Effects of 0.2% brimonidine and 0.2% brimonidine-0.5% timolol on intraocular pressure and pupil size in normal equine eyes.
Authors: Von Zup M, Lassaline M, Kass P H, Miller P E, Thomasy S M
Journal: Equine veterinary journal
Summary
# Brimonidine and Equine Glaucoma: A Safety and Efficacy Study Whilst brimonidine, an α2-adrenergic agonist that reduces aqueous humour production, shows promise in human glaucoma management, its effects in equine eyes remained unknown prior to this 2017 investigation. Von Zup and colleagues used a balanced crossover design across 16 healthy horses, with one eye receiving either 0.2% brimonidine or 0.2% brimonidine-0.5% timolol and the contralateral eye serving as control (treated with balanced salt solution), measuring intraocular pressure three times daily via rebound tonometry over two 10-day study phases separated by an 18-day washout. Neither drug produced clinically meaningful IOP reductions in normal eyes (brimonidine averaged 25.6 mmHg versus 24.5–24.6 mmHg in controls—a difference of ≤1 mmHg), and no adverse effects or pupil changes occurred with either formulation. The authors conclude that normal equine eyes may not demonstrate the same drug sensitivity as glaucomatous ones, suggesting that whilst brimonidine and its combination are well tolerated, further investigation in clinically glaucomatous horses is essential before recommendations can be made for therapeutic use in this population.
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Practical Takeaways
- •Brimonidine and brimonidine-timolol are safe and well-tolerated in normal horses with no clinically significant IOP reduction, so they may not be effective for routine IOP management in non-glaucomatous eyes
- •These findings suggest efficacy testing in glaucomatous equine eyes is warranted before dismissing these drugs as treatment options for equine glaucoma
- •Practitioners should not expect meaningful IOP lowering with these medications in horses with normal baseline pressures
Key Findings
- •Brimonidine alone produced average IOP of 25.6 mmHg compared to 24.5 mmHg in control eyes (BSS), a difference of ≤1 mmHg that is not clinically significant
- •Brimonidine-timolol combination produced average IOP of 24.6 mmHg, also not significantly different from controls
- •No adverse effects or significant changes in pupil size were observed with either brimonidine or brimonidine-timolol treatment in normal equine eyes
- •Normal horses appear less sensitive to IOP-lowering effects of these medications compared to glaucomatous animals, limiting treatment efficacy in this population