Coloboma is a general term used to describe conditions where the normal tissue in or around the eye is missing from birth. The missing tissues may cause gaps in one of several parts of the eye, including the iris, retina, the choroid, or the optic nerves. The condition may occur one or both the eyes and may even affect a person's vision. The condition affecting the iris generally does not lead to vision loss. However colobomas in the retina may result in vision loss.
Colobomas large in size or affecting the optic nerve can cause low vision which may not be corrected even with glasses. Some people with coloboma also have microphthalmia where one or both eyeballs are abnormally small. People with coloboma may also have other eye abnormalities, including clouding of the lens of the eye, vision problems such as nearsightedness, increased pressure inside the eye that can damage the optic nerve, involuntary back-and-forth eye movements, or separation of the retina from the back of the eye.
Some individuals have coloboma as part of a syndrome that affects other organs and tissues in the body causing syndromic colobomas. When coloboma occurs by itself, it is usually termed as nonsyndromic or isolated. The condition arises from abnormal development of the eye. During the second month of development before birth, a seam called the optic fissure closes to form the structures of the eye. When the optic fissure does not close completely, the result is a coloboma. Colobomas involving the eyeball should be distinguished from gaps that occur in the eyelids. While these eyelid gaps are also called colobomas, they arise from abnormalities in different structures during early development.
Coloboma is a rare condition that occurs only in approximately 1 in 10,000 people. However, since the condition does not always cause vision loss or any change in the appearance of the eye, many people with this condition usually go undiagnosed. The risk of coloboma may also be increased by environmental factors that affect early development, such as exposure to alcohol during pregnancy. People born with coloboma usually have other health problems in addition to the condition. Coloboma is usually not inherited; however, there is still a higher risk of passing on the condition into the family. In cases when it is passed down in families, coloboma may have different inheritance patterns. Isolated coloboma is sometimes inherited in an autosomal dominant pattern, where one copy of an altered gene in each cell is sufficient to cause the disorder.
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