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.