Canine Glaucomas

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Canine Glaucomas


We have used the term glaucomas because this group of ocular diseases, which affects the entire eye, includes at least 25 different types of glaucoma documented in humans and nearly as many types reported in the dog. Traditionally, the glaucomas are divided into primary, secondary, and congenital forms. The human and canine glaucomas generally have elevated intraocular pressure (IOP), optic nerve damage, and loss of retinal ganglion cells. In canine patients presented to veterinary college hospitals in North America over nearly a 40‐year span, glaucoma patients represented about 2% of the total canine population and occurred in 45 breeds of dogs. Primary and secondary types occurred with equal frequency. The primary glaucomas are divided into narrow/closed angle glaucomas and open angle glaucomas. The majority of the breed‐related primary glaucomas have not been investigated in any detail to document the distinction. In the USA, the most frequent type of primary glaucoma in the dog is narrow/closed angle glaucoma; in humans in the USA, the most frequent primary glaucoma is open angle. In Asian countries, the most frequent type of glaucoma in humans is narrow/closed angle. In secondary glaucomas, the ocular hypertension is attributed to another disease. In the dog, the most frequent cause of secondary glaucoma is lens displacement (lens luxation). The rarest form of glaucomas are congenital, affecting young puppies with anomalous outflow pathways and sometimes other ocular anomalies.


Diagnostic aids essential in the clinical management of the canine glaucomas include tonometry, gonioscopy (to examine the anterior chamber, or iridocorneal, angle) and ophthalmoscopy. In general, medical and surgical therapy must anticipate that the underlying outflow disease is progressive, therefore the maintenance of “safe” IOP changes over time.


Surgical therapy of the primary glaucomas is aimed at either decreasing aqueous humor production (usually by laser cyclophotocoagulation) or increasing aqueous humor outflow (anterior chamber shunts). Treatment for the secondary glaucomas requires concurrent treatment of the underlying etiology and the ocular hypertension.


Optic Nerve Head Changes in the Glaucomas


The optic nerve head/optic disc/optic papilla is the focus of injury with elevated IOP as it mostly contains the axons of the retinal ganglion cells as they exit the scleral sieve known as the lamina cribrosa. The peripheral retinal ganglion cells in the ocular fundus are the most sensitive to elevated IOP as they travel the greatest distance to converge at the optic nerve head and lamina cribrosa, and bend the greatest angle at the disc.


The normal canine optic nerve head is quite variable in appearance. It is located in the tapetal fundus, nontapetal fundus, or at their junction. When the optic disc is located in the tapetal fundus, it is usually surrounded by a pigmented ring. The canine optic disc is myelinated, but variably so, which accounts for its shape and size. From its surface and periphery emerge 15–20 primary arterioles (light red), and 3–4 primary retinal veins (darker red). Often, a complete or incomplete venous circle is near the center of the disc, which is the physiologic pit (optic cup) and remnants of the primary hyaloid system (Bergmeister papilla).


The optic nerve head or papilla changes in glaucoma appear to be influenced by the degree of IOP elevation and duration of the elevation. In primary open angle glaucoma, the papilla develops an enlarging central optic cup that signals progressive degeneration (Figure 9.1). In narrow/closed angle glaucoma, the rise in IOP often appears acutely (even within hours) and with considerably higher IOPs (50–80 mmHg) (Figure 9.2). The result is optic nerve atrophy. Infarcts in the peripapillary region and ischemia of individual short ciliary arteries, which supply the posterior segment (retina and choroid), have been associated with these high spikes in IOP. These infarcts appear as wedge‐shaped hyperreflective areas in the tapetal ocular fundus. When viewing the ocular fundus by ophthalmoscopy in a potential glaucoma patient, it is important to measure IOP by tonometry before any interpretations are made.

Fundus photo of a dog displaying early optic nerve head changing in primary open angle glaucoma.
Fundus photo of a dog displaying more advanced optic nerve head changing in primary open angle glaucoma with its optic nerve smaller than usual, its myelin lost and pigmented.

