Introduction
Optic nerve sheath meningiomas (ONSMs) represent the most common tumors of the optic nerve sheath and account for one-third of primary optic nerve tumors.1 The diagnosis is usually made by a combination of clinical and neuroimaging findings and is rarely confirmed by a biopsy. The natural history of ONSMs typically shows progressive, painless visual decline, although long periods of stability have been reported.
Treatment options include observation for patients with minimal visual loss or nonprogressing disease. Stereotactic fractionated radiation therapy is the treatment of choice for patients with more advanced disease or progressive visual loss. Surgical treatment is associated with a high risk of blindness and is rarely recommended. It should be reserved for a select group of patients with primarily exophytic tumors.
Primary and Secondary Optic Nerve Sheath Meningioma
Primary ONSMs represent 1%-2% of all meningiomas and are the second most common optic nerve tumors after gliomas. They arise from arachnoid cap cells and can develop anywhere along the course of the optic nerve. The vast majority (90%) of ONSMs are secondary and represent anterior extension of intracranial meningiomas arising from the dura near or on the planum sphenoidale.1,4
In a review by Dutton of 5000 orbital meningiomas, 95% of ONSMs were unilateral.1 Tumors confined to the optic canal (8%) had a higher incidence of bilaterality (38%). Bilateral and multifocal ONSMs occur most commonly in patients with neurofibromatosis type 2.
Clinical Characteristics
The average age at presentation is reported as 41 years, with a female preponderance of 61%1. Only 4% of ONSMs occur in the pediatric population. However, the childhood form of ONSM is often characterized by a more aggressive course with rapid growth and higher incidence of intracranial extension.2
Patients typically present with slowly progressive vision loss. Other visual complaints include double vision, transient visual obscurations, and gaze-evoked amaurosis. On occasion, patients will describe periocular or retrobulbar pain.
Common clinical findings include ipsilateral relative afferent pupillary defect, dyschromatopsia, and visual field defect as well as optic disc swelling, optic atrophy, or both. Optociliary collateral vessels occasionally develop and can be seen in both swollen and atrophic nerves. The triad of visual loss, optic atrophy, and optociliary collateral vessels is almost pathognomonic for ONSM. Other ophthalmoscopic findings include choroidal folds and contiguous macular edema. External signs such as proptosis, lid edema, and limitation of ocular motility can also be found on examination.
Neuroimaging Characteristics
The diagnosis of ONSM is usually supported by neuroimaging findings. Several radiographic growth patterns have been described: tubular, globular, fusiform, focal, and cystic.2,3 High-resolution computed tomography (CT) scanning and thin-section magnetic resonance imaging (MRI) represent the most commonly used modalities. On thin-section axial CT images, hyperdense enhancement of the meninges surrounding an enlarged hypodense nerve ("tram track" sign) is highly suggestive of ONSM. On noncontrast CT imaging, linear calcifications along the length of the nerve can also be seen.
MRI is currently the procedure of choice to identify and determine the extent of ONSMs. T1-weighted images of the brain and orbits, with fat suppression and gadolinium, allow excellent visualization of ONSM. The tumor is typically isointense or slightly hypointense to the brain on T1-weighted imaging, and it enhances with gadolinium. Imaging will often show the tumor to be separated from the optic nerve on coronal views, and it provides excellent details of the intracanalicular and intracranial portion of the optic nerve.
The radiographic differential diagnosis includes orbital inflammatory disorder, optic perineuritis, sarcoid infiltration, lymphoma, carcinomatous meningitis, and metastatic infiltration of the optic nerve and optic nerve sheath.
Clinical Course
Although visual loss will generally occur if the tumor is left untreated, it typically occurs slowly and is not associated with mortality. A rapid growth can occasionally be observed during pregnancy, most likely related to estrogen or progesterone receptors expressed by meningioma cells. In the presence of an atypical clinical course characterized by sudden and severe or rapidly progressive visual loss, a biopsy limited to the dural sheath may be needed. However, the biopsy should be reserved for patients with poor vision who have undergone serial lumbar punctures to exclude inflammatory and neoplastic processes as well as for patients for whom corticosteroid therapy has failed to improve vision.
