INTRODUCTION

Meningiomas arising in the sphenoid region are frequently encountered in neurosurgical practice. They account for 11% to 18% of cases in large surgical series.^1–6 Presumably radiation-induced sphenoid meniningiomas have been described.^7,8 Jacobs and co-workers have reported a nonrandom association between sphenoid meningiomas, breast cancer, and genital cancer.⁹ Other than that, there are no data implicating specific causes or genes and genetic pathways specifically in the formation of sphenoid wing meningiomas.¹⁰ Epidemiologists have generally not distinguished between meningiomas occurring at different sites.^11,12

Sphenoid meningiomas (and other meningiomas of the anterior skull base) may be more often of the meningiothelial than the transitional or fibroblastic histologic subtype possibly reflecting a lesser rate of NF2 gene disruptions in these tumors.¹³ Histologically malignant and atypical tumors of the sphenoid have been reported, but histologically aggressive meningioma variants are less frequently observed in the cranial base than the convexity.¹⁴ Overall, meningiomas more commonly affect women. This is particularly true for meningiomas en plaque of the sphenoid wing. Between 77% and 94% of these growths are diagnosed in females (Table 32-1).

This chapter describes the surgical treatment of meningiomas of the sphenoid wing and its results.

SURGICAL ANATOMY

The sphenoid bone joins the orbit and facial region with the cranial convexity and the skull base. The lesser wing (ala minor sv. parva) of the sphenoid bone forms part of the orbital roof, and the ala major part of the lateral wall of the orbit. Its external surface is related to the temporal, infratemporal, and pterygopalatine fossa. The medial part of the sphenoid ridge is formed by the ala minor, and the lateral part by the ala major. The sphenoid ridge separates the anterior from the middle cranial fossa and is related to the sphenoid segment of the sylvian fissure and the M1 segment of the middle cerebral artery. Its lateral end joins the frontal, parietal, and temporal bone. This landmark is called the pterion. Hence, meningiomas of the lateral sphenoid wing are often referred to as pterional meningiomas.

The optic nerve and the ophthalmic artery enter the orbit through the optic canal. The roof of the optic canal is formed by the lesser wing of the sphenoid bone and proximally by the falciform ligament. A dural envelope (the optic sheath) follows the optic nerve into the orbit. Its outer layer is in continuity with the periorbita, and its inner layer surrounds the optic nerve up to the level of the globe. The oculomotor, trochlear, and abducens nerves; the first division of the trigeminal nerve; and the optic vein pass through the superior orbital fissure. The borders of the superior orbital fissure are the ala minor and major, and medially the body of the sphenoid bone. Its dural covering blends with the periorbita and the dura of the cavernous sinus. These dural relations explain much of the growth pattern (i.e., growth into the orbital canal, orbit, and cavernous sinus) observed in some sphenoid meningiomas, although transosseous tumor growth clearly occurs.

Operating on some medially located sphenoid meningiomas requires an understanding of the anatomy of the cavernous sinus. The anterior clinoid process of the sphenoid bone overlies the anterosuperior aspect of the cavernous sinus. The carotid artery exits from the cavernous sinus medially to and at the level of the anterior clinoid process, passing through two dural rings. Only at the level of the more distal ring the carotid artery truly enters the intradural compartment. The external layer of the wall of the cavernous sinus is a continuation with the dura of the middle fossa. The inner layer encloses the third and fourth cranial nerves and the first and second divisions of the trigeminal nerve. The abducens nerve courses medial to the carotid artery in the cavernous sinus. Inferolaterally, the cavernous sinus extends up to but does not include the margins of the foramen rotundum and ovale. The maxillary nerve (i.e., the second division of the trigeminal nerve) exits the cavernous sinus and the neurocranium through the foramen rotundum, passes through the pterygopalatine fossa, and enters the orbit through the inferior orbital fissure. The mandibular nerve (third division of the trigeminal nerve, including its motor portion) leaves the skull through the foramen ovale. These relationships are important because sphenoid wing meningiomas frequently infiltrate the dura of the temporal fossa and the sphenoidal wing.

