Optic Disc and Chorioretinal Coloboma

An optic disc coloboma is characterised by a yellow white bowl-shaped excavation of the optic nerve, typically affecting the inferonasal part of the nerve, but in some cases involving the entire disc. Often associated with an optic disc coloboma are chorioretinal colobomas. These are typically found in the inferior retina and have a yellow-white appearance, often with hyperpigmented margins. OCT imaging shows excavation of the retinal and choroidal layers.

Optic nerve head colobomas arise from incomplete closure of the embryonic fissure prior to birth. This closure occurs when the embryo is approximately 10mm in size and at this stage, no retinal differentiation has occurred. Following this period of closure, the retina starts to develop distinct layers. The cells that lie over the top of a coloboma are actually still undifferentiated retinal cells - the cells that would otherwise in normal development develop into ganglion, amacrine, and Müller cells. These cells are referred to as the intercalary membrane.

Studies have shown that 23% to 42% of those with colobomas will eventually develop retinal detachment (Jesberg et al (1961) and Schepens et al (1983), frequently in the second decade of life. Choroidal neovascularisation can also occur at the edge of a coloboma with vessels growing in towards the intercalary membrane.

Kyoko Ohno-Matsui, Akito Hirakata, Makoto Inoue, Masahiro Akiba, Tatsuro Ishibashi; (2013) Evaluation of Congenital Optic Disc Pits and Optic Disc Colobomas by Swept-Source Optical Coherence Tomography. Invest. Ophthalmol. Vis. Sci;54(12):7769-7778.

Nakamura, K. M., Diehl, N. N., & Mohney, B. G. (2011). Incidence, ocular findings, and systemic associations of ocular coloboma: a population-based study. Archives of ophthalmology (Chicago, Ill. : 1960), 129(1), 69–74. https://doi.org/10.1001/archophthalmol.2010.320