Update on glaucoma in dogs (Proceedings)
The glaucomas are a group of conditions in dogs characterized by increased intraocular pressure and progressive optic nerve and retinal degeneration. This can be one of the most devastating and frustrating ophthalmic conditions, both for the general practitioner and the ophthalmologist. This article will discuss the current clinical understanding of glaucoma and the treatment options.
Glaucoma can be either a primary or a secondary condition, and the distinction is critical for proper management. Secondary glaucoma is an increase in intraocular pressure that is a direct consequence of a pre-existing condition, such as chronic uveitis, lens luxation, trauma, hyphema, or delevopmental anomalies. This form of glaucoma can occur in any breed and age. Treatment is primarily by correcting the underlying abnormality. Primary glaucoma occurs as a genetic condition due to the structural anatomy of the iridocorneal angle and the ciliary cleft. This is a bilateral disease, although it typically affects each eye at different times, and it will be the primary focus of this article.
A basic understanding of the physiology of aqueous humor production and outflow is required to understand glaucoma and the relevant treatments. Aqueous is produced in the ciliary body, specifically the non-pigmented epithelium. The reversible carbonic anhydrase catalyzed reaction H2O + CO2 ←→ HCO3 - + H+ is essential for the production of aqueous humor. Bicarbonate and sodium are actively transported into the posterior chamber and water follows by osmosis. The majority of the aqueous flows through the pupil and exits the eye through the iridocorneal angle, where it must pass through the pectinate ligaments and trabecular meshwork before entering the angular aqueous plexus. This plexus joins with the intrascleral venous plexus which then leads to the vortex venous plexus for passage out of the eye. An alternative pathway for aqueous drainage is referred to as the unconventional or uveoscleral pathway. Aqueous passes directly through the iris stroma into the suprachoroidal space and out through the choroidal veins. The significance of this pathway varies with the species (dog – 15%, cat – 3%). Glaucoma in dogs is almost exclusively due to decreased aqueous outflow. A "sink" analogy where the drain is clogged works well to describe what is happening with glaucoma.Signalment is very important in identifying primary glaucoma. While it has been observed in many breeds, certain breeds, such as Cocker Spaniels, Shar Peis, Chow Chows, Siberian Huskies, Basset Hounds, and Beagles are afflicted with much greater frequency. Females are more affected than males, on balance, and most are middle aged (5-7 years old). Clinical sign recognition is also important. True glaucoma is nearly always associated with some combination of clinical signs, including scleral injection, diffuse corneal edema, elevated third eyelid, mydriasis, blepharospasm, buphthalmos, corneal striae, or vision loss. If an elevated tonometry value is obtained without any of the above clinical signs, the restraint and tonometry techniques should be evaluated and the reading repeated as falsely elevated pressures are common with improper techniques.
Buphthalmos is an exclusive sign of chronic glaucoma in adult dogs, but this is often confused with exophthalmos. The latter is anterior globe displacement due to a space occupying orbital mass. In this situation, the globe is typically normal in most ways apart from being anteriorly displaced. There is typically minimal scleral injection, a normal cornea, normal PLR, and normal visual reflexes. The third eyelid is elevated and there is a reduced ability to retropulse the globe. Ophthalmoscopy may reveal a scleral indentation which confirms an orbital lesion. If there is still doubt, the horizontal diameter of the cornea can be measured with calipers and compared to the fellow eye.
Other diagnostic tests utilized in the ophthalmic examination include a menace response, dazzle reflex (bright light used to elicit a blink), pupilary light reflex, assessment of resting pupil size, and tonometry. It is important to remember that these tests are not perfect for evaluating the presence of vision as some normally visual dogs may not have a positive menace or dazzle, especially if they are stressed. Gonioscopy (evaluation of the iridocorneal angle) can be performed on the contralateral eye to assess the anatomic predisposition to developing glaucoma.