Glaucoma in dogs
Glaucoma and glaucoma therapy
Glaucoma is a combination of several pathological conditions of the eye, which progressively advance, leading to weakening of sight, and resulting in characteristic changes in the optic nerve, and are always associated with increased intraocular pressure. In veterinary medicine, it is one of the most common causes of vision loss.
In glaucoma, there is a reduced outflow of the aqueous humor, increased intraocular pressure, reduced blood supply to the head of the optic nerve, structural changes in the head of the optic nerve and inflammation, and damage to the retinal ganglion cells. Due to the elasticity of the sclera, high intraocular pressure leads to enlargement of the eye globe (buphthalmos), which is especially noticeable in young animals, puppies, and kittens, or in chronic glaucoma of adult animals. The cornea becomes thicker and cloudy (bluish) due to stromal edema. The optic nerve is effaced by hypoxia resulting in axon death and gliosis of the optic nerve head.
Glaucoma is classified into:
• congenital,
• primary
• secondary.
Congenital glaucoma
Congenital glaucoma is a hereditary and congenital disease, which means, it is present at birth. The patient shows signs of glaucoma soon after birth, which is a couple of weeks or months. Congenital glaucoma affects both eyes, but glaucomatous changes do not have to be symmetrical. Clinical signs of glaucoma are increased intraocular pressure, mydriasis, corneal edema with possible appearance of stretch marks (Haab’s striae), secondary ulcerative keratitis, congestion of episcleral blood vessels, buphthalmos, lens subluxation, secondary uveitis, cortical retinal cataract, and degeneration optic nerve.
Primary glaucoma
Primary glaucoma is hereditary glaucoma that occurs due to mutations in various genes, and most commonly occurs in pure breed dogs. Breeds prone to primary glaucoma are Basset Hounds, Beagles, Cocker Spaniels, Siberian Huskies, Boston Terriers, Chow Chow, Miniature Poodles… Clinical signs of primary glaucoma typically manifest in young dogs; however, clinical signs may be manifested in middle age or older dogs. Dogs with primary glaucoma may have an “open” or “closed” iridocorneal angle (iridocorneal angle is evaluated by gonioscopy), but the canal system that drains the ocular humor into the capillary system (ciliary cleft – examined by HF ultrasound) must be continuously evaluated in order to diagnose primary glaucoma. Glaucoma is present in both eyes; however, glaucomatous changes in the eye do not have to be symmetrical. If a dog is diagnosed with glaucoma in one eye, additional tests should be performed to determine whether it is primary or secondary glaucoma. This information is important in choosing an adequate therapy for the eye exhibiting glaucoma symptoms, but also for the remaining, “healthy” eye in which the symptoms still did not appear.
Clinical signs of glaucoma are increased intraocular pressure, mydriasis (dilated pupil), edema with possible appearance of stretch marks (Haab’s striae), secondary ulcerative keratitis, congestion of episcleral blood vessels, buphthalmos, lens subluxation, secondary uveitis, cortical vitreous degeneration retina and atrophy of the optic nerve head.
Secondary glaucoma
The cause of secondary glaucoma is a preexisting eye disease that impairs the outflow of the aqueous humor. Preexisting diseases can be primarily eye diseases or systemic diseases that affect the eye. In addition, preexisting diseases which result in glaucoma can be categorized as infectious (Toxoplasma, Neospora, Bartonela, Brucella, Distemper, Adenovirus, FIP, FeLV, Blastomycosis, Cryptococcosis…), metabolic (diabetes, systemic hypertension, hyperlipidemia), neoplastic (lymphoma, melanoma, myeloma, sarcoma), immune-mediated (phacolytic and phacoclastic uveitis, uveodermatologic syndrome), idiopathic (lymphocytic plasmacytic) and traumatic (blunt or penetrating eye trauma). All of these conditions initiate uveitis, and secondary glaucoma results from uveitis. Secondary glaucoma is often diagnosed as a consequence of lens luxation, cataracts, phacoemulsification, iris bombe, intraocular pigment deposits, retinal detachment and many other ocular pathological processes. Clinical signs of glaucoma are increased intraocular pressure, mydriasis, miosis, corneal edema with possible appearance of stretch marks (Haab’s striae), secondary ulcerative keratitis, uveitis, congestion of episcleral blood vessels, buphthalmos, lens subluxation, cortical cataract, degenerative cataract, degeneration heads of the optic nerve and reduced vision or blindness. Secondary glaucoma can occur in one or both eyes.
Dog with a secondary glaucoma caused by perforating corneal ulcer. Buphthalmos, corneal edema and corneal scars can be observed 
Secondary glaucoma in a dog due to phacoclastic uveitis which was caused by cat scratch and laceration of the lens
Diagnosis of glaucoma
Diagnosis of glaucoma is made by use of tonometry, gonioscopy, and high-frequency ultrasound. If glaucoma was diagnosed in one eye, it is necessary to examine the remaining eye which appears to be healthy at the time. This is because, in dogs with primary glaucoma, the remaining, “healthy” eye will be eventually affected by the disease, and therefore it is necessary to initiate the therapy in both eyes. Therapy may involve the application of antiglaucoma drugs (typically in the form of drops), or dogs can be treated surgically. Surgical procedures which can be done in our hospital include laser transscleral or endoscopic cyclophotocoagulation (in order to reduce the production of aqueous humor), or placement of various shunts in the anterior chamber that serves to drain aqueous humor. In some dogs, both surgical procedures may be combined. In dogs with secondary glaucoma, it is important to determine the primary cause of glaucoma and use therapy that will eliminate the primary cause of the disease.