Literally meaning a mutilation, the term ‘coloboma’ is used to denote a gap within or absence of a tissue or structure. Intraocular colobomas are caused by imperfect closure of the embryonic fissure and therefore occupy an inferior nasal position. They are non-progressive, congenital malformations, which, depending on the degree of imperfection, may affect singly or together the iris, ciliary body, lens, choroid, retina, optic nerve head and optic nerve. They are bilateral in over 60 per cent of cases. In most patients, the coloboma is sporadic in nature and, when inherited, an autosomal dominant pattern predominates.
Other congenital ocular malformations have been described in association with colobomas such as microphthalmos, lenticonus, cataract, persistent hyaloid artery and posterior embryotoxon. Serous non-rhegmatogenous and rhegmatogenous retinal detachments have been reported in some cases, as has choroidal neovascularization occurring at the border of the colobomatous area.
Colobomas may occur as an isolated defect or together with a variety of systemic conditions. Examples include the CHARGE syndrome (coloboma, heart disease, atresia choanae, retarded growth, genital hypoplasia and ear anomalies), renal hypoplasia, midline facial abnormalities and trans-sphenoidal encephalocele.
Visual acuity may be normal despite a seemingly extensive area being involved with the ophthalmoscope. The determining factor is the degree of damage to the retinal nerve fibres. If the latter is severe, visual acuity will be poor. Color vision may be abnormal.
Outwardly, the patient may show a 72 characteristic, inferior, nasally pointing iris coloboma if that tissue is also involved. When unilateral, strabismus may be a presenting sign. Retinochoroidal colobomas typically present as a yellowwhite, semi-lunate or comet-shaped area in the fundus. Scleral thinning may result in a considerable posterior ectasia and the edge is usually sharply defined and pigmented. The retinal vessels pass over the colobomatous zone normally. The cardinal sign is that the lesion is directly below the optic disc. If the embryonic fissure partially closes a circular or oval self-contained lesion is seen surrounded by normal retina. When affecting the optic nerve head, the optic disc appears abnormally large, white and excavated, the latter being greatest inferiorly. Peripapillary pigmentation is common and retinal vessels frequently emerge from the inferior aspect of the coloboma like the spokes of a bicycle wheel. A superior visual field defect is usually found and, in colobomas of the optic nerve head, this often has glaucoma-like characteristics such as an arcuate shape and in respect of the horizontal midline.
Intraocular colobomas are uncommon. When they do occur, retinochoroidal coloboma is the most frequently encountered.
Other conditions where the optic nerve head appears abnormally large or excavated such as megalopapilla, morning glory optic disc and optic disc pit.
Advice and review Although there is no treatment for retinochoroidal and optic nerve head colobomas, the practitioner should keep potential systemic associations, albeit rare, in mind.
Left: mild optic nerve head coloboma in a 16-year-old white boy. Vision was
normal although there was an extensive visual field loss in the superior arcuate nerve fibre bundle. The anterior eye was normal. Right: severe optic nerve head coloboma.
Left: coloboma affecting the right optic disc and fundus. Vision was normal. Right: large coloboma below the right optic disc. Vision was normal. The coloboma also affected the anterior uveal tract.