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).