From the Department of Neuro-Ophthalmology, Aravind Eye Hospital and Post Graduate Institute of Ophthalmology, Pondicherry, India.
The authors have no financial or proprietary interest in the materials presented herein.
Address correspondence to Sabyasachi Sengupta, DO, Resident, Aravind Eye Hospital, Pondicherry 605007, India.
Optic nerve entrance coloboma is a rare condition frequently associated with amblyopia. The clinical picture and visual field defects may be indistinguishable from glaucoma. The present report describes an extremely unusual variant, in which the primitive choroid was seen to traverse across the colobomatous defect. Optical coherence tomography may be a useful aid in recognizing this entity.
A 30-year-old man presented with poor vision in his right eye since childhood. There were no other ocular or systemic complaints. On examination, he was found to have a best-corrected distance visual acuity of 6/18 in the right eye and 6/6 in the left eye. A 20 prism diopter right exotropia could be measured with poor binocular single vision. The anterior segment examination was unremarkable in both the eyes with no relative afferent pupillary defect. The intraocular pressure was normal as well. The dilated fundus examination of the right eye revealed a large optic disc excavation, an entrance coloboma, involving the inferior three-quarters of the disc with preservation of the superior neuroretinal rim (Fig. 1A). The vasculature was seen to arise from the supero-temporal pole of the optic disc and traverse the floor of the excavation. A pigmented band traversed the optic disc horizontally from 3-o’clock to 9-o’clock, margin to margin, in a tram-track manner. The peripapillary area and retinal nerve fiber layer (RNFL) were normal. The rest of the retina including the macula was also normal. An optical coherence tomographic (OCT) analysis of the optic nerve head using the fast optic disc scan protocol demonstrated a normal-sized disc with distorted architecture, with the contour of the coloboma increasing progressively from the superior to the inferior pole (Fig. 1B). The colobomatous excavation showed high-intensity reflections resembling the retinal pigment epithelium and chorio-capillary complex. The RNFL was normal in thickness. The macular configuration and thickness was normal as well. A superior altitudinal field defect corresponding to the inferior disc coloboma was documented using Humphrey’s field analyzer 24-2 (SITA-Standard; Carl Zeiss Meditec, Dublin, CA) strategy (Fig. 2). The optic nerve head was normal in the left eye (Fig. 3A). The OCT of the nerve head and central visual fields were also within normal range (Fig. 3B). Results of cardiovascular and neurological evaluations were normal, and there was no family history of a similar disease.
Figure 1. (A) Optic Nerve Head of the Right Eye Demonstrating the Coloboma Involving the Inferior Two-Third of the Disc Sparing the Superior Neuroretinal Rim. The Central Pigmented Band is Seen Cutting the Disc into Two. (B) Optical Coherence Tomography Demonstrating the Deepening Contour of the Coloboma from the Superior (left of the Figure) to the Inferior Pole, Extending Slightly Beyond the Inferior Disc Margin, with High Reflectivity at the Base of the Coloboma. Arrows Indicate the Irregular Area of the Coloboma.
Figure 2. Central Visual Fields Done Using the HFA 24-2 Program Demonstrate the Superior Altitudinal Defect Corresponding to the Inferior Coloboma.
Figure 3. (A) Normal Optic Nerve Head in the Left Eye and (B) Normal Disc Contour of the Left Eye on the OCT.
Developmental anomalies of the eye in the region of the optic disc are commonly divided into 2 groups: (a) those in which failure of coaptation occurs within the optic nerve sheath and (b) those showing lesions of the peripapillary choroid or sclera.1
Only very rarely is a coloboma found to be restricted to the optic nerve entrance, the majority affecting the adjacent choroid as well.2 It occurs due to incomplete or abnormal coaptation of only the extreme proximal end of the embryonic fissure, whereas the rest of the fissure closes normally. Some authors have favored the view that the defect, similar that in an ordinary coloboma, is of the fetal fissure but restricted to its posterior part.3 Others have suggested that the defect differs from that in an ordinary coloboma by being more primitive and by representing a deformity of the primitive epithelial papilla, ie the primary optic vesicle or its stalk.4 Another possible explanation is, perhaps, that pressure on the eye from without might have caused the sclera to give way so that the retina prolapsed; this mechanism was suggested by Parsons and Coats as the explanation for a cystic excavation beside the optic nerve that they found in the eye of an infant who had an orbital encephalocoele.5 When the defect lies entirely within the nerve sheath, as in our case, the embryological defect cannot be deduced with certainty.
The choroid develops from the mesoderm earlier than the adjacent sclera. Thus, the primitive choroid can invaginate into the optic nerve head colobomatous defect at its entrance, especially after the intraocular pressure develops. We postulate a similar mechanism to explain the broken disc anomaly in this case, in which the primitive choroid is seen as a pigmented band running horizontally across the entire length of the disc. The answer as to why this has occurred selectively across the horizontal meridian is difficult to explain. Most probably, the basic defect could have been a failure of maturation in the posterior sclera, ie it had failed to fuse with the optic nerve and its sheath completely, resulting in an annular space surrounding the optic nerve as it left the globe (described as the optic nerve entrance coloboma). As the normal fusion failed to occur, the posterior choroid was driven posteriorly through the resultant hiatus.
Visual acuity, which depends on the integrity of the papillomacular bundle, may be mildly to severely hampered, but the level of visual function is difficult to predict from the mere appearance of the disc.6 The poor vision in our patient was attributed to the high myopia and hence anisometropic amblyopia.
Even though the axons appear normal in thickness at the level of the RNFL on the OCT, they traverse through the colobomatous defect at the optic nerve entrance (which lies distal to the RNFL) and are the cause for the corresponding field defect seen in this case.
In general, patients with optic disc coloboma should be kept on long-term follow-up for fundus examination. These eyes with isolated optic disc colobomas are prone to develop serous macular detachments, in contrast to the rhegmatogenous detachments that complicate retino-choroidal coloboma.7
About 50 case reports exist describing optic nerve entrance colobomas that are confined to the optic nerve sheath.1–9 Some authors have reported peripapillary masses resembling glioma and retinoblastoma for which a few eyes have been enucleated in the past.1,8 Rao et al. have reported situs inversus in association with optic nerve entrance coloboma.9 Our patient did not have any of these features. However, such an optic nerve appearance with primitive choroid running across the disc cutting it into half (the broken disc) has never been reported to the best of our knowledge. This is also the first instance where such an occurrence has been sighted after the advent of the OCT, making it the first report of an entrance coloboma documented using the OCT.
- Pedler C. Unusual coloboma of the optic nerve entrance. Br J Ophthalmol. 1961;45:803–807. doi:10.1136/bjo.45.12.803 [CrossRef]
- Duke-Elder S, ed. System of Ophthalmology, St. Louis: CV Mosby; 1963;3:472–481,677.
- Steinberg T.Amer J Ophthal. 1943;26:846.
- Mann I. Developmental Abnormalities of the Eye, 2nd ed. London:British Medical Association; 1957:113.
- Coats G. Royal London. Ophthal. Hosp. Rep. 1906;17:178.
- Brodsky MC. Congenital optic disc anomalies. Surv Ophthalmol. 1994;39:89–112 doi:10.1016/0039-6257(94)90155-4 [CrossRef]