, df = 8, 184, p < 0.0001), being significantly greater during the dark phase compared with the light phase (mean difference = 4.47 μm, 95% CI: 2.11 to 6.84; p = 0.0003). Intraocular pressure varied significantly over time (F = 2.886, df = 8, 184, p = 0.0047). The largest difference was observed between 0 h (7 a.m.) and 15 h (10 p.m.), with IOP being higher at 15 h by 3.96 mmHg (95% CI: 0.90 to 7.01). There was no significant correlation between CCT and IOP at any time point (β = -0.044 mmHg per μm increase in CCT, 95% CI: -0.100 to 0.013; p = 0.14). CONCLUSIONS: In healthy horses, CCT showed minimal circadian variation that was statistically significant but not clinically relevant and was not associated with intraocular pressure. Intraocular pressure exhibited a modest diurnal rhythm, peaking at the end of the light phase, supporting species-specific differences in circadian ocular physiology.