Effect of topical ophthalmic dorzolamide(2%)-timolol(0.5%) solution and ointment on intraocular pressure in normal horses

Objective: To compare the effect of commercially available solution and compounded ointment formulations of dorzolamide(2%)-timolol(0.5%) on intraocular pressure (IOP) of normal horses. Animals: Eighteen clinically normal horses. Procedures: A randomized, masked prospective design was used with horses divided into two equal groups. One eye of each horse was selected for topical ophthalmic treatment with either 0.2mL of dorzolamide(2%)-timolol(0.5%) solution or 0.2g of dorzolamide(2%)-timolol(0.5%) ointment every 12h for 5days. The contralateral eye of horses in both groups was untreated. Rebound tonometry was performed every 6h starting 2days prior to and ending 2days after the treatment period. Results: The mean IOP reduction in eyes treated with the solution or ointment formulations was 13%. Untreated eyes in both groups experienced a lesser but still statistically significant reduction in IOP. The IOP values did not return to baseline within 48h of the last treatment. Conclusions and Clinical Relevance: The commercially available solution and compounded ointment formulations of ophthalmic dorzolamide(2%)-timolol(0.5%) had similar effects on IOP in normal horses. Persistent IOP reduction following cessation of treatment may indicate prolonged drug effect or acclimation of horses to tonometry.