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Veterinary ophthalmology2026; 29(4); e70194; doi: 10.1111/vop.70194

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

Abstract: 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. Methods: 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.
© 2026 American College of Veterinary Ophthalmologists.
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Publication Date: 2026-05-20 PubMed ID: 42157424DOI: 10.1111/vop.70194Google Scholar: Lookup
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Summary

This research summary has been generated with artificial intelligence and may contain errors and omissions. Refer to the original study to confirm details provided. Submit correction.

Overview

  • This study examined daily fluctuations in central corneal thickness (CCT) and intraocular pressure (IOP) in healthy horses over a 24-hour period.
  • The goal was to determine whether changes in corneal thickness correlate with eye pressure variations, which is important for interpreting clinical IOP measurements.

Background

  • Central corneal thickness (CCT) is a measure of how thick the cornea is at its center, and it can affect intraocular pressure (IOP) readings.
  • IOP is a key parameter in eye health, especially relevant for diagnosing conditions like glaucoma.
  • In humans and some animals, CCT and IOP are known to show circadian (daily) variations, but such data were lacking for horses.
  • Understanding if CCT changes across the day and influences IOP can help veterinary ophthalmologists interpret tonometry results more accurately.

Methods

  • The study involved 12 healthy adult horses: 6 gelded males and 6 intact mares, with measurements taken from both eyes (24 eyes total).
  • Measurements were taken every 3 hours for 24 hours, covering both daylight (approximately 14 hours) and dark phases during summer.
  • IOP was measured using rebound tonometry, a quick and noninvasive method to assess eye pressure.
  • CCT was measured using ultrasonic pachymetry, which uses ultrasound to precisely measure corneal thickness.
  • Statistical analysis tested whether CCT and IOP changed significantly throughout the day and whether the two parameters correlated.

Results

  • CCT showed statistically significant variation across different times of day (p<0.0001), being thicker during the dark phase compared to the light phase by an average of ~4.47 micrometers.
  • Despite statistical significance, the magnitude of CCT change was minimal and likely not relevant clinically.
  • IOP also varied significantly during the day (p=0.0047), with the largest difference observed between 7 a.m. and 10 p.m.—the IOP was about 3.96 mmHg higher at night.
  • There was no significant correlation between CCT values and IOP at any measured time point, implying that corneal thickness does not meaningfully influence eye pressure readings in these horses.

Conclusions

  • Healthy horses have small but statistically significant circadian changes in corneal thickness, with the cornea being slightly thicker at night.
  • Intraocular pressure follows a modest diurnal rhythm, peaking toward the end of the light period and into the night, indicating species-specific regulation of eye pressure.
  • No clinically important association between variations in corneal thickness and intraocular pressure was found, suggesting that IOP measurements in horses are not confounded by minor fluctuations in CCT.
  • This information may assist veterinarians in timing and interpreting IOP measurements in horses without needing to adjust for corneal thickness changes across the day.
  • Species-specific differences highlight the importance of evaluating ocular parameters directly in horses rather than extrapolating from other species.

Cite This Article

APA
Iturregui C, Liu CC, Carter RT, Camacho-Luna P. (2026). Evaluation of Circadian Variations of Central Corneal Thickness and Its Correlation With Intraocular Pressures in Healthy Horses. Vet Ophthalmol, 29(4), e70194. https://doi.org/10.1111/vop.70194

Publication

Veterinary ophthalmology
ISSN: 1463-5224
NlmUniqueID: 100887377
Country: England
Language: English
Volume: 29
Issue: 4
Pages: e70194

Researcher Affiliations

Iturregui, Carolina
  • Department of Veterinary Clinical Sciences, School of Veterinary Medicine, Louisiana State University, Baton Rouge, Louisiana, USA.
Liu, Chin-Chi
  • Department of Veterinary Clinical Sciences, School of Veterinary Medicine, Louisiana State University, Baton Rouge, Louisiana, USA.
Carter, Renee T
  • Department of Veterinary Clinical Sciences, School of Veterinary Medicine, Louisiana State University, Baton Rouge, Louisiana, USA.
Camacho-Luna, Pilar
  • Department of Veterinary Clinical Sciences, School of Veterinary Medicine, Louisiana State University, Baton Rouge, Louisiana, USA.

