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Magnetic resonance imaging of colobomatous optic hypoplasia
  1. MICHAEL C BRODSKY
  1. University of Arkansas for Medical Sciences, Little Rock, Arkansas
  1. Arkansas Children’s Hospital, 800 Marshall, Little Rock, AR 72202, USA.

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Editor,—Retinochoroidal coloboma is a common ocular malformation that can occur as an isolated finding in an otherwise healthy individual or as part of a complex malformation syndrome.1 Histologically, it is a well demarcated, excavated, infrapapillary area of absent retina, pigment epithelium, Bruch’s membrane, and choriocapillaris, with variable attenuation of the choroid.2 Some retinochoroidal colobomas incorporate the optic disc and cause the inferior aspect of the optic disc to appear retruded or absent within the excavation.3The purpose of this study was to determine whether such malformations are associated with hypoplasia of the intracranial optic nerve.

Five patients with unilateral retinochoroidal coloboma involving the optic disc underwent magnetic resonance imaging (MRI) of the head to rule out associated intracranial malformations. Patients consisted of two males and three females with ages at the time of MRI ranging from 2 weeks to 4 years. All patients had large unilateral retinochoroidal colobomas that incorporated the optic disc (Fig 1).

Figure 1

(A) Retinochoroidal coloboma incorporating the segmentally hypoplastic right optic disc (open arrows). The major retinal vessels delimit the lower margin of the optic disc. (B) T1 weighted coronal MR image (case 1) demonstrating hypoplasia of the right optic nerve (small arrow). The area of the right optic nerve is approximately half the size of the normal left optic nerve (large arrow).

MRI consisted of sagittal T1 weighted images, axial T2 weighted images, and coronal T1 weighted thin section images (with 3 mm slice thickness and 0.3 mm gaps) through the chiasm, intracranial optic nerves, and orbits. T1 weighted coronal MR images of the intracranial optic nerves were examined to compare the size of the intracranial optic nerve corresponding to the colobomatous eye with that corresponding to the normal eye. Cases 1–3 had no associated systemic or intracranial malformations. Case 4 had Goldenhar syndrome with hemifacial microsomia, cerebral hemispheric asymmetry without disorganisation, and colobomatous microphthalmos on the involved side. Case 5 had Aicardi syndrome with agenesis of the corpus callosum and bilateral grey matter heterotopia.

In all patients, coronal MRI showed a smaller intracranial optic nerve on the side corresponding to the retinochoroidal coloboma (Fig 1). The degree of intracranial optic nerve hypoplasia varied according to the ophthalmoscopic configuration of the optic disc. When the inferior aspect of the optic disc was present but retruded posteriorly within the colobomatous defect (case 1), the corresponding intracranial optic nerve was only slightly diminished in size relative to the normal optic nerve. When only the inferior aspect of the optic disc was absent within the colobomatous defect (cases 2–5), a moderate reduction in intracranial optic nerve size was seen.

In 1988, Novakovic et al demonstrated that focal retinal lesions can produce segmental hypoplasia of corresponding sectors of optic disc.4 In 1990, Brodskyet al showed that T1 weighted MRI can be used to confirm the clinical diagnosis of optic nerve hypoplasia by showing a reduction in size of the intracranial optic nerve(s).5 In the present study, MRI showed that colobomatous retinochoroidal malformations involving the optic disc are consistently associated with hypoplasia of the ipsilateral intracranial optic nerve, corresponding to the inferior segmental hypoplasia observed ophthalmoscopically. The nosological overlap between colobomatous derangement of the optic nerve and segmental hypoplasia, which has gone largely unrecognised, reflects the timing of colobomatous dysembryogenesis early in gestation and implicates a primary developmental failure of inferior retinal ganglion cells.4 MRI of other segmental optic disc malformations (for example, congenital tilted disc syndrome, unilateral high myopia) may disclose similar reductions in intracranial optic nerve size.

Acknowledgments

Supported in part by a grant from Research to Prevent Blindness, Inc.

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