Two important congenital disorders that affect the optic nerve and optic disk in dogs are coloboma which is localized absence of optic nerve tissue, and optic nerve hypoplasia marked by underdevelopment of the optic nerve. The nerve may also be impaired by inflammation (optic neuritis), swelling (papilledema), or shrinkage (optic atrophy). Optic nerve hypoplasia is a relatively rare condition, and may be uni- (one eye is affected) or bilateral (both eyes are affected). In this condition the optic nerve appears extremely small because of its underdevelopment. Either eye can be affected by this congenital abnormality, and affected eyes frequently are blind. Because blindness in one eye is compensated by sight in the other, hypoplasia affecting only one eye often will go undetected. Affected eyes have reduced numbers of retinal ganglion cell axons. Resting pupil size may be normal, being regulated from the opposite eye, or dilated, with bilateral optic nerve hypoplasia. Examination reveals a small or barely detectable optic nerve. The retinal blood vessels are present; but the number of arterioles and veins may be reduced. There is no effective therapy for this condition. Saint Bernards are affected by optic nerve hypoplasia
Breeds affected by the optic nerve hypoplasia include the Dachshund, Great Pyrenees, Irish Setter, Miniature Schnauzer, Norfolk terrier, Toy Poodle, Miniature Poodle, Standard Poodle, and Saint Bernard. It is best to withhold affected dogs from breeding programs, since the condition is known to be inherited in the Miniature Poodle, while in others it is associated with inherited eye disorders and a broad range of disorders of the central nervous system.
- UC Davis Book of Dogs : The Complete Medical Reference Guide for Dogs and Puppies. Mordecai Siegal
- Color Atlas of Veterinary Ophthalmology, Kirk N. Gelatt