favorable prognosis, whereas younger age at presentation is associated with a poorer prognosis [5].

31.2 Optic Nerve Sheath Meningiomas

Meningiomas constitute approximately 20% of intracranial neoplasms and have a total annual incidence of 7.8 per 100,000. Optic nerve sheath meningiomas (ONSMs) represent 1–2% of all meningiomas. The frequency of ONSM among orbital tumors is 8–14% in larger series. After gliomas, these are the second most common type of optic nerve tumor. There is a relationship between optic nerve meningiomas and neurofibromatosis type 2 [10]. A study by Bosch et al. showed that 27% of patients with neurofibromatosis 2 developed optic nerve meningioma [11].

ONSMs can be divided into two types:

(i)Primary optic nerve meningiomas are those that arise from the meningothelial cells of the arachnoid villi surrounding the intraorbital or intracanalicular portions of the optic nerves. Primary meningiomas may grow intracranially to involve the optic chiasm, contralateral optic nerve, internal carotid artery, cavernous sinus, and sella tursica.

(ii)Secondary optic nerve meningiomas arise intracranially usually from the sphenoid ridge, tuberculum sellae, or olfactory groove, and later on invade the optic canal and orbit by extending between the dura and arachnoid of the optic nerve [10].

31.2.1Incidence

Primary ONSMs represent only 10% of all ONSMs; the other 90% are secondary ONSMs. Of the primary ONSMs, it is estimated that approximately 96% are true primary ONSMs and only 4% are considered ectopic, i.e., those arising from ectopic arachnoid cells within the orbital interstitial tissues or along the orbital nerves [10].

Of all primary ONSMs 92% arise intraorbitally and only 8% intracanalicularly. Therefore, when one sees what appears to be an ONSM in the region of the optic canal, one should look carefully for a secondary ONSM, in that there may be the primary lesion in the region of the anterior clinoid or tuberculum or other. Most of these tumors are unilateral with 5% presenting bilaterally. There is no strong evidence for predilection for left or right laterality. Canalicular meningiomas have a higher incidence of bilaterality than ONSM within the orbit [12].

As with other intracranial meningiomas, primary ONSMs typically develop in middle-aged women with remarkably consistent gender ratio with the proportion of females in several series ranging from 70 to 80%. Males tend to present at a younger

age, while the mean age at presentation in women is typically in their fourth to fifth decades; men present 10–15 years earlier. Primary ONSM can occur in children as well. Primary ONSM in children behave more aggressively, are characterized by faster growth, tend to be bilateral and show intracranial involvement, and exhibit a more invasive growth [10].

31.2.2 Histology and Pathophysiology

ONSMs arise from accumulations of meningothelial cells. Within the orbit, such accumulations of meningothelial cells are called the arachnoid villi. ONSM are regarded to arise from cap cells of these arachnoid villi. They grow within the subarachnoid space, the intact arachnoid and dura acting as a tumor “capsule.” Their spread usually results in a mass encircling the optic nerve while respecting its dural sheath, exerting increasing pressure on the nerve itself as well as a progressive impairment of its vascular supply. Though the pattern of growth is mainly one of growth along the preexistent anatomic pathways, invasion of the optic nerve along fibrovascular septae and vessels has been shown on a histological basis. Infiltration of the dura and extension into the orbital tissue is a rare event. The tumor may extend from the globe to the optic canal and eventually exhibit continued growth into the middle cranial fossa and involve the chiasm or even the contralateral nerve. Whether cases of bilateral ONSM represent two separate tumors or continuous growth of tumor cells along the tuberculum sellae remains a matter of debate [10].

31.2.3 Clinical Presentation

The symptoms and signs of ONSM depend on whether they have arisen within the orbit, within the optic canal, or intracranially. Slowly progressive visual loss is the hallmark of an ONSM. A relative afferent pupillary defect and dyschromatopsia invariably are present. The optic disc may be swollen or atrophic, have an element of both, or be normal in appearance. The optic disc is usually swollen when the tumor surrounds or compresses the intraorbital portions of the optic nerve and rarely shows peripapillary hemorrhages. When the tumor originates at the apex of the orbit or within the optic canal, there is slowly progressive visual loss without orbital signs, usually with a normal-appearing optic disc, although atrophy or slight swelling of the nerve may be seen. With time the disc becomes pale. Optociliary collateral vessels and retinal and choroid folds may be evident on fundus examination. The triad of visual loss, optic atrophy, and optociliary shunt vessel is most commonly caused by meningiomas. In some cases the tumor may invade the optic nerve by growing along the fibrovascular septa. Extraocular motility dysfunction is present in at least half of the patients, but most patients may not notice diplopia as their vision is impaired [4].

Fig. 31.4 MRI meningioma OS (post-gadolinium): (a) axial and (b) coronal

CT scans, but only gadolinium-enhanced MRI reliably demonstrates a meningioma that involves the intracanalicular optic nerve, and MRI shows the intracranial portion with more accuracy, especially around bony structures. T1-weighted MRI obtained following intravenous injection of gadolinium-based contrast material demonstrates moderate to marked contrast enhancement of meningiomas. Post-contrast-enhanced fat suppression T1 MRI is most valuable for defining the extent of the optic nerve pathology. Studies with high-quality MRI demonstrate that, even with small tumors, intracranial extension is the rule rather than the exception [13].

31.2.5 Treatment

Treatment of ONSM is conservative, because these tumors usually grow very slowly. Observation, serial automated visual fields, and regular MRI scans with gadolinium enhancement are appropriate for patients who have good vision and no evidence of intracranial or intracanalicular extension of tumor. An undisputed decline in visual function or any intracranial extension often results in treatment of the ONSM. The treatment of choice for a tumor confined to the orbit is stereotactic fractionated radiation. Stereotactic fractionated radiation uses multiple small doses of radiation using tight margins. A reasonable alternative, three-dimensional conformal fractionated radiation uses CT guided planning but usually requires wider margins [14]. Proton radiotherapy has also been used at our center for treatment of ONSM and is a particularly attractive option for younger patients, given the more focused delivery of radiation with proton therapy and theoretically fewer expected side effects in the surrounding intracranial structures such as the pituitary axis (see Chapter 32). Conventional radiation uses much wider margins and would not be recommended for the treatment of ONSM. The radiation can be administered during 5–6 weeks in 28 daily fractions of 1.8–2 Gy/fraction to a total of 50.4–56 Gy. Many patients have improvement or stabilization of their visual function. Gamma knife radiosurgery does not have a role in ONSM because the required dose is toxic to