Purpose: 

To report a case of optic nerve meningocele simulating massive, recurrent extraocular extension of choroidal melanoma.

Method: 

Case report.

Results: 

A 53-year-old white man with choroidal melanoma in his left eye of 7.3-mm thickness was treated with plaque radiotherapy and transpupillary thermotherapy. On 1-year follow-up examination, visual acuity was 20/20 in the right eye and 20/30 in the left eye. The regressed choroidal melanoma scar in the left eye measured 1.5 mm in thickness with stable margins. The optic disk was normal. Ultrasonography demonstrated regressed echogenic choroidal scar, with an echolucent multilobulated retrobulbar mass, suspicious for extraocular extension. On magnetic resonance imaging, the retrobulbar mass corresponded to a distended and kinked optic nerve sheath, filled with extensive subarachnoid fluid and normal-size optic nerve with apposition against the posterior globe. There was no extraocular extension of tumor. Similar but less distended right optic nerve sheath was documented, consistent with optic nerve sheath meningocele in both eyes. Observation was advised and the findings remained stable.

Conclusion: 

Optic nerve sheath meningocele is a benign dilatation of the optic nerve sheath that can simulate orbital tumor or extraocular extension of intraocular tumor. Magnetic resonance imaging can reliably differentiate these conditions.