Lysine

For cats that suffer frequent bouts of recrudescent disease, lysine may help. While lysine alone will not eliminate a herpesvirus infection, lysine can decrease the severity of clinical signs and increase the time between recurrences.¹² Lysine is typically dosed at 250 mg orally twice daily for kittens and lifelong at 500 mg orally twice daily for adult cats. Lysine is safe; however, it should be administered with food since it may induce vomiting if given on an empty stomach.¹² Several veterinary and human formulations (e.g. powder, paste, gel, tablet) of lysine are available, and ease of administration should be considered when selecting which formulation to use.

GLAUCOMA MEDICATIONS

Topical carbonic anhydrase inhibitors

Topical carbonic anhydrase inhibitors offer the same decreases in IOP that systemic carbonic anhydrase inhibitors such as acetazolamide and methazolamide do but without the systemic side effects. Topically administering 2% dorzolamide (Trusopt—Merck) or 1% brinzolamide (Azopt—Alcon) every eight hours significantly decreases IOP by decreasing the rate of aqueous humor formation.¹³ Therapy will continue as long as the IOP can be controlled. Many patients with glaucoma undergo enucleation once vision is lost and the IOP cannot be adequately controlled.

The combination of dorzolamide with oral methazolamide does not result in any greater decrease in IOP than administration of either product alone,¹⁴ so only administer a topical carbonic anhydrase inhibitor.

Topical beta-blockers

Topical beta-blockers can also be used to decrease aqueous humor production and, thereby, decrease IOP. The two most commonly used beta-blockers in veterinary medicine are 0.5% betaxolol (beta₁-selective) and 0.5% timolol (nonselective). These medications may be administered every eight to 12 hours. Since beta-blockers can produce bradycardia and heart block (beta₁ effects) and bronchospasm (beta₂ effect), use these medications with caution in patients with cardiac or pulmonary disease. Cosopt (Merck) combines dorzolamide and timolol, which may increase ease of administration and, thus, improve owner compliance. Cosopt may be given every eight hours.

Prostaglandin analogues

Prostaglandin analogues, an exciting new drug class for glaucoma therapy, are highly effective ocular hypotensive agents with a rapid onset.¹⁵ Prostaglandin analogues decrease IOP by increasing outflow through the uveoscleral or unconventional outflow pathway.¹⁶ In my experience, this IOP reduction can occur within 45 to 60 minutes; thus, prostaglandin analogues may replace mannitol as the initial therapy during management of a glaucomatous crisis.

The two prostaglandin analogues with proven comparable efficacy in dogs are latanoprost 0.005% (Xalatan—Pharmacia & Upjohn) and travaprost 0.004% (Travatan—Alcon) ophthalmic solutions.¹⁷ An advantage travaprost offers over latanoprost is increased stability over a wide range of temperatures. Prostaglandin analogues cause marked miosis and mild breakdown of the blood-aqueous barrier and result in a low-grade anterior uveitis, so these drugs should be used with caution in patients with secondary glaucoma due to uveitis or lens luxation.

While effective in dogs and horses, prostaglandin analogues have not proved as effective in decreasing IOP in cats.^15,18 Prostaglandin analogues are labeled for use in people for administration once a day; however, administration every eight to 12 hours may be required in certain cases to maintain control of the IOP.

SUMMARY

New pharmaceuticals for ophthalmologic problems become available every year. Veterinary ophthalmologists frequently use many of these drugs in referred cases. General practitioners may want to try these medications in their practices as well.

SUGGESTED READING

Veterinary Clinics of North America: Small Animal Practice on Ocular Therapeutics edited by Cecil Moore, DVM, MS, DACVO, May 2004.