Coloboma - External and Internal Eye
The embryonic fissure develops as an invagination of the optic vesicle and leaves a gap inferonasally to allow the entrance of the hyaloid artery. Fusion begins at the equator and then proceeds anteriorly and posteriorly. As such, colobomas are often found inferonasally and either in the anterior structures or posterior structures of the eye. This process is complete by the fifth week of gestation. Other relevant etiologic factors include nutrition and environmental exposure. Rat fetuses made deficient in retinol (Vitamin A) fail to close the optic fissure; this was prevented by supplementing with retinol. Colobomas can also be seen in children with fetal alcohol syndrome. Genetically, the regulation of globe development is complex. Recently, zebra fish deficient in FGF-Ras pathway components have been shown to develop ocular colobomas. Colobomas have been seen to demonstrate autosomal dominant, autosomal recessive, and X-linked inheritance in family studies.
Some syndromes associated with eye colobomas include CHARGE syndrome, cat eye syndrome, Patau syndrome, and Treacher-Collins syndrome. Occasionally, colobomas co-present with microphthalmos, a condition in which one or both eye(s) is abnormally small.
Colobomas of the eyelid are also holes in orbital structures, but arise from failure of fusion of epidermal progenitor cells, not components of the choroid fissure. Colobomas of the upper eyelid in particular can result in severe exposure keratopathy in infants. They can be seen as a component of the Fraser or Goldenhar syndromes.
Q13.0 – Coloboma of iris
93390002 – Congenital ocular coloboma
Differential Diagnosis & Pitfalls
- Iris nevi – melanotic macules on iris
- Iris trauma – classic "teardrop" appearance
- Aniridia – severe iris tissue hypoplasia
- Morning glory disc – large, funneled disc with peripapillary elevation and pigmentation with straight radially exiting retinal vessels reminiscent of a "flower" pattern.
- Congenital optic pits – herniation of dysplastic retina through defects in the lamina cribrosa resulting in yellowish depression of optic disc, commonly associated with posterior vitreous detachment and serous retinal detachment.
- Optic nerve staphylomata – defect in sclera near the optic nerve resulting in bowl-shaped excavation of retina and choroid, often atrophic in appearance.
The eyelid coloboma differential includes eyelid trauma and entropion (abnormal inward turning of the eyelid margin).