Topical medications, such as prostaglandins, miotics, β-blocking adrenergics, and topical carbonic anhydrase inhibitors, are the primary drugs for treatment of glaucoma (see Glaucoma), but these are often supplemented with systemic drugs.
In the emergency treatment of acute glaucoma, intraocular pressure must be reduced urgently. This is done pharmacologically using osmotic diuretics such as mannitol or glycerol in combination with other topical and systemic drugs. Osmotic diuretics are large-molecular-weight molecules that increase the osmotic pressure of plasma relative to the aqueous and vitreous. Most of the water in the eye is in the vitreous. Dehydration of the vitreous allows the lens and iris to move posteriorly, opening the iridocorneal angle. The other effect is to decrease formation of aqueous humor. Mannitol is given (1–1.5 g/kg, IV over 20–30 min), with the effect peaking in 2–3 hr and lasting up to 5 hr. Mannitol is not metabolized and thus can be used in diabetic animals. Glycerol (1–2 g/kg, PO) can be used but is unpalatable, and most dogs vomit. With both drugs, water should be withheld for 3–5 hr, and the animal should be given regular opportunities to urinate. Kidney and cardiac function should be checked before treatment, and cardiac function monitored during treatment. Mannitol can be used again within 8–12 hr if initial control of intraocular pressure is not maintained; longterm control is unlikely if intraocular pressures do not stay within the normal range after two treatments.
Oral carbonic anhydrase inhibitors are also used to treat and manage acute glaucoma. These inhibit the enzyme carbonic anhydrase in the nonpigmented ciliary epithelium responsible for catalyzing the following reaction: CO2 + H2O ← carbonic anhydrase → H2CO3 ↔ H+ + HCO3–
The bicarbonate and sodium ions are actively transported into the anterior chamber, leading to passive movement of water. This mechanism produces 40%–60% of aqueous humor. Drugs used include methazolamide (2–4 mg/kg, PO, bid-tid), acetazolamide (5–8 mg/kg, PO, bid-tid), and dichlorphenamide (2–4 mg/kg, PO, bid-tid). Methazolamide is the drug of choice.
Maximal effect occurs 3–6 hr after administration. The most common adverse effect is a metabolic acidosis that causes panting. Other effects can include vomiting, diarrhea, and hypokalemia. Acetazolamide commonly causes anorexia. Potassium supplementation can be given with potassium bicarbonate or citrate (1–2 g/day) added to the food. Cats are more sensitive than dogs to the adverse effects of these drugs and need careful monitoring. In general, systemic carbonic anhydrase inhibitors are not recommended for use in cats.