Effects of 0.2% brimonidine and 0.2% brimonidine–0.5% timolol on intraocular pressure and pupil size in normal equine eyes

Abstract

Summary Background Brimonidine is an alpha 2-adrenergic agonist that decreases aqueous humor production and may increase uveoscleral outflow. It has not been evaluated in normal or glaucomatous equine eyes. Objectives To evaluate the IOP-lowering efficacy and safety of brimonidine, alone and in conjunction with timolol, as a treatment for equine glaucoma by comparing IOP in normal equine eyes treated with brimonidine and brimonidine-timolol to control eyes. Study design A balanced crossover design with 16 horses receiving one of two treatments, brimonidine and brimonidine-timolol, during each of two 10-day study phases, was used. Four horses were randomly assigned to each of four combinations of treated eye (right or left) and drug order within the two 10-day study phases (brimonidine first or brimonidine-timolol first). Methods Pupil size and conjunctival hyperemia were assessed twice daily and IOP was measured three times daily using rebound tonometry in both eyes of 16 normal horses throughout two 10-day study periods (brimonidine and brimonidine-timolol) followed by an 18-day washout period. One eye of each horse was treated with brimonidine or brimonidine-timolol while the opposite eye was treated with balanced salt solution. Results There were no adverse effects and no significant changes in pupil size in normal equine eyes treated with brimonidine or brimonidine-timolol. Average IOP in normal equine eyes treated with brimonidine (25.6 mm Hg) was statistically higher than in eyes treated with brimonidine-timolol (24.6 mm Hg) or balanced salt solution (24.5 mm Hg) however IOP differences were 1 mm Hg or less and thus not clinically important. Main limitations Horses with normal eyes may not be as sensitive to IOP-lowering effects of treatment as horses with glaucoma. Conclusions Brimonidine and briminonidine-timolol are well tolerated in normal horses but do not decrease IOP.

Comparison of two rebound tonometers in healthy horses

“…It has been previously noted that physiological variables, such as blood pressure, pulse, respiration, and anxiety, can cause major changes in IOP over short periods and can at least be partially diminished with sedation 13 . The variation in readings caused by changes in the alertness and attitude of the animal could have been minimized by an acclimation period, during which the horses would have become adapted to rebound tonometry using positive reinforcement as described by Von Zup et al 38 …”

Clinical insights: Ophthalmology in the eye of the beholder: Are we properly caring for horses with ocular disease?

“…Other studies in Equine Veterinary Journal have focussed on optimal devices and protocols for evaluating 10 and treating 11 increased intra‐ocular pressure (IOP). The rebound tonometer (Tonovet ™ , iCare) was found to be more reliable both for the same, and between, observer(s) than the applanation tonometer (Tonopen ™ , Reichert) in horses.…”

“…Von Zup et al 11 found that topical brimonidine (0.2%) caused an increase in IOP in normal horses, while brimonidine‐timolol topically had no statistically significant effect compared to topical balanced salt solution (placebo). Neither preparation had a clinically significant effect (changes were less than 1 mm Hg) but were demonstrated to be well tolerated.…”