CHAPTER 33 Primary Optic Nerve Sheath Meningiomas

INTRODUCTION

Optic nerve sheath meningiomas (ONSMs) represent 1% to 2% of all meningiomas, 1.7% of all orbital tumors, and about 35% of all intrinsic tumors of the optic nerve.¹^–³ Thus ONSMs represent a rare but nevertheless important entity due to their natural course of slowly progressive and unremitting loss of vision. Classical primary ONSM arise from the meningothelial cap cells of the arachnoids surrounding the intraorbital optic nerve and may extend along the optic canal intracranially. Secondary ONSM extends from the planum sphenoidale into the subdural or subarachnoid spaces surrounding the nerve within the optic canal and, ultimately, within the orbit.^1,^2,⁴^–⁷ Most ONSMs are unilateral, with 5% manifested bilaterally.¹ ONSM occurs predominantly in middle-aged women or in children.

The treatment of ONSM remains controversial, but includes surgery, radiotherapy, and plain observation. Complete surgical excision is thought to result in blindness in almost all cases.⁸ When intracranial extension that threatens the optic chiasm or the contralateral optic nerve is present, complete excision of the intracranial part via craniotomy has been described.^3,^7,⁹ Complete neurectomy and tumor resection are also performed in cases of severe unilateral loss of vision accompanied by disfiguring proptosis. As surgical reports revealed a high morbidity in terms of visual loss and recurrences due to incomplete removal, other treatment options were debated.¹⁰^–¹⁷ In adults with good vision, observation was often chosen as first method because of the slow-growing nature of these tumors.¹⁸ During the past several years, treatment plans for ONSM changed with intervention of precise fractionated and stereotactic radiotherapy. The role of radiation became more and more important as several authors have reported stabilization or even improvement of vision.^8,^14–17,^19,²⁰ Radiation therapy is increasingly being offered to adults as primary therapy once mild to moderate vision loss develops. Despite the positive results reported in the literature, concerns regarding secondary complications have limited the acceptance of radiotherapy.²¹

In 2004, we introduced a new classification system in order to the possible manifestations and recommended optimal treatment modalities where surgical intervention and radiotherapy play a supplementary role.²² We overview a single center series of 90 optic nerve sheath meningiomas treated over a 17-year period (n = 70 surgery only, n = 5 radiation only, n = 18 surgery and postoperative radiation, n = 2 observation).

CLASSIFICATION

Our classification system based on the tumor location and extent is provided to clarify the possible manifestations of ONSM (Fig. 33-1).²² Treatment modalities are derived from the different types and subtypes.

FIGURE 33-1 Schematic drawing of different types and subtypes of ONSM according to their location. Type I, purely intraorbital; Ia, flat tumor extension around the optic nerve; Ib, large extension around the optic nerve; Ic, exophytic tumor upon the optic nerve. Type II, intraorbital with extension through the optic canal or superior orbital fissure; IIa, intraorbital tumor with tumor growth through the optic canal; IIb, tumor of the apex, superior orbital fissure, and cavernous sinus. Type III, intraorbital and large intracranial tumor extension; IIIa, intraorbital and intracranial extension to the chiasm; IIIb, intraorbital and large intracranial extension to the chiasm, contralateral optic nerve, and planum sphenoidale.

(From Schick U, Dott U, Hassler W. Surgical management of meningiomas involving the optic nerve sheath. J Neurosurg 2004;101:951–9. Reproduced with permission.)

Type I is located purely intraorbital (n = 12). Type Ia is restricted to a flat extension around the optic nerve (n = 2). Type Ib is manifested as a large bulbiform mass (n = 7), growing concentrically around the optic nerve with marked proptosis. Type Ic shows exophytic tumor growth upon the optic nerve (n = 3).

Type II is located intraorbitally with extension through the optic canal or superior orbital fissure (n = 41; see Fig. 33-1). Type IIa is manifested as an intraorbital tumor with tumor growth through the optic canal (n = 37; Figs. 33-2A and 33-3B). Type IIb involves the orbital apex (n = 4) and the superior orbital fissure, and sometimes even infiltrates the cavernous sinus.

FIGURE 33-2 A, Intraoperative aspect of meningioma type IIa coming out of the optic canal, surrounding the optic nerve, and preserved arachnoid. B, Intraoperative aspect after intradural decompression of the optic canal and tumor removal around the optic nerve. C, Intraoperative aspect following extradural bony decompression of the optic canal and intradural tumor removal.

(From Lee J. Meningiomas, Diagnosis, Treatment and Outcome. New York: Springer; 2008. p. 355–62.)

Type III is located intraorbitally with large intracranial tumor extension (>1 cm; n = 37). Type IIIa extends to the chiasm (n = 29). Type IIIb involves the chiasm up to the contralateral optic nerve and planum sphenoidale (n = 8). Intracranial extension could be diffuse grass-like (66%) or nodular (34%).

CLINICAL FEATURES

Slowly progressive, painless loss of vision and proptosis are the cardinal features of ONSMs.^2,^3,^9,²³ Commonly reported other clinical findings include afferent pupillary defect, color vision disturbance, visual field defect, optic disc edema, optic atrophy, and motility disturbance.²³ Ophthalmoscopic examination may reveal optic nerve head swelling, contiguous macular edema, nerve pallor, or choroidal folds.⁸ Opticociliary shunt vessels may develop from compression of the central retinal vein in about one third of patients.^3,⁸ The classic triad of optic atrophy, visual loss, and opticociliary shunt vessels is present only in a minority of patients. Mild to moderate proptosis (2–5 mm) is commonly present and may be the presenting sign.⁵ Chemosis, lid edema, and limitation of motility may also be found. Visual deterioration in ONSM usually is a question of time. In our series, visual acuity became worse with longer duration of preoperative symptoms and longer follow-up period. Optic atrophy is another negative predictor for recovery.