Treatment
Observation
Observation is appropriate in patients with no significant optic nerve dysfunction, no significant progression, and no significant intracranial extension. However, these patients should be monitored closely with repeat clinical assessment and neuroimaging every 6 months for the first 1-2 years. If the disease remains unchanged, clinical and radiographic reassessment can then be scheduled at less frequent intervals.4
Radiation therapy
Stereotactic fractionated radiotherapy (SFR) is currently accepted as the best vision-preserving therapy for the management of ONSM in patients with progressive visual loss or advanced disease. Several published articles report stability or improvement of optic nerve function following SFR.5-9 The major concern with radiotherapy is late toxicity affecting the optic nerve, the retina, and the pituitary gland. Patients with diabetes are more challenging to treat since they have an increased susceptibility to the vascular complications associated with radiation therapy.5
Surgery
Given its tendency to cause postoperative blindness, surgery for ONSM has largely been replaced by radiation therapy. Surgical excision should be limited to a select group of patients with primarily exophytic tumors.4 Turbin and colleagues10 reported visual improvement after optic nerve decompression in 2 patients with severe disc edema and rapid vision loss. Until more data are available regarding this surgical option, it should be reserved for a carefully chosen group of patients and should be used only in conjunction with radiation therapy.
Summary
The diagnosis of ONSM is usually based on a constellation of clinical findings supported by neuroimaging. Biopsy is rarely needed, as surgical intervention carries a high risk of morbidity. SFR represents the current treatment of choice for patients with progressive visual loss. However, the optimum timing of radiation therapy (before, during, or after visual changes) remains uncertain at this point. Longer follow up is needed for a definite conclusion concerning the long-term efficacy as well as late toxicity of SFR.
References
1. Dutton JJ. Optic nerve sheath meningiomas. Surv Ophthalmol. 1992;37(3):167-183.
2. Saeed P, Rootman J, Nugent RA, White VA, et al. Optic nerve sheath meningiomas. Ophthalmology. 2003;110(1):2019-2030.
3. Rosca TI, Carstocea BD, Vladescu TG, St Tihoan C, et al. Cystic optic nerve sheath meningiomas. J Neuroophthalmol. 2006;26(2):121-122.
4. Miller NR. New concepts in the diagnosis and management of optic nerve sheath meningioma. J Neuroophthlamol. 2006;26(3):200-208.
5. Turbin RE, Thompson CR, Kennerdell JS, Cockerham KP, et al. A long-term visual outcome comparison in patients with optic nerve sheath meningioma managed with observation, surgery, radiotherapy, or surgery and radiotherapy. Ophthalmology. 2002;109(5):890-899.
6. Baumert BG, Villa S, Studer G, Mirimanoff RO, et al. Early improvement in vision after stereotactic radiotherapy for primary optic nerve sheath meningioma. Radiother Oncol. 2004;72(2):169-174.
7. Landert M, Baumert BG, Bosch MM, Lutolf UM, et al. The visual impact of fractionated stereotactic conformal radiotherapy on seven eyes with optic nerve sheath meningiomas. J Neuroophthalmol. 2005;25(2):86-91.
8. Richards JC, Roden D, Harper CS. Management of sight-threatening optic nerve sheath meningioma with fractionated stereotactic radiotherapy. Clin Exp Ophthalmol. 2005;33(2):137-141.
9. Jeremic B, Pitz S. Primary optic nerve sheath meningioma: stereotactic fractionated radiation therapy as an emerging treatment of choice. Cancer. 2007;110(4):714-722.
10. Turbin RE, Wladis EJ, Frohman LP, Langer PD, et al. Role for surgery as adjuvant therapy in optic nerve sheath meningioma. Ophthal Plast Reconstr Surg. 2006;22(4):278-282.
Author Disclosure
Dr. Falardeau states that she has no financial relationship with the manufacturer of any product discussed in this article or with the manufacturer of any competing product.