Meningiomas are neoplasms arising from arachnoid cap cells in the meninges. They can arise in the orbit as primary orbital meningiomas (ie. optic nerve sheath) or extend into the orbit from intracranial structures (ie. sphenoid wing), as secondary orbital meningiomas.
Meningiomas are typically slow growing, benign neoplasms. Primary orbital meningiomas arise from within the orbit and include: optic nerve sheath meningiomas (ONSM), which are the most common tumors of the optic nerve sheath, and primary ectopic meningiomas.
Secondary orbital meningiomas arise from adjacent structures listed below and extend into the orbit via the optic canal or the superior orbital fissure: sphenoid wing (SOM), anterior clinoid, cavernous sinus, tuberculum sellae, and olfactory groove.
Meningiomas are neoplasms arising from arachnoid cap cells and can arise anywhere arachnoid cells are found. In primary orbital meningiomas, they arise from the meningothelial cells around the optic nerve sheath. Or rarely, ectopic meningeal tissue in the orbit .
Meningiomas are the most common primary intracranial tumors that account for approximately 19% of primary intracranial tumors. However, orbital meningiomas account for only approximately 4-8% of total orbital lesions where approximately 2-4% are optic nerve sheath meningiomas and 2-4% are secondary orbital meningiomas. Primary ectopic orbital meningiomas are very rare with a few case reports.
Incidence of orbital meningioma is much higher in women compared to men, similar to intracranial meningiomas. The average age of onset is approximately 45 years old in orbital meningiomas. However, approximately 7% of ONSM occur it patients younger than 20 years of age and are often more aggressive in the pediatric population .
Although the risk is low with advances in techniques in radiation therapy, there are reports of orbital meningioma after radiation therapy. Radiation induced meningiomas are the most common brain tumors caused by radiation .
Patients with neurofibromatosis type 2 have a mutation in the tumor suppressor gene merlin, and are susceptible to developing multiple meningiomas. Deletion of chromosome 22 is found in more than 50% of meningiomas. 
There is significant heterogeneity in the histopathology of meningiomas as can be appreciated above by the several subtypes described in the WHO classification system. In general, whorls of meningothelial cells are present. Cells may be spindle shaped and separated by fibrous connective tissue. Calcified whorls, or psammoma bodies, may also be present. On immunohistochemistry, meningiomas are positive for vimentin, which stains for mesenchymal tissue. Estrogen and progesterone receptors may also be present, which might explain the growth of meningiomas during pregnancy.
The World Health Organization has established criteria to grade meningiomas from benign to malignant based on histologic features. According to the most recent edition (2007), Grade I meningiomas are considered benign and lack anaplastic features; Grade II meningiomas are defined by having 4+ mitoses per 10 high power fields (HPFs), brain infiltration, and 3 or more of the following features: small cell change, increased cellularity, prominent nucleoli, sheet like growth, or necrosis; and Grade III meningiomas have 20+ mitoses per 10 HPFs with histologic features of malignancy.
|WHO Grade 1 (benign) subtypes include:||Meningiothelial, fibrous (fibroblastic), transitional (mixed), psammomatous, angiomatous, microcytic, secretory, lymphoplasmacyte-rich, metaplastic|
|WHO Grade 2 (anaplastic) subtypes include:||Choroid, clear cell, atypical|
|WHO Grade 3 (malignant) subtypes include:||Papillary, rhabdoid, anaplastic|
Orbital meningiomas are most commonly WHO Grade I. The most common subtype in the orbit was meningothelial and transitional was the second most common subtype .
Progressive painless vision loss with proptosis
Patients may present with the following signs due to either mass effect and compression or infiltration of the optic nerve. On exam, patients may have proptosis, which should be measured using an exophthalmometer, optic nerve pallor or optic nerve head edema, decreased visual acuity, visual field deficits, globe displacement, extraocular movement restriction, or optociliary shunt vessels.