Figure 9.1 Optic nerve head and primary open angle glaucoma in the Beagle. (A) Early optic nerve head changes in primary open angle glaucoma. Note the depression or enlargement of the central optic nerve head nerve cup. (B) More advanced optic nerve head changes in primary open angle glaucoma. The optic nerve head is smaller than normal, its myelin lost, and has become pigmented.

Fundus photo of a dog displaying corneal edema developing when intraocular pressure (IOP) will exceed about 40 mmHg.
Fundus photo of a dog displaying the optic nerve head appearing atrophied and most of the retinal blood vessels have disappeared in advanced PCAG.

Figure 9.2 Optic nerve head changes in primary narrow/closed angle glaucoma (PCAG) in the dog. (A) Corneal edema develops when intraocular pressure (IOP) exceeds about 40 mmHg, which makes direct ophthalmoscopy more difficult because of the hazy view. In acute PCAG, the optic nerve head is swollen (papilledema) because of impaired axoplastic flow. Primary retinal blood vessels are constricted. (B) In advanced PCAG, the optic nerve head appears atrophied and most of the retinal blood vessels have disappeared. Because of the retinal degeneration, the tapetal fundus is hyperreflective.


Congenital Glaucoma


The congenital glaucomas affect young puppies which usually present with a unilateral rapidly enlarging globe (Figures 9.3 and 9.4). Anomalous aqueous outflow pathways are usually present, therefore both eyes should be evaluated closely; gonioscopy can be very helpful.

Photo displaying a Dalmatian puppy with congenital glaucoma, with its enlarging globe, and ulcerated cornea due to exposure.

Figure 9.3 Congenital glaucoma in a Dalmatian puppy. The globe is enlarged and the cornea ulcerated due to exposure.

Photo displaying a Jack Russell Terrier puppy with congenital glaucoma and with its enlarging globe and edematous cornea.

Figure 9.4 Congenital glaucoma in a Jack Russell Terrier puppy. The globe is enlarged and the cornea is edematous.


Upon presentation, megaloglobus (or buphthalmia) is present, the pupil is usually dilated, the episcleral vessels are enlarged, and lens luxation or subluxation is frequent. The puppy sclera is highly elastic (accounting for the greater degree of buphthalmia than that seen in an adult animal), but advanced retinal and optic nerve degeneration are usually present.


Short‐term medical therapy is recommended to determine if the size of the globe reduces and if return of vision is possible. With an enlarged globe, lagophthalmia and persistent exposure keratitis occur, often necessitating enucleation. Prophylactic therapy by the medical reduction of IOP for the fellow eye is also recommended.


Primary Narrow/Closed Angle Glaucoma


Primary closed angle glaucoma (PCAG) is the most common form in the canine and has been reported in at least 11 breeds including the American Cocker Spaniel, Samoyed, and Chow Chow. This condition is bilateral, but usually presents asymmetrically and months may separate recognition of clinical signs in both eyes (Figure 9.5).

Photo displaying a Siberian husky dog with advanced glaucoma in the right eye and early glaucoma in the left eye.
Photo displaying a Chinese Shar Pei dog with its left eye having an absolute glaucoma.
Photo displaying an acute congestive PCAG in the American Cocker Spaniel.
Photo displaying an acute congestive PCAG in the American Cocker Spaniel with an IOP of 66 mmHg noting the corneal edema, dilated pupil, and episcleral congestion.
Fundus photo of an American Cocker Spaniel dog with enlarged globe, prominent and congested episcleral blood vessels, dilated pupil, and a cataractous and luxated lens within the anterior chamber.
Gonioscopic appearance of an eyeball of the American Cocker Spaniel dog displaying the entire aqueous outflow apparatus being obstructed from direct observation by the base of the iris.
Image described by caption.
Image described by caption.

