Among the many hereditary eye disorders that can appear in your canine is an unusual condition known as an ocular coloboma. Colobomas are a congenital anomaly in which some of the structures of the eye are missing. This occurs when tissues fail to fuse and/or form completely while the puppy is still in the mother's womb. In ocular colobomas, for example, there is a visible black hole or cleft in the ocular nerve.
Colobomas also can be found in other structures within the eye, including the iris, choroid, ciliary body, eyelid, lens, and retina, where they again are present as a hole, split or cleft in the affected structure. In iris coloboma the condition can appear like a black notch at the edge of the pupil, giving it an irregular shape. They sometimes can appear in conjunction with other inherited eye problems, including juvenile and senior cataracts, detached retinas, persistent pupillary membrane, distichiasis, and progressive retinal atrophy. These conditions can affect human beings, cattle, and other mammals, in addition to dogs.
The cause of colobomas isn't yet known, although they have been associated with hereditary eye diseases, trauma, and injury to the eye. In some cases, they appear as a complication following eye surgery. The defect in these cases also can encompass several of the eye's structures, specifically the retina, choroid and optic nerve.
Most colobomas are usually diagnosed at birth or shortly after birth. Pet owners should contact their veterinarian if they notice a hole in the iris of their dog's eye or an unusually-shaped pupil. Many pets with the condition have blurred or double vision, therefore the owner may notice signs of a vision problem as well.
Your veterinarian will diagnose the problem after a detailed physical examination, including an ophthalmic exam of the dilated eye. In some cases, an MRI also will be performed to provide an image of the brain and nerves connecting the eye to the brain. The more information you can provide about your pet's lineage, the more help this will be to the veterinarian in making a diagnosis.
Colobomas (and other inherited eye diseases) can be found in any breed of dog, and are fairly common in Basenjis, Australian Shepherds, and other types of herding dogs, such as Collies and Sheepdogs. The only way to prevent the condition is for pet owners to be responsible in their breeding. They can do this by not breeding affected animals and having annual eye exams performed on all animals that are bred. The only dogs that should be bred are those whose eyes have been certified as normal by a board-certified ophthalmologist. Agencies such as the Canine Eye Registration Foundation monitor eye problems in all purebred dogs, and can provide owners with certification that their dog's eyes are normal. Breeders also should have their puppies' eyes examined by an ophthalmologist before placing them in new homes; this can be done as early as six weeks of age.