The optic nerves--located at the back of the eyes where they establish a direct connection to the brain--are responsible for vision. Should embryologic development not proceed normally, however, bits of the optic nerve may be left out of the process. This creates a "pitted" or "cratered" area within or adjacent to the optic disc (where the optic nerve attaches to the eye). We call this lesion an optic nerve coloboma. If the defect is large enough, normal nerve conduction related to vision can be disrupted and vision impairment ensues.
In dogs, optic nerve colobomas can arise not only from a simple failure in embryologic development (which can be either genetic or environmental in origin and affect one or both eyes), but more commonly as a result of a genetic disease known as Collie Eye Anomaly (CEA). In CEA, optic disc colobomas are but one of several potential problems observed in both eyes of affected dogs.
CEA is inherited as an autosomal recessive trait.
Symptoms and Identification
Optic nerve colobomas, whether inherited as CEA or acquired congenitally via embryologic aberration, are present at birth and do not progress. Affected dogs' vision will be altered to varying degrees (generally mildly and sometimes not at all). Hence, some puppies will never be diagnosed and will continue to pass down the genetic trait associated with either CEA or embryologic anomaly.
Optic nerve colobomas are diagnosed via fundoscopic examination (observing the back of the eye with a lens). The coloboma typically appears as a misshapen optic disc, usually in the area located at 6 o'clock.
Diagnosis usually occurs either in the course of normal puppy evaluation, because vision impairment is suspected or when the puppy is of a CEA-prone breed. For CEA breeds, genetic testing is now available to determine whether dogs carry the genetic trait that may lead to optic nerve colobomas.
Up to 75% of Collies may be affected by CEA. This includes both rough and smooth-coated collies, the Shetland Sheepdog, Australian Shepherd, Lancashire Heeler, and Border Collie. Other dogs may be affected by optic nerve colobomas though more rarely and sporadically. Of these CEA dogs, an estimated 30% will suffer optic disc colobomas.
For the non-CEA associated form of optic nerve coloboma, there appears to be a hereditary predisposition in the Basenji breed.
No treatment is available for this condition.
The cost of diagnosis is relatively minor, though ophthalmologist consultation is strongly recommended and may increase the cost of diagnosis into the low hundreds of dollars.
Preventing optic nerve colobomas is best achieved through CEA testing of all susceptible breeding animals. Affected and carrier individuals should be removed from the breeding pool. Additionally, any dog with a non-CEA related optic nerve coloboma should not be bred.
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