Figure 9.5 (A) Bilateral PCAG in the Siberian Husky. Advanced glaucoma in the right eye; early glaucoma in the left eye. (B) PCAG in the Chinese Shar Pei. The left eye has absolute glaucoma. (C) Acute congestive PCAG in the American Cocker Spaniel. Note the corneal edema, dilated pupil, and episcleral congestion. IOP is 66 mmHg. (D) Chronic PCAG in an American Cocker Spaniel. Note the buphthalmos, corneal edema, and episcleral injection. (E) Advanced PCAG in the American Cocker Spaniel; note the enlarged globe, prominent and congested episcleral blood vessels, dilated pupil, and a cataractous and luxated lens within the anterior chamber. (F) Gonioscopic appearance of PCAG. The entire aqueous outflow apparatus is obstructed from direct observation by the base of the iris. (G) Fundus appearance of PCAG patient with optic nerve head atrophy. The papilla is smaller than normal, depressed, and some of the lamina cribrosa is exposed. (H) Fundus appearance of PCAG patient with advanced optic disc degeneration. The disc is reduced in size, myelin has been nearly lost, and the numbers of retinal blood vessels reduced to only the large primary arteries and veins.


Owners may observe the onset of PCAG as a series of self‐limiting attacks of mydriasis, corneal edema, episcleral congestion, and perhaps increased sensitivity to touch about the eye and head. Presumably during these episodes IOP is also elevated, but within hours returns to normal levels. However, eventually the ocular hypertension and the clinical signs persist, initiating presentation to the veterinarian. Gonioscopy reveals an iridocorneal angle that is either narrow or closed. The opposite eye, which is often still asymptomatic, may also have narrow iridocorneal angle.


Chronic PCAG results in blindness. Unfortunately, it is not unusual for a patient to present with chronic, end‐stage disease in one eye and acute disease in the other. Signs of the advanced state include vision loss, fixed and dilated pupil, episcleral venous congestion, moderate to advanced megaloglobus, corneal edema and striae, lens luxation, cataract formation, vitreal degeneration and liquefaction, and optic nerve head and retinal degeneration. The optic nerve appears dark and depressed. The atrophied retina in the tapetal areas appears as hyperreflective areas with variably pigmentation and generalized loss of retinal vasculature.


Treatment depends on the stage of the disease and in the IOP and the response to medical therapy. Medical treatment usually involves polypharmacy with various combination of prostaglandins, miotics, beta antagonists, alpha agonists, carbonic anhydrase inhibitors (both systemic and topical), and corticosteroids (iridocyclitis is often present concurrently). Acute disease can be treated with intravenous mannitol. Laser cyclophotocoagulation and anterior chamber shunts are the currently recommended surgical approaches. Prophylactic therapy of the asymptomatic eye with miotics or beta antagonists can delay the onset of the glaucoma for several months to as long as 2½ years.


Primary Closed Angle Glaucoma with Pectinate Ligament Dysplasia


PCAG with pectinate ligament dysplasia (persistent mesodermal remnants or goniodysgenesis) is typified in the glaucoma in the Basset Hound (Figure 9.6). Predisposed breeds include the Bouvier des Flandres, Flat Coated Retriever, Great Dane (in England), among others. The effect of this anomaly on the genesis of ocular hypertension is probably related to the extent or percentage that this anomaly affects the outflow pathways; in any event, only the most severely affected eyes appear to develop glaucoma. There can be additional iridocorneal angle abnormalities behind these anomalous pectinate ligaments. The clinical and histopathologic significance of a few dysplastic pectinate ligaments in an asymptomatic eye is not known.

Photo of a Basset Hound dog with pectinate ligament dysplasia affecting both eyes of advanced glaucoma affecting one eye and early glaucoma in the opposite eye.
Photo of a Basset Hound glaucomatous eye having concurrent iridocyclitis.
Image described by caption.
Image described by caption.

Figure 9.6

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Jul 24, 2020 | Posted by in INTERNAL MEDICINE | Comments Off on Canine Glaucomas

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