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

Equine IOP shows modest circadian variation peaking at 10 p.m.; CCT changes are statistically insignificant clinically.

This study investigated circadian variations in central corneal thickness (CCT) and intraocular pressure (IOP) in healthy horses, as these relationships remain unexplored in equine ophthalmology. Twelve horses (six geldings, six mares) underwent rebound tonometry and ultrasonic pachymetry measurements every 3 hours over a 24-hour period during summer photoperiod conditions. Results demonstrated that CCT varied significantly across the 24-hour cycle, with measurements being approximately 4.47 μm thicker during dark phases compared to light phases. However, this variation, while statistically significant, was deemed clinically irrelevant. IOP also exhibited significant circadian variation, with peak pressures occurring at 10 p.m. (15 hours), approximately 3.96 mmHg higher than morning readings. Notably, no significant correlation was identified between CCT and IOP at any measured time point. The findings suggest that equine ocular physiology demonstrates species-specific circadian rhythms distinct from other species, with modest but predictable IOP fluctuations peaking near the end of the light phase. These results have implications for standardizing IOP measurement protocols and interpretation in equine clinical practice.

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