What is the definition of Coloboma of Optic Nerve?

Coloboma of the optic nerve is a congenital eye abnormality in which the optic nerve (which carries images of what the eye sees to the brain) is incompletely formed. The condition may occur in one or both eyes. The degree of visual impairment varies widely depending on the severity and structures involved. Serous detachments of the retina commonly occur in affected people, with a high risk for extensive retinal detachment. The coloboma may be associated with other features, such as a small eye (microphthalmia) with or without a cyst; small cornea (microcornea); or coloboma of other eye structures. Although the condition is present from birth, diagnosis may be delayed since the coloboma is inside the eye and not visible by simple inspection. Coloboma of the optic nerve may occur sporadically, may be due to a genetic mutation and be inherited, or may occur as a feature of an underlying syndrome or other genetic condition. There is no treatment to correct an optic nerve coloboma, but low vision aids may be helpful for some people.

What are the alternative names for Coloboma of Optic Nerve?

  • Optic nerve head pits, bilateral congenital
  • Congenital coloboma of the optic nerve
  • Optic nerve coloboma

What are the causes for Coloboma of Optic Nerve?

Colobomas are due to incomplete development of the eye. During the second month of development in utero, a seam-like structure called the optic fissure closes up to form the structures of the eye. When it doesn't fuse normally, it results in a coloboma in the affected location of the eye.

A coloboma may occur sporadically, it may be inherited and due to mutations in various genes, or may occur as a feature of an underlying syndrome or other genetic condition. When an underlying syndrome or genetic condition is present, the cause is assumed to be the same as that of the underlying condition. For example, when an optic nerve coloboma occurs as part of renal coloboma syndrome, it is caused by mutations in the PAX2 gene. Certain environmental factors affecting early development may also increase the risk for colobomas in general.

Many genes involved in early eye development may be responsible for colobomas in general. While some of these genes have been identified, most of them remain unknown. Most genetic mutations associated with colobomas have been identified only in very small numbers of affected people. Autosomal dominant inheritance has been reported. One gene that has been associated with coloboma of the optic nerve specifically is the PAX6 gene.

The PAX6 gene is part of a "family" of genes that are needed for forming tissues and organs during embryonic development, and for maintaining normal function of certain cells after birth. The genes in this family give the body instructions to make proteins that help control the activity of particular genes. It is thought that the PAX6 protein turns on (activates) genes involved in forming the eyes before birth, and regulating the actions of genes within many eye structures.

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