Current glaucoma therapy (Proceedings)
During this talk, we will be discussing currently available medical and surgical therapies for glaucoma in veterinary medicine, with emphasis on what is new and updated, as well as when their use is appropriate. Treatment of canine glaucoma is highlighted, although species variations will be touched upon where appropriate. We are fortunate in some ways that glaucoma is a prevalent human condition and so the many research dollars have been spent, and are being spent, on the study of its treatment. Although the precise differences between the primary glaucomas of humans and dogs are beyond the scope of this lecture, it is important to realize that there are some key differences between the two conditions, as well as between the ocular anatomies of the species, and so the efficacies of different therapies may vary. Common denominators of most glaucomas include elevated intraocular pressure, retinal ganglion cell death, and subsequent vision loss, and so surgical and medical therapies are aimed at one or more of these areas. Some surgical procedures, like cyclophotocoagulation and goniovalve implantation, can be translated across species, while others like trabeculoplasty are less useful. The pharmaceutical industry has provided a number of heavily researched anti-glaucoma drugs. Most are effective in domestic animal species. Some of these drugs act to lower aqueous humor production, some act to improve aqueous humor outflow, some have a combined intraocular pressure (IOP)-lowering effect, and more recently some are being used to provide a neuroprotective effect.
Anti-glaucoma drugs which act to decrease aqueous production include beta-blockers (timolol, betaxolol) and carbonic anhydrase inhibitors (methazolamide, dorzolamide). Although some systemic beta-blockers have an IOP-lowering effect, only topical agents are used for this purpose. Even these may have side effects. Timolol is a non-specific beta-blocker, and so could precipitate a feline asthma attack or aggravate heart disease in predisposed patients. Although this risk appears to be low, the risk could be obviated by the use of a beta-1-selective agent like betaxolol. Timolol appears more effective in the cat than dog. Carbonic anhydrase inhibitors effectively lower IOP in the dog and cat by competitive inhibition of the carbonic anhydrase II within the ciliary body epithelium which is critical for aqueous prodction. The advent of effective topical versions has been advantageous. The topical CAIs act only on ciliary body CA-II, whereas oral versions like methazolamide act on this ubiquitous enzyme within other body tissues. Oral CAIs can result in noticeable side effects including hypokalemia, acidemia, and hyperpnea. The side effect profile and advent of effective alternatives explain the decreased availability of systemic formulations. Although there may be some benefit in rapidity of onset, at least beyond the first several days there should not be a significant benefit to co-administration of systemic and topical CAIs. A combination drug of topical 2% dorzolamide and 0.5% timolol exists and its twice daily use is particularly useful for cats, which are resistant to frequent drop administration.