Horses have large prominent corneas, the surface of which is predisposed to a variety of diseases such as superficial keratitis and epithelial keratopathy. Clinically normal corneas have a precorneal tear film that protects and nourishes the ocular surface and helps provide an optically clear surface for vision. In horses, keratopathies without obvious etiologies (eg, infectious, parasitic, or neoplastic1,2) are often presumed to be immune mediated on the basis of their variable response to topical administration of corticosteroids.3,4 The underlying cause frequently cannot be identified for many of those presumptive immune-mediated keratopathies,4 which suggests that those conditions would be more aptly described as idiopathic keratopathies. In humans, many corneal abnormalities such as Sjögren and non-Sjögren dry eye,5 anterior blepharitis,5 meibomian gland dysfunction,6 superior limbic keratoconjunctivitis,7 diabetic keratoepitheliopathy,8 and graft-versus-host disease dry eye9 cause pathological changes similar to those in horses with idiopathic keratopathies and are associated with changes in the tear film osmolality and composition. We theorized that some keratopathies in horses may also be associated with tear film abnormalities.
Deficiencies in the aqueous component of the tear film are referred to as quantitative deficiencies, are diagnosed on the basis of STT results, and have been infrequently described in horses.10–12 Abnormalities of the biochemical or biophysical properties of the tear film are referred to as qualitative deficiencies, can cause tear film instability, and are more subtle clinically than are quantitative deficiencies This makes them difficult to diagnose13 and likely under recognized.14 Simple and easily repeatable methods for assessment of the quantitative and qualitative properties of the tear film in horses would facilitate the diagnosis and monitoring of subtle tear film and corneal abnormalities and provide a basis for the development of appropriate treatment regimens. In human medicine, tear osmolarity or osmolality (which are interchangeable parameters in aqueous solutions such as the tear film) is considered to be an objective variable for the assessment of qualitative and quantitative tear film characteristics.15,16 In addition to osmolality, the ionic composition of the tear film may affect its qualitative properties. Divalent cation concentrations have been linked to increased stability in the tear film of rabbits but not of humans.17 Because of the relatively large exposed surface area of the equine cornea, horses require tear film stability for ocular health. We hypothesized that the tear film in horses would have a relatively low osmolality and high divalent cation concentration to promote tear film stability. The purpose of the study reported here was to determine the tear film osmolality and electrolyte composition in healthy horses.
Supported by the University of Tennessee Companion Animal Fund.
Presented as a poster at the 45th Annual Conference of the American College of Veterinary Ophthalmologists, Fort Worth, Tex, October 2014.
Schirmer tear test
Indoor humidity monitor 613, AcuRite, Lake Geneva, Wis.
Osmometer 5520, Wescor Inc, Logan, Utah.
Cobas c 501, Roche Diagnostics, Indianapolis, Ind.
Korth RME, Romkes G, Eule JC. Tear film osmolarity as a diagnostic tool in small animal and equine medicine? (abstr) Vet Ophthalmol 2010;13:349.
TearLab Osmolarity System, OcuSense, San Diego, Calif.
2. Andrew SE, Willis AM. Diseases of the cornea and sclera. In: Gilger BC, ed. Equine ophthalmology. St Louis: Elsevier Health Sciences, 2005;157–251.
4. Gilger BC, Michau TM, Salmon JH. Immune-mediated keratitis in horses: 19 cases (1998–2004). Vet Ophthalmol 2005; 8: 233–239.
5. Ousler GW, Gomes PJ, Welch D, et al., Methodologies for the study of ocular surface disease. Ocul Surf 2005; 3: 143–154.
6. Abelson MB, Ousler GW, Nally LA, et al., Alternative reference values for tear film break up time in normal and dry eye populations. Adv Exp Med Biol 2002; 506 (Pt B): 1121–1125.
8. Inoue K, Kato S, Ohara C, et al., Ocular and systemic factors relevant to diabetic keratoepitheliopathy. Cornea 2001; 20: 798–801.
9. Khanal S, Tomlinson A. Tear physiology in dry eye associated with chronic GVHD. Bone Marrow Transplant 2012; 47: 115–119.
10. Van Kampen KR, James LF. Ophthalmic lesions in loco-weed poisoning of cattle, sheep, and horses. Am J Vet Res 1971; 32: 1293–1295.
11. Spurlock SL, Spurlock GH, Wise M. Keratoconjunctivitis sicca associated with fracture of the stylohyoid bone in a horse. J Am Vet Med Assoc 1989; 194: 258–259.
12. Spiess BM, Wilcock BP, Physick-Sheard PW. Eosinophilic granulomatous dacryoadenitis causing bilateral keratoconjunctivitis sicca in a horse. Equine Vet J 1989; 21: 226–228.
14. Ollivier FJ. The precorneal tear film in horses: its importance and disorders. Vet Clin North Am Equine Pract 2004; 20: 301–318.
15. Lemp MA, Bron AJ, Baudouin C, et al., Tear osmolarity in the diagnosis and management of dry eye disease. Am J Ophthalmol 2011; 151: 792–798.
16. Sullivan BD, Whitmer D, Nichols KK, et al., An objective approach to dry eye disease severity. Invest Ophthalmol Vis Sci 2010; 51: 6125–6130.
17. Wei XE, Markoulli M, Millar TJ, et al., Divalent cations in tears, and their influence on tear film stability in humans and rabbits. Invest Ophthalmol Vis Sci 2012; 53: 3280–3285.
18. Tomlinson A, Khanal S, Ramaesh K, et al., Tear film osmolarity: determination of a referent for dry eye diagnosis. Invest Ophthalmol Vis Sci 2006; 47: 4309–4315.
19. Mudgil P, Millar TJ. Surfactant properties of human meibomian lipids. Invest Ophthalmol Vis Sci 2011; 52: 1661–1670.
20. Dartt DA. Formation and function of the tear film. In: Leven LA, Nilsson SFE, VerHoeve J, eds. Adler's physiology of the eye. 11th ed. St Louis: Elsevier Health Sciences, 2011;350–362.
21. Davis K, Townsend W. Tear-film osmolarity in normal cats and cats with conjunctivitis. Vet Ophthalmol 2011; 14: 54–59.
22. Chen T, Ward DA. Tear volume, turnover rate, and flow rate in ophthalmologically normal horses. Am J Vet Res 2010; 71: 671–676.
23. Beech J, Zappala RA, Smith G, et al., Schirmer tear test results in normal horses and ponies: effect of age, season, environment, sex, time of day and placement of strips. Vet Ophthalmol 2003; 6: 251–254.
25. Chakraborti S, Mandal M, Das S, et al., Regulation of matrix metalloproteinases: an overview. Mol Cell Biochem 2003; 253: 269–285.
26. Wang C, Zhan C, Cai Q, et al., Expression and characterization of common carp (Cyprinus carpio) matrix metalloproteinase-2 and its activity against type I collagen. J Biotechnol 2014; 177: 45–52.
27. Ollivier FJ, Gilger BC, Barrie KP, et al., Proteinases of the cornea and preocular tear film. Vet Ophthalmol 2007; 10: 199–206.
28. Maidment DC, Kidder DE, Taylor MN. Electrolyte and protein levels in bovine tears. Br Vet J 1985; 141: 169–173.
32. Maïssa C, Guillon M. Tear film dynamics and lipid layer characteristics—effect of age and gender. Cont Lens Anterior Eye 2010; 33: 176–182.
33. Maruyama K, Yokoi N, Takamata A, et al., Effect of environmental conditions on tear dynamics in soft contact lens wearers. Invest Ophthalmol Vis Sci 2004; 45: 2563–2568.
34. Piccione G, Giannetto C, Fazio F, et al., Daily rhythm of tear production in normal horse. Vet Ophthalmol 2008; 11 (suppl 1): 57–60.
35. Terry JE, Hill RM. Human tear osmotic pressure: diurnal variations and the closed eye. Arch Ophthalmol 1978; 96: 120–122.
36. Niimi J, Tan B, Chang J, et al., Diurnal pattern of tear osmolarity and its relationship to corneal thickness and deswelling. Cornea 2013; 32: 1305–1310.