Coloboma is a congenital developmental deformity of the upper eyelid, which appears as a cleft in the eyelid. The defective eyelid is often unable to function properly, leaving the eye exposed and at risk for inflammation and the development of ulcers unless it is surgically repaired. There are two forms of coloboma. Typical coloboma lies in the fetal fissure, a region which extends from the optic disk to the cornea (the 6 o'clock area). Atypical coloboma occurs in a region other than the fetal fissure.
Coloboma is seen in the Collie, Shetland Sheepdog, Australian Shepherd, Labrador Retriever, and Saint Bernard as part of their ocular maldevelopment syndrome, or in other breeds as isolated lesions. As an inherited disorder, it represents part of Collie Eye Anomaly disorder, but also occurs sporadically as non-inherited birth defect. In both cases, the size of the defect varies from dog to dog. The deformity is non-progressive and can be found in one or both eyes. If the defect is large, vision will be compromised. There is no effective treatment and it is best to avoid breeding affected dogs.