VN September 2023

September 2023 37 In all species, this group of disorders is unified in their final common pathway of characteristic optic nerve and retinal pathology resulting in loss of vision. Glaucoma is therefore widely considered a neurodegenerative disease. Carbonic anhydrase inhibitors [CAIs] These are the most used antiglaucoma agents indicated for use in cats. The two commercially available products are dorzolamide and brinzolamide. They are sulphonamide-based CAIs. Carbonic anhydrase (CA) catalyzes the following reaction: CO2 + H2O ↔ HCO3 – + H + In the ciliary body, the formation of bicarbonate moves sodium and secondarily water into the eye, forming aqueous humor. Carbonic anhydrase inhibition decreases bicarbonate production and subsequently reduces the synthesis of aqueous humor. Dorzolamide 2.0% and brinzolamide 1 % significantly reduce both IOP and aqueous humor flow when administered to eyes q 8 h. The maximum IOP decrease occurs between 5 and 6.5 h post-treatment. • Indications Dorzolamide and brinzolamide are indicated in the treatment of both primary and secondary glaucoma in cats. • Cautions/adverse drug reactions Oral carbonic anhydrase inhibitors are classically known for their systemic inhibition of carbonic anhydrase. Carbonic anhydrase is found in high concentrations in red blood cells and kidney nephrons. Side effects reported inhumans includemetabolic acidosis, general malaise, fatigue, depression, hypokalaemia, blood dyscrasias, loss of appetite, gastrointestinal disturbances, weight loss, paresthesias, and renal calculi. The most frequent side effects associated with topical dorzolamide use in humans are ocular stinging, burning, or discomfort upon instillation. In cats, nearly 30% of cases develop hypokalaemia with long-term use of dorzolamide. No studies have been done exploring this side effect for brinzolamide, but it is safe to assume that this will also occur with long-term brinzolamide dosing in cats. Cats on topical CAIs should be monitored and possible oral supplementation of potassium started if hypokalaemia develops. The little to no ocular burning and stinging upon instillation, along with the same efficacy as dorzolamide makes brinzolamide a good candidate for use in animal patients. Beta-blockers Timolol, a nonselective beta-adrenergic blocking agent, reduces IOP by lowering the rate of aqueous humour production. No significant additive ocular hypotensive effect is observed in normal cats treated with a combination of the CAI, dorzolamide 2% and timolol 0.5%, relative to the effect of treatment with dorzolamide 2% alone. Beta-blockers also are ineffective at lowering IOP during sleep, due to lower sympathetic tone, which may reduce their efficacy in felines that nap frequently during the day and exhibit peak IOP during the nocturnal phase. Timolol may cause bradycardia and bronchoconstriction and is probably contraindicated in cats with feline asthma or cardiac disease. Adrenergic agonists Alpha-2 agonists such as apraclonidine and brimonidine may reduce IOP by increasing conventional outflow, decreasing aqueous humour production and reducing episcleral venous pressure. However systemic toxicity following topical application of 0.5% apraclonidine characterised by a mean reduction in heart rate of about 12%, and vomiting is very common precluding the use of the commercially available formulation in cats. Prostaglandin analogues Prostoglandin analogues include latanaprost, bimatoprost and travaprost. These prostaglandin analogues are prostanoid selective FP-receptor (receptors specific for PGFs) agonists that reduce IOP by increasing aqueous humor outflow through the uveoscleral pathways. Technical I Ophthalmology Column >>>38