Optic Nerve Sheath Meningioma

A benign tumour arising from arachnoid cap cells of the optic nerve sheath within the orbit or optic canal, or growing into the orbit from the sphenoid wing or intracranial compartment. It accounts for about 1 to 2 percent of orbital tumours and about one third of orbital meningiomas.

• Typically women (60 to 70 percent) aged 30 to 60 years. Occurs earlier and more aggressively in children and in neurofibromatosis type 2, where bilateral or multiple meningiomas may occur.
• Slow growth, but compresses nerve and its blood supply causing progressive visual loss.
• Most are low grade (grade 1) tumours of meningothelial or transitional type; inactivating mutations of NF2 are frequent.

• Classic triad: slowly progressive painless visual loss, optic atrophy, and optociliary shunt (retinochoroidal collateral) vessels on the disc. The triad is present in a minority; many have only one or two features.
• Disc swelling (early), pallor (late), relative afferent pupillary defect, colour desaturation, visual field constriction or central scotoma, transient visual obscurations on gaze.
• Mild proptosis (late), restricted motility, diplopia, choroidal folds, and hyperopic shift.
• Intracranial extension through the canal can produce chiasmal syndromes; hyperostosis of the sphenoid wing in en plaque meningioma causes proptosis.

• MRI orbits with contrast and fat suppression: diffuse or fusiform tubular enhancement of the nerve sheath with a non-enhancing optic nerve centrally ("tram track" sign on axial images, "doughnut" sign on coronal images); CT shows nerve sheath calcification, hyperostosis, thick tubular nerve.
• DOTATATE (somatostatin receptor) PET-CT confirms meningioma and defines extent.
• Visual acuity, colour vision, visual fields, optical coherence tomography (retinal nerve fibre layer, ganglion cell layer), fundus photographs, and serial measurements.
• Biopsy is generally avoided because it can cause blindness; reserved for atypical cases.
• Differential: optic neuritis with perineuritis, sarcoidosis, lymphoma, glioma, metastasis, idiopathic orbital inflammation, IgG4 disease.

• Observation with regular acuity, fields and MRI follow up (every 3 to 6 months) for patients with good vision and mild or stable features.
• Fractionated stereotactic radiotherapy (50.4 to 54 Gy in 28 to 30 fractions of 1.8 Gy, with image guidance or intensity modulated or proton therapy) is the preferred treatment when there is decline or threatened vision; it stabilises or improves vision in 80 to 95 percent and gives local control above 90 percent at 5 years. Late risks: radiation retinopathy, optic neuropathy, dry eye, cataract, pituitary dysfunction.
• Surgery: complete removal of the optic nerve sheath tumour usually sacrifices vision and carries risk of infarction of the nerve; limited to blind eyes with disfiguring proptosis or intracranial extension threatening the opposite nerve (resection of the optic nerve and tumour, craniotomy or orbitotomy).
• Systemic therapy: experimental; somatostatin analogues, bevacizumab and mTOR inhibitors for NF2-related disease.
• Optic nerve sheath fenestration is not indicated.
• Children: earlier intervention with radiotherapy, close follow up.