In this VETgirl online veterinary continuing education blog, Dr. Shelby Reinstein, DVM, DACVO reviews medical management of glaucoma in dogs and cats. Make sure to check out our general guide for glaucoma and surgical options for glaucoma in Part 2 and 3 of these VETgirl blogs!
The goals of medical treatment of glaucoma therapy in dogs are:
- To normalize IOP to preserve or regain vision, if possible
- To alleviate pain by controlling IOP if vision has been permanently lost
Medical therapy of glaucoma either reduces the formation of AH or improves the outflow of AH. Unfortunately, in most cases medical therapy eventually becomes ineffective at controlling the IOP, and surgery is necessary to control discomfort.
Hyperosmotic agents reduce the formation of aqueous humor by reducing plasma flow through the ciliary body, and also cause dehydration of the vitreous. The main indication for the use of hyperosmotic agents in glaucoma is in cases of acute glaucoma with a positive dazzle reflex and / or consensual PLR, where vision is salvageable. For maximum efficacy, water should be withheld for 4 hours after hyperosmotic treatment.
Mannitol is an osmotic diuretic that has been shown to significantly reduce IOP within 15 minutes of administration and can remain effective for 6 – 10 hours. Mannitol can be used safely in most dogs but should be used with caution in dogs with cardiac or renal disease, or in dehydrated patients. The use in cats is not well documented, likely due to the rarity of acute glaucoma. An appropriate starting dose is 1 gram/kg IV over 30 – 45 minutes.
Oral glycerin causes a significant decrease in IOP within 30 minutes of administration and can persist for 10 hours. Glycerin should not be used in dogs with diabetes mellitus. The most common side effect of oral administration is gastrointestinal upset. The reported effective dose is 1-2 gram/kg PO; however this treatment is rarely used.
Carbonic Anhydrase Inhibitors (CAIs)
Both systemic and topical CAIs are available. Inhibition of carbonic anhydrase decreases aqueous humor production by reducing synthesis of bicarbonate in the ciliary body.
Acetazolamide is an oral CAI that is no longer recommended due to the high incidence of systemic side effects. Methazolamide is an alternative oral CAI, however, still has potential to cause unwanted systemic effects, including gastrointestinal upset, metabolic acidosis, and hypokalemia. The author uses this medication for treatment of glaucoma in dogs that are unable to be treated topically.
Topical CAIs have the advantage of providing adequate ciliary body concentrations of the drug but reduce the risk of systemic adverse effects. Brinzolamide (Azopt®) and Dorzolamide (Trusopt®) are commercially available and have both been shown to reduce IOP effectively in dogs and cats. Dorzolamide is available in a generic form, which makes it more cost effective. The degree of IOP reduction observed with topical CAI is comparable to that of the oral CAI, and no additional decline in IOP is obtained from the combination of the two. The most common adverse effect of topical dorzolamide is transient blepharospasm after instillation, and this is less commonly seen with brinzolamide. Topical CAI are most often used every 8-12 hours.
A solution of 2% dorzolamide and 0.5% timolol (Cosopt®) is available in generic form. This combination therapy is as efficacious in reducing IOP as concurrent use of each drug, but the commercially available combination improves client compliance as it only requires twice daily administration.
Beta-blockers reduce the formation of AH via their effects on beta receptors present in the ciliary body. Undesirable cardiac and respiratory effects can be seen with topical beta-blockers, including bradycardia, and bronchoconstriction. Thus, these medications should be avoided in patients with cardiovascular disease and asthma.
Betaxolol is a selective β1-antagonist and has been shown to prolong the onset of glaucoma in the fellow eye when used twice daily. Timolol is a non-selective β-antagonist and is also often used as glaucoma prophylaxis. The degree of IOP reduction with beta-blockers is mild, thus these medications are often combined with other anti-glaucoma therapy.
Prostaglandin analogs are the newest modality of topical glaucoma therapy used in dogs. These medications are ineffective in cats due to their lack of appropriate receptor types in their ciliary body epithelium. Prostaglandin analogs are thought to lower IOP primarily by increasing AH outflow via their action on iris and ciliary body musculature. They induce a profound miosis and may physically open the ICA and improve flow. Prostaglandin analogs should be avoided in cases of glaucoma secondary to anterior lens luxation or uveitis.
Latanoprost (Xalatan®) is a selective prostaglandin-F2α receptor agonist that results in a dramatic decrease in IOP within 20 minutes. Often used to treat acute glaucoma, this is the most commonly utilized prostaglandin analog and is available in generic form. Travaprost and bimatoprost are newer prostaglandin analogs that have also been shown to be effective in the dog. Prostaglandin analogs are administered every 8-24 hours.