A 66-year old male patient presented to our department to investigate progressive loss of vision over the last five years, associated with a visual field defect on clinical ophthalmologic examination. There was no evidence of other systemic diseases.
Ultrasound examination of the right eye revealed a retrobulbar isoechoic mass with well- defined margins.
Magnetic Resonance (MR) imaging confirmed a solid, encapsulated, intraconal mass along the optic nerve course, eccentrically positioned. A slight right bulbar protrusion was perceptible. The mass was hypointense on T1-weighted images, slightly hyperintense on T2 weighted and T2 fat saturated images. After administration of gadolinium there was homogeneous enhancement. There was no sign of invasion of the retrobulbar fat neither of other orbital structures.
The patient underwent surgery via a trans-sphenoidal approach. The histological diagnosis of the excised mass was a schwannoma of the optic nerve.
Schwannomas are slowly progressive, benign proliferations of Schwann cells that occur more often in middle-aged individuals . Orbital schwannomas account for 1-2% of all orbital tumours . They may arise from trigeminal nerve branches or less commonly from peripheral branches of the oculomotor, trochlear, abducens nerves, or autonomic fibers . The optic nerve is exceptionally affected by Schwannomas as its sheath is formed by oligodendrocytes; however multiple cases in literature demonstrate that schwannomas may arise from ectopic Schwann cells , from mesenchymal cells which undergo differentiation  or, more often, from Schwann cells that accompany sympathetic nerves which are tightly adherent to the optic nerve sheath .
Clinical signs and symptoms are nonspecific: lesions may be asymptomatic or associated with progressive painless proptosis, visual field reduction, restriction of ocular movements, diplopia and pain . The differential diagnosis should include malignant peripheral nerve sheath tumour, cavernous malformation, glioma, neurofibroma.
US is useful in the first instance to evaluate the eyeball and retrobulbar space and, as in our case, to identify the presence of a retrobulbar mass. MR is the imaging modality of choice to identify, characterize and locate the mass, revealing its exact extent and its relationship with nearby structures. On T1-weighted images, schwannomas typically appear hypointense, which is nonspecific. On T2-weighted images they usually demonstrate mild hyperintense signal, which can be heterogeneous reflecting the mixed solid and cystic components of the tumor, as well the prevalence of Antoni A or B cellular patterns. Schwannomas of the Antoni B histologic type usually appear hyperintense, whereas those of the Antoni A type tend toward hypointensity on T2-weighted images . After intravenous contrast material administration, early homogeneous uptake is typically observed. Information obtained from T2W and contrast enhanced sequences is useful to discriminate between schwannomas and cavernous malformations which, unlike schwannomas, show homogeneous, marked T2 hyperintensity and typical progressive enhancement on delayed venous phases .
Use of MR imaging greatly facilitates successful surgical excision. In literature different surgical approaches are described, as transcranial or transorbital surgery, or a less invasive endoscopic endonasal approach . In some cases radiotherapy could be an adjunctive therapy. The treatment of choice depends on the location of the mass and its relationship with adjacent structures. Our patient underwent surgical decompression of the optic nerve by trans-sphenoidal approach, with complete resolution of the clinical symptoms.
Differential Diagnosis List
Schwannoma of the optic nerve.
Vasculogenic orbital masses (Cavernous malformation
Optic nerve glioma
Optic nerve sheath meningioma
Malignant peripheral nerve sheath tumor
Schwannoma of the optic nerve.