MeSH Terms

  • Animals
  • Horses / physiology
  • Horses / anatomy & histology
  • Intraocular Pressure / physiology
  • Circadian Rhythm / physiology
  • Male
  • Cornea / anatomy & histology
  • Cornea / physiology
  • Female
  • Tonometry, Ocular / veterinary
  • Corneal Pachymetry / veterinary

References

This article includes 28 references
  1. Chan‐Ling T, Efron N, Holden BA. Diurnal Variation of Corneal Thickness in the Cat. Investigative Ophthalmology and Visual Science 26, no. 1 (1985): 102–105.
  2. Katz RS, Henkind P, Weitzman ED. The Circadian Rhythm of the Intraocular Pressure in the New Zealand White Rabbit. Investigative Ophthalmology 14, no. 10 (1975): 775–780.
  3. Martín‐Suárez E, Molleda C, Tardón R, Galán A, Gallardo J, Molleda J. Diurnal Variations of Central Corneal Thickness and Intraocular Pressure in Dogs From 8:00 am to 8:00 pm. Canadian Veterinary Journal 55, no. 4 (2014): 361–365.
  4. Zhou Q, Gao TY, Fan SJ. Intraocular Pressure, Age, and Central Corneal Thickness in a Healthy Chinese Children Population: The Handan Offspring Myopia Study. Ophthalmic Epidemiology 29, no. 5 (2022): 499–506.
  5. Park YW, Jeong MB, Kim TH. Effect of Central Corneal Thickness on Intraocular Pressure With the Rebound Tonometer and the Applanation Tonometer in Normal Dogs. Veterinary Ophthalmology 14, no. 3 (2011): 169–173.
  6. Pang K, Lennikov A, Yang M. Hypoxia Adaptation in the Cornea: Current Animal Models and Underlying Mechanisms. Animal Models and Experimental Medicine 4, no. 4 (2021): 300–310.
  7. Barleon L, Hoffmann EM, Berres M, Pfeiffer N, Grus FH. Comparison of Dynamic Contour Tonometry and Goldman Applanation Tonometry in Glaucoma Patients and Healthy Subjects. American Journal of Ophthalmology 142 (2006): 583–590.
  8. Fogagnolo P, Rossetti L, Mazzolani F, Orzalesi N. Circadian Variations in Central Corneal Thickness and Intraocular Pressure in Patients With Glaucoma. British Journal of Ophthalmology 90 (2006): 24–28.
  9. Kotecha A, Crabb DP, Spratt A, Garway‐Heath DF. The Relationship Between Diurnal Variations in Intraocular Pressure Measurements and Central Corneal Thickness and Corneal Hysteresis. Investigative Ophthalmology and Visual Science 50 (2009): 4229–4236.
  10. Shah S, Chatterjee A, Mathai M. Relationship Between Corneal Thickness and Measured Intraocular Pressure in a General Ophthalmology Clinic. Ophthalmology 106 (1999): 2154–2160.
  11. Doughty MJ, Zaman ML. Human Corneal Thickness and Its Impact on Intraocular Pressure Measures: A Review and Meta‐Analysis Approach. Survey of Ophthalmology 44, no. 5 (2000): 367–408.
  12. Tonnu PA, Ho T, Newson T. The Influence of Central Corneal Thickness and Age on Intraocular Pressure Measured by Pneumotonometry, Non‐Contact Tonometry, the Tono‐Pen XL, and Goldmann Applanation Tonometry. British Journal of Ophthalmology 89, no. 7 (2005): 851–854.
  13. Bertolucci C, Giudice E, Fazio F, Piccione G. Circadian Intraocular Pressure Rhythms in Athletic Horses Under Different Lighting Regime. Chronobiology International 26, no. 2 (2009): 348–358.
