Ectasia of the optic nerve sheath is a rare finding in imaging studies, and various terms have been used to desribe this entity: arachnoid cyst,4
patulous subarachnoid space,6
Concerning pathogenesis and associated diseases, Shanmuganathan et al7
reviewed the literature and identified a patient group with associated progressive hyperopia and choroidal folds.
Lövblad et al
reported three patients with neurofibromatosis type 1 and tube‐like ectasia of the optic nerve sheath.8
In neurofibromatosis type 1, dural ectasias are a typical finding and are most often present in the spinal canal.9,10
In contrast to an enlargement of the optic nerve sheath by optic gliomas, which are also common in neurofibromatosis type 1, the ectasias are isointense to cerebrospinal fluid.
Hansen and Helmke1
investigated the optic nerve sheath response to pressure during CSF absorption studies in 12 patients undergoing neurological testing, and found that changes in the diameter of the sheath followed changes in the intracranial pressure.
In our case, neither progressive hyperopia, choroidal folds, nor features of neurofibromatosis were present. The intracranial pressure was at least temporarily raised, but the morphological changes remained after normalisation of the intracranial pressure.
The morphology of the optic nerve sheath ectasia in our case is different from what has been reported previously. Whereas previous reports describe a tube‐like enlargement, in our case it was aneurysm‐like.
Vascular aneurysms usually arise from weak parts of the vessels, from the vascular division sites. In our case the aneurysm‐like ectasia of the optic nerve may have been pre‐existing and only accidently detected. Alternatively, one may hypothesise that a focal weakness in the optic nerve meninges may have been induced by some indirect trauma to the orbit. The increased pressure within the optic nerve sheath may then have contributed to the formation of the optic nerve sheath aneurysm.