Retrobulbar Optic Nerve Diastasis Caused by Intraoptic Nerve Cyst

A 56-year-old woman was referred for evaluation of an optic disc abnormality in the left eye. She had already noticed a black shadow in the upper paracentral visual field of the left eye for many years. She had no history of systemic or ophthalmologic disease. On examination, visual acuity was 20/20 in each eye with normal color vision and pupillary testing. Slit lamp exam was unremarkable and intraocular pressures were 14 mm. Hg bilaterally. Ophthalmoscopy showed a peripapillary rod-shaped band of white tissue on the inferior edge of the left optic disc extending into the vitreous. Retinal nerve fiber layer (RNFL) revealed inferior thinning in the left eye (Fig. 1A). B-scan ultrasonography of the left optic nerve head detected a fibrous structure extending into the vitreous, consistent with a Bermeister papilla. Automated perimetry showed a paracentral area of superior depression in the left eye (Fig. 1B). MRI of the orbits demonstrated that the left optic nerve had a diastatic appearance located approximately 12 mm. behind the optic disc (Fig. 2A). This diastatic space had both low and high signal intensity, with no enhancement, which was consistent with fluid content. In addition, serial coronal short tau inversion recovery (STIR) images showed that the fluid cavity was located inferiorly and appeared to divide the nerve into upper and lower segments on sections closer to the globe (Fig. 2B). The cystic structure was separated from the cerebrospinal fluid in the subarachnoid space surrounding the optic nerve.FIG. 1.: In the left eye, there is a rod-shaped band of tissue on the inferior edge of the optic disc extending into the vitreous. OCT images show inferior RNFL thinning in an accurate pattern. The right eye is normal. B. In the left eye, visual field testing (Humphrey 30-2) shows a paracentral area of superior depression. The right visual field is intact. OCT, Optical coherence tomography; RNFL, retinal nerve fiber layerFIG. 2.: MRI of the left orbit. A. T2 axial (left), sagittal (middle) and coronal (right) images demonstrate that the retrobulbar optic nerve is split into two segments by a cystic lesion. B. Serial coronal STIR images of the left optic nerve at 2.5 mm. (lower left), 5 mm., 10 mm. and 12.5 mm. behind the globe show that the cyst is located inferiorly and splits the nerve into two segments. The cyst does not connect to the subarachnoid space.Repeat ophthalmologic examinations including visual fields and OCT as well as orbital MRI scans showed no change in the diastasis of the optic nerve in three years of follow-up. Interpretation of Findings Optic nerve cysts are rare. Most previous reports have documented this optic nerve anomaly following trauma or penetration by an aneurysm.1 There is one case report of congenital optic nerve diastasis with optic nerve hypoplasia2 and others with3 or without optic nerve pit.4 In our patient, MRI demonstrated that the abnormality was filled with fluid consistent with a cyst, but this was not proven by histopathologic examination. nerve cysts. Some of them coexisted with an optic pit. Only 4 cases of primary intraorbital optic nerve cyst without optic pit have been described.2 Arachnoid cyst, which has arachnoidal layer or meningoepithelial cell lining (that distinguishes it from primary cyst) along the inner cystic wall, may also involve a portion of the intraorbital optic nerve.3 The cause of the optic nerve cyst is unclear. Kim et al4 proposed two hypotheses on the histogenesis of the cyst. The first is a congenital developmental anomaly of the optic nerve occurring during the 4–5-mm sized embryonic stage with invagination of the optic vesicle and stalk. The second is a formation by budding from the subarachnoid system in the optic nerve. MRI of the orbit in the present case demonstrated that the optic nerve cyst did not seem to contact with subarachnoid space along the optic nerve on coronal STIR images. Thus, the cause of the intraoptic nerve cyst in the present case might be a congenital developmental anomaly of the optic nerve occurring during the embryonic stage. One possibility may account for the relationship between the optic nerve anomaly and the reduction of RNFL thickness in our patient. The serial coronal MR images showed that optic nerve cyst was located in the inferior retrobulbar optic nerve. In addition, OCT showed inferior arcuate RNFL thinning compatible with visual field depression. Therefore, optic nerve cyst might compress the inferior optic nerve fibers resulting in the reduction of the inferior RNFL. Over time, optic nerve cysts may lead to progressive visual field loss or proptosis.4,5 Although our patient has remained stable for three years, careful observation will be required in the future.