Determination of tear break-up time reference values and ocular tolerance of tetracaine hydrochloride eyedops in healthy horses

Reasons for performing study: Tetracaine hydrochloride (THCl) has been reported to cause irritation in dogs. In man, some topical anaesthetics have been shown to disrupt the tear film. Tear break‐up time (TBUT) is a useful test allowing an assessment of the quality of the precorneal tear film. Only one TBUT value has been reported in horses with no information on the technique used. Objectives: To provide a method for performing the TBUT in horses and to report any side effects of a single application of THCl in clinically normal horses, particularly on the stability of the tear film. Methods: In Study 1, one drop of 0.5 or 1% THCl was applied to one eye of 20 horses divided in 2 groups. Treated eyes were assessed for the development of side effects 2.5 and 5 min after treatment. In Study 2, the TBUT was measured in both eyes of 2 groups of 10 horses, before and 2.5 and 5 min after, instillation of one drop of either 0.5 or 1% THCl. Results: No animals developed any ocular side effect after instillation. Basal TBUT was 8.3 ± 1.3 s. TBUT decreased from baseline 5 and 2.5 min after application of one drop of 0.5% THCl and one drop of 1% THCl, respectively. Conclusions: A technique to measure the TBUT in healthy horses is described and normal range values that could be used as a reference were obtained. Potential relevance: THCl is well tolerated in horses but lowers the TBUT.