coloboma

Canine Coloboma

The term ‘Coloboma’ in Greek means ‘unfinished.’ Essentially, Coloboma occurs when the eye stops growing before it is fully developed resulting in some of the structures of the eye to be missing. It is an inherited disorder, and represents part of Collie Eye Anomaly, but also occurs sporadically as a noninherited birth defect. The size of the defect can vary from dog to dog. The lesion can be found in one or both eyes. Depending on the severity and location of the coloboma, eye defects such as cataracts, lens displacement and reduction of light entering the eye may result. A coloboma, if found, will need to be carefully monitored by your veterinarian. If the defect is large, vision will be compromised.

It may be a congenital developmental deformity of the upper eyelid, which appears as a cleft in the eyelid. This occurs when tissues fail to fuse and/or form completely while the puppy is still in the mother's womb. 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. Some patients with a minor defect may develop pigment across the affected area but will appear normal. For this reason, if a coloboma is suspected, early examination of your dog in the first six to eight weeks of life is highly recommended.

Colobomas also can be found in other structures within the eye, including the iris, choroid (the fine web of blood vessels that feed the retina), ciliary body, lens and retina or optic disc (the area at the rear of the eyeball from which the optic fibres exit to carry information to the brain), where they again present as a hole, split or cleft in the affected structure.

There are several types of coloboma:

Typical Coloboma
This lies in the fetal fissure - this is a region which extends ventrally from the optic disk to the cornea - would be considered the 6 o'clock area. Most colobomas involve or are near the optic disk and are always accompanied by Choroidal Hypoplasia (CH). This is the inadequate development of the choroid, which is a thin layer of blood vessels that deliver oxygen and nutrients to the retina.

Atypical Coloboma
This occurs in a region other than fetal fissure.

In mild cases, the dog can see with little or no problem, however a large coloboma can force a dog to squint in bright light because the iris is incapable of contracting to reduce the amount of light entering the eye. This can cause minor discomfort as well as temporarily reducing the range of vision while squinting. In the more severe cases, the sight is not only impaired, but the Coloboma may actually cause retinal detachment. Retinal detachment is a condition where the choroidal area of the eye peels away from the sclera with a resulting blindness. Another possible side effect of colobomas is retinal hemorrhaging. As the vessels begin to bleed, they can cause tearing and cut off of blood supply to the retina which then causes the area to become necrotic (dead). Depending on the amount of bleeding and area involved, retinal hemorrhaging can also cause blindness.

When having eye exams performed, it is important that the irises be examined before dilation. Some small colobomas may not be apparent when the eye is dilated and thus missed.

Signs of this disease can be seen as early as at the opening of the eyelids at 14 days of age. The absence of a sector of the iris may sometimes give the appearance of a 'keyhole' in the pupil. The pupil may appear to extend into the iris often with a jagged edge, slightly increasing the risk of retinal tearing. The eye may also be dramatically reduced in size in severe cases. Dogs with a large coloboma may be forced to squint in bright light due to the iris being unable to contract to reduce the amount of light entering the eye. Symptoms in an affected dog can range from corneal ulcers and pigmentation, constant squinting or excess production of tears, to abnormal behaviour.

The mode of inheritance for iris coloboma is unknown. Affected animals should not be bred. Unaffected individuals which produce it repeatedly, particularly with multiple mates, should be pulled from breeding.

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