CLASSIFICATION OF SPHENOID MENINGIOMAS

Growth patterns of sphenoid meningiomas reflect the complicated anatomy detailed in the preceding text and may therefore vary considerably. On the end of the spectrum of sphenoid meningiomas are some tumors of the lateral sphenoid wing (or the pterion; see earlier) which closely resemble convexity meningiomas in most aspects of clinical presentation and surgical treatment (Fig. 32-1). On the other hand, many tumors of the medial sphenoid wing are typical skull-base tumors infiltrating into skull-base structures. The proximity of vital and eloquent neurovascular structures such as the carotid artery and its branches and the optic apparatus (Figs. 32-2, 32-3, and 32-4) can pose formidable surgical challenges. Sometimes, tumors not only infiltrate the dural layer but also may grow into the cavernous sinus itself. Orbital involvement and extracranial growth in the subcranial region is a prominent finding in certain sphenoid meningiomas (spheno-orbital meningiomas). Bone invasion and hyperostosis usually associated with en plaque growth along the underlying dura are a characteristic finding in spheno-orbital meningiomas (Figs. 32-5 through 32-8). Excision of the dural origin of the tumor is generally required for a permanent cure of meningiomas.¹⁵ This can be regularly achieved when operating on lateral sphenoid wing tumors. In contrast, tumor growth into the cavernous sinus, orbital tumor extensions, and even carpet-like involvement of the convexity dura (see Fig. 32-8) encountered in tumors of the middle and inner thirds of the sphenoid wing may prevent a truly complete and radical resection.

FIGURE 32-1 Large lateral sphenoid wing (pterional) meningioma. The tumor was diagnosed in a 67-year-old female patient who presented with flattened affect, cognitive decline, a possible seizure, and a mild hemiparesis. The tumor could be completely removed. The patient’s hemiparesis and neuropsychological deficits improved after surgery.

A, Triplanar T1-weighted contrast enhanced MRI scans showing the origin and size of the tumor. Note that the patient presented only few symptoms quite in contrast to the tumor’s mass effect.

B, Axial T1-weighted contrast-enhanced (upper panel) and T2-weighted (lower panel) MR images. The course of the middle cerebral artery and its branches is indicated by arrows. At surgery, the middle cerebral artery and the M2 branches could be relatively easily identified and preserved. However, several M3 branches were engulfed by the tumor, necessitating careful dissection.

FIGURE 32-2 Large medial sphenoid wing meningioma.

A, MR appearance of a large meningioma of the right medial sphenoid wing (upper panel: coronal and sagittal T1-weighted images, lower panel: axial T1- and T2-weighted images). Hyperostosis and secondary bone formation at the origin of the tumor in the region of the anterior clinoidal process are indicated by arrowheads. The internal carotid artery is engulfed by the tumor already proximal to the internal carotid bifurcation (arrows). The MR study fails to depict the bifurcation and the course of the middle and the anterior cerebral artery through the tumor.

B, Conventional digital subtraction (from left to right: lateral views after injection of the common carotid and external carotid artery, AP view) and MR angiograms (AP view, far right) show that the tumor has virtually occluded the internal carotid bifurcation and the proximal segments of the anterior and the middle cerebral artery (arrows). The tumor derives its blood supply mainly from branches of the internal carotid artery.

FIGURE 32-3 Medium-sized medial sphenoid wing meningioma.

This tumor was diagnosed in a 39-year-old woman after a seizure. Visual acuity and visual fields were normal. Please compare Figure 32-4 for the corresponding intraoperative findings.

A, Coronal (upper panel) and axial T1-weighted contrast-enhanced MR images. The tumor encroaches upon and slightly compresses the optic nerve (arrow). The carotid and the middle cerebral artery are located medially and anterosuperiorly to the tumor (arrowheads). The meningioma involves the lateral wall of the cavernous sinus but there is no obvious intracavernous growth (arrowheads).

B, A sagittal and axial T2-weighted MR scans depict the course of the anterior circulation arteries even more precisely (arrowheads).

FIGURE 32-4 Surgery for a medium-sized meningioma of the left medial sphenoid wing. Please compare Figure 32-3 for radiologic findings.

A, The neck is slightly retroflexed, and the head is rotated 45 degrees to the right and fixed in a Mayfield clamp. The burr holes and saw cuts for the craniotomy are outlined in violet.

B, A pterional craniotomy including thorough drilling of the lesser sphenoid wing has been performed.

C, Initial subfrontal inspection showing the tumor (arrow on left) