Choice of dosing volume and frequency for topical administration of ophthalmic drugs used in the treatment of horses with ocular diseases has historically been largely empirical. A major impediment to the development of more quantitatively legitimate dosing regimens is that the volume and turnover rate of tears in healthy horses are unknown. Such information is important because the tear fluid has a dilutional effect on exogenously applied drugs. Therefore, determination of tear volume and flow rate in equine eyes will help establish more rigorous scientific standards for topical ophthalmic treatment regimens through better estimation of drug kinetics in tear film.
Most of the tear film is produced by the main lacrimal gland and the gland of the nictating membrane and ejected onto the ocular surface through multiple small ductules. Tears are distributed across the corneal surface by the sweeping actions of the eyelids and nictitating membrane, and they are removed from the ocular surface by movement into the nasolacrimal system, absorption into ocular tissues, and evaporation.1,2
The most commonly used method of assessing the lacrimal system in domestic animals is the STT, which involves use of a small strip of filter paper imbued with dye and placed in the inferior conjunctival cul-de-sac for 60 seconds. However, the STT does not measure the tear flow rate per se. It does yield a relative measure of tear film volume through comparison of the distance by which tears wick up the paper strip in a given animal to published reference values, but it does not measure an absolute volume.3 Studies4,5 in which the STT was used to assess relative tear volumes in horses have been performed to establish reference limits and to assess the influence of environmental factors and signalment on those values.
Whereas various techniques, including lacrimal scintigraphy,6 lacrimal streak dilution,7 and mathematical modeling,8,9 have been used to quantify absolute tear turnover and flow rates in humans, fluorophotometry has emerged as the gold standard.10–13 Mishima et al10 used fluorophotometry to measure rates of fluorescein decay in human eyes,10 and the mathematical treatment of the data has been corroborated by several investigators and remains the basis for most fluorophotometric tear flow studies. The purpose of the study reported here was to use fluorophotometry to measure the tear volume, turnover rate, and flow rate in ophthalmologically normal horses.
Natural logarithm of the fluorescein concentration
Minimal concentration required to inhibit the growth of 90% of organisms
Schirmer tear test
Seabuck Complete, SeaBuck Equine LLC, Salt Lake City, Utah.
Microcapillary tube, Fischer Brand, Fischer Scientific, Pittsburgh, Pa.
Ak-Fluor, Akorn, Buffalo Grove, Ill.
1.5-mL Eppendorf tube, Fischer Brand, Fischer Scientific, Pittsburgh, Pa.
Fluorotron Master, OcuMetrics, Mountain View, Calif.
Microsoft Excel 2008 for Mac, version 12.2.3, Microsoft Corp, Redmond, Wash.
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Hartley C, Williams DL, Adams VJ. Effect of age, gender, weight, and time of day on tear production in normal dogs. Vet Ophthalmol 2006;9:53–57.
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