‘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
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
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.
are several types of 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.
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.
This occurs in a region other than fetal fissure.
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.
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