ABSTRACT
Optic nerve colobomas are congenital defects in the posterior globe of eye with typical funduscopic appearances. We report a two-year-old child suspected of morning glory disc anomaly who underwent thin-section 3D steady-state magnetic resonance imaging which showed characteristic findings of a tiny defect and discontinuity in the optic nerve head.
KEYWORDS: Optic nerve coloboma, magnetic resonance imaging, 3D MRI
Photo essay
A two-year-old boy presented with symptoms of divergent strabismus and visual impairment of his right eye. The child did not cooperate with vision testing. Right eye fundus examination revealed a ‘morning glory disc anomaly’ with a deepened optic disc suggestive of optic nerve coloboma. The macula was normal and there was no retinal detachment. Magnetic resonance imaging (MRI) was done to rule out other associated ocular and/or brain anomalies. The imaging was performed on a 1.5-T MRI scanner (HDX GE Healthcare, USA), using routine brain neuroimaging sequences. Fast Imaging Employing STeady-state Acquisition (FIESTA), a steady-state 3D sequence, was acquired at the level of orbits with the following parameters; TR, 4.8 ms; TE, 1.4 ms; slice thickness, 0.5 mm; FOV, 21 × 18.9 cm; matrix, 352 × 192; NEX, 4. A small defect of 1.5 mm diameter was noted in the optic nerve head with herniation of vitreous content into the defect confirming the coloboma (Figure 1a,b). The brain and left orbit were normal.
Figure 1.
Axial magnetic resonance imaging with a heavily T2-weighted 3D steady-state (FIESTA) sequence (a) and sagittal reformation (b) showing a small funnel-like defect in the optic nerve head and discontinuity in the normal uveoscleral coat with an extension of vitreous contents into the defect, suggesting the diagnosis of coloboma
The imaging abnormalities in optic nerve coloboma vary from near normal studies to posterior colobomas extending into the optic nerve sheath.1,2 Optic nerve colobomas result from incomplete closure of the embryonic optic fissure. Common co-existing intracranial abnormalities are basal encephalocoeles, moyamoya type of changes in the cerebral vasculature and congenital vascular syndromes.3,4 Though ‘morning glory anomaly’ fundoscopic findings may be seen in optic nerve colobomas, they may be seen in other conditions such as basal encephalocoeles and midfacial malformations.
3D steady-state MRI has been described to be of use in various cystic lesions in neuroimaging and orbital imaging. Steady-state sequences such as 3D FIESTA are highly T2 weighted and hence accentuate the signal intensity of water. This is of advantage in delineating the anatomy of fluid-filled structures like the globe.5 Other uses are in identifying a scolex in neurocysticercosis or findings related to the optic nerve-sheath space. The use of steady-state sequences with 3D slab acquisition and thin slices (0.2–1.0 mm) has the advantage of identifying structures not identified by routine MRI sequences.6,7 In this child, 3D steady state revealed the optic nerve coloboma that was very subtle and could have been easily missed by routine MRI imaging using T1- and T2-weighted sequences. This finding in steady-state imaging may be of help in funduscopically doubtful cases or when anterior globe abnormalities preclude fundus examination.
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