Coloboma is an eye abnormality that occurs before birth. They're missing pieces of tissue in structures that form the eye.
Colobomas affecting the iris, which result in a "keyhole" appearance of the pupil, generally do not lead to vision loss.
Colobomas involving the retina result in vision loss in specific parts of the visual field.
Large retinal colobomas or those affecting the optic nerve can cause low vision, which means vision loss that cannot be completely corrected with glasses or contact lenses.