RADIOLOGIC FINDINGS

Magnetic resonance imaging (MRI) has become the gold standard for diagnosis of ONSMs, thus obviating the need for a tissue biopsy in most cases.^3,^9,¹⁰ High-field T1-weighted images with fat suppression and gadolinium contrast remain the procedure of choice for diagnosis of ONSM. The tumor is typically isointense to slightly hypointense to brain and optic nerve on T1-weighted images and hyper- or hypointense on T2-weighted images.¹⁶ The classic “tram-tracking” sign consists of the thickened optic nerve sheath containing the lesion surrounding the nonenhancing optic nerve. Neuroimaging characteristics may also show tubular, globular, or even fusiform enlargement of the optic nerve.²⁴

On thin computed tomography (CT) images there is hyperdense enhancement of the meninges surrounding a hypodense optic nerve. On noncontrast CT images linear or diffuse calcifications may be present.

C-scan ultrasound imaging as a noninvasive, quantitative, and inexpensive method provided optic nerve sheath diameters similar to those obtained by CT of the orbits.²⁵ This ultrasound could image the optic nerve up to 15 mm behind the globe.

PATHOLOGY AND PATHOGENESIS

Primary ONSMs arise from the arachnoid cap cells surrounding the intracanalicular or intraorbital portion of the nerve and are almost always intimately associated with the nerve tending to surround the nerve.^5,^9,^24,²⁶ This results in a concentric thickening of the optic nerve diameter. ONSMs extend posteriorly into the annulus of Zinn. Therefore, ONSMs cannot be resected completely without compromising the integrity of the optic nerve.²⁷

There are different mechanisms for preoperative optic nerve injury: ischemia, compression, demyelization and tumor invasion.^10,²⁷ Compressive mechanical injury leads to small vessel compromise and demyelization, especially in patients with a long duration of visual loss before surgery. Assuming that there is no additional intraoperative trauma to the optic nerve, incomplete or no recovery of visual function after surgery may imply chronic severe preoperative ischemic or compressive damage and demyelization. In our study, preoperative disc pallor and location in the optic canal were negative prognostic factors for visual improvement. The bony optic canal is not enlarged in ONSMs and the tumor in the canal compresses the optic nerve. This compressive injury can at least be reversed by surgical bony decompression of the optic canal. This is the main argument in favor of surgery.

Aggressive ONSMs are known with infiltration of the globe or optic nerve. Thus, deterioration of visual acuity may also result from direct tumor invasion into the intracranial optic nerve (n = 4, own series). Infiltration of the globe occurred in two patients and infiltration of the cavernous sinus in four patients (type IIb). Irregular margins in the orbit implied local invasion.²⁶

LOCATION

Tumor location seems to have an important impact on the evolution of visual function. Onset of visual loss in ONSM at or near the orbital apex is thought to be followed by rapid and progressive further loss and a higher risk of intracranial extension.^2,²⁸ We were also able to identify intracanalicular location as a negative factor for visual acuity. Saeed and co-workers²⁶ demonstrated that tumors with posterior components in the orbit had more frequent intracranial involvement. Intracranial extension was more frequent and had a greater growth rate in younger patients.²⁶ In cases in which ONSM involves the intracanalicular portion of the nerve, up to 38% incidence of contralateral nerve involvement was reported.³ Bilateral presentations are most commonly found in patients with neurofibromatosis type 2.^3,²⁴

SURGICAL TECHNIQUES

Almost all operations were performed via a unilateral frontotemporal approach. This pterional approach is well described in the literature.²⁹^–³¹ Craniotomy is extended to the middle of the orbital rim and 1 cm above the margin. For intradural procedures, the sylvian fissure was routinely opened. Drainage of cerebrospinal fluid (CSF) was performed by opening the basal cisterns and by lumbar drainage. The ipsilateral optic nerve and carotid artery were identified and the intracranial tumor was first coagulated and resected along the dura around the optic canal. Preservation of the small feeding vessels between the carotid artery and the optic nerve is important and can be reached by sucking the tumor within the arachnoidal plane. At this step mainly irrigation instead of coagulation should be used. The ipsilateral optic canal should always be opened. We favor early lateral opening of the dural part of the optic canal to avoid narrowing of the optic nerve through this band. The dura was resected around the optic canal and the bony optic canal was decompressed. The drilling should begin laterally until the floor of the optic canal is reached to prevent any contact to the nerve. Finally, the optic canal was unroofed. In case of opening the medially located sphenoid sinus or ethmoidal cells, subcutaneous tissue with fibrine glue were inserted. The tumor around the optic nerve and the dura was carefully removed. In tumors infiltrating the nerve, resection was limited to the exophytic part. In blind patients with disfiguring painful proptosis, a prechiasmatic transsection of the optic nerve was performed intradurally and the intraorbital part was removed as well. The intraorbital optic nerve was transected just behind the globe and deep in the apex.

In tumors without intracranial extension, an extradural pterional approach was favored with sphenoid ridge drilling, posterior orbitotomy, and decompression of the superior orbital fissure and unroofing of the optic canal. Tumors with exophytic intraorbital mass were amenable to excision via a lateral orbitotomy.

During the last few years we changed our technique of decompression and performed an extradural pterional decompression of the optic canal in the last 10 patients. This extradural drilling is much easier, more extensive, and safer for the optic nerve, avoiding any damage. The second step remained intradurally with tumor removal.

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