  14. Del Sole MJ, Sande PH, Bernades JM, Aba MA, Rosenstein RE. Circadian Rhythm of Intraocular Pressure in Cats. Veterinary Ophthalmology 10, no. 3 (2007): 155–161.
  15. Trim CM, Colbern GT, Martin CL. Effect of Xylazine and Ketamine on Intraocular Pressure in Horses. Veterinary Record 117, no. 17 (1985): 442–443.
  16. van der Woerdt A, Gilger BC, Wilkie DA, Strauch SM. Effect of Auriculopalpebral Nerve Block and Intravenous Administration of Xylazine on Intraocular Pressure and Corneal Thickness in Horses. American Journal of Veterinary Research 56, no. 2 (1995): 155–158.
  17. Holve DL, van der Woerdt A, Gilger BC, Wilkie DA, Strauch SM. Effect of Sedation With Detomidine on Intraocular Pressure With and Without Topical Anesthesia in Clinically Normal Horses. Journal of the American Veterinary Medicine Association 240, no. 3 (2012): 308–311.
  18. Ko YC, Liu CJ, Hsu WM. Varying Effects of Corneal Thickness on Intraocular Pressure Measurements With Different Tonometers. Eye 19, no. 3 (2005): 327–332.
  19. Plummer CE, Ramsey DT, Hauptman JG. Assessment of Corneal Thickness, Intraocular Pressure, Optical Corneal Diameter, and Axial Globe Dimensions in Miniature Horses. American Journal of Veterinary Research 64, no. 6 (2003): 661–665.
  20. Kikkawa Y. Diurnal Variation in Corneal Thickness. Experimental Eye Research 15, no. 1 (1973): 1–9.
  21. Madigan MC, Gillard‐Crewther S, Kiely PM. Corneal Thickness Changes Following Sleep and Overnight Contact Lens Wear in the Primate (Macaca fascicularis). Current Eye Research 6, no. 6 (1987): 809–815.
  22. Knickelbein KE, Lassaline ME, Kim S, Scharbrough MS, Thomasy SM. Corneal Thickness and Anterior Chamber Depth of the Normal Adult Horse as Measured by Ultrasound Biomicroscopy. Veterinary Ophthalmology 25, no. Suppl 1 (2022): 17–24.
  23. Liu JH, Dacus AC. Endogenous Hormonal Changes and Circadian Elevation of Intraocular Pressure. Investigative Ophthalmology & Visual Science 32, no. 3 (1991): 496–500.
  24. Qureshi IA, Huang YB, Xi XR, Wu XD. Hormonal Changes Associated With Menstrual Cycle Have no Definite Influence on Ocular Pressure. Proceedings of the National Science Council, Republic of China. Part B, Life Sciences 21, no. 2 (1997): 49–53.
  25. Qureshi IA, Xi XR, Wu XD, Pasha N, Huang YB. Variations in Ocular Pressure During Menstrual Cycle. Journal of the Pakistan Medical Association 48, no. 2 (1998): 37–40.
  26. Binesh S, Ghasemzadeh‐nava H, Raoofi A, Rajaei SM. Reproductive Status Effects on Intraocular Pressure in Mares. Veterinary and Animal Science 30 (2025): 100542.
  27. Mori K, Ando F, Nomura H, Sato Y, Shimokata H. Relationship Between Intraocular Pressure and Obesity in Japan. International Journal of Epidemiology 29, no. 4 (2000): 661–666.
  28. Meekins JM, Overton TL, Rankin AJ, Roush JK. Effect of Oral Administration of Carprofen on Intraocular Pressure in Normal Dogs. Journal of Veterinary Pharmacology and Therapeutics 39, no. 4 (2016): 244–249.

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