Patients may experience the following symptoms to varying degrees based on severity of disease and location of tumor compression/ infiltration. Symptoms include vision loss, diplopia, headache, and retrobulbar pain.
Meningiomas typically appear isodense to the optic nerve and hyperdense to brain on CT, and will have homogeneous enhancement on contrast studies. Approximately 20% have calcification. “Tram track” sign can be seen on axial imaging of the enhancing tumor surrounding the optic nerve. Hyperostosis of surrounding bone can be seen as well. Attention should be paid to canal involvement and correlated with the clinical exam. On MRI, meningiomas are isointense on T1 weighted imaging, and mildly hyperintense on T2 weighted imaging. On contrast studies, meningiomas exhibit homogeneous enhancement. Helpful signs include a dural tail (this can be seen in other conditions and is not pathognomonic) and sunburst vessels .
Management of orbital meningiomas are primarily determined by patient symptoms. In patients with visual acuity of 20/50 or better patients are usually managed conservatively  with biannual neuroimaging and ophthalmologic examinations with visual field testing and color plates. Frequency of neuroimaging and exams can be decreased once tumor is shown to be stable. Younger patients should be examined more frequently.
The goals of surgery in orbital meningiomas includes long term tumor control, recovery of optic nerve and cranial nerve function, and cosmesis. Surgery for ONSM is typically reserved for those with intracranial extension to the optic chiasm or in a patient with a blind eye with significant disfiguring proptosis. Management of orbital meningiomas involves an interdisciplinary team with an oculoplastic surgeon and neurosurgeon. Surgical resection or debulking of secondary orbital meningiomas are determined by location of the tumor and associated symptoms. Complete surgical resection is often not possible due to the delicate structures involved, and patients with orbital meningiomas often require periodic debulking surgeries. Mariniello et al. points out that visual function in patients with SOMs mainly depends on the presence and extent of compression of the optic nerve at the entrance or within the optic canal. They emphasize that optic canal decompression is important for relieving optic nerve compression . Studies show that about 75% of patients have stable or improved visual acuity after surgery. Improvement in visual acuity is reported to range from 30-50%. Proptosis is improved in 85-90% of patients after surgery .
Recent data suggests radiation therapy should be considered prior to surgery in symptomatic patients with ONSM. The optic nerve and chiasm are the most sensitive structures in the area even compared to adjacent cranial nerves. In single dose radiosurgery, damage to the optic nerve has been shown to be dose dependent with very low incidence of optic neuropathy with doses <10 Gy and up to a 78% incidence with doses >15 Gy. Incidence of damage to the optic chiasm and nerve is extremely low with fractionated therapy up to a total dose of <60 Gy, as long as the fraction dose is <2 Gy. 
A study by Turbin et al. compared visual outcome of four treatment groups in patients with ONSM: radiation only, surgery only, radiation and surgery, and observation. Radiation alone had the best visual outcome and authors recommend fractionated radiotherapy of 50-55 Gy as initial treatment in patients with ONSM . Similarly, it is thought that adjunctive radiation therapy for residual tumor after surgical resection may improve outcomes with decreased recurrence and stable or reduced tumor size. A study by Peele et al. showed there was no recurrence in 42 patients treated with radiation during a 4 year follow up period after subtotal resection or recurrent tumor. In the non-irradiated group, there was recurrence in 42% of patients with subtotal resection of primary tumors.
Some tumors that are benign on initial histological analysis may undergo malignant transformation following radiation treatment and then can become clinically much more aggressive with rapid growth and regrowth after surgical debulking.
Post-operative complications include trigeminal hypesthesia, oculomotor palsy, and facial nerve palsy. Ocular complications associated with radiation therapy include radiation retinopathy, iritis, cataracts and retinal vascular occlusions. 
Rate of recurrence ranges from about 17%  to 42% . The recurrence rate is lower in patients with radiation therapy after surgery. However, this is also dependent upon the WHO grade of the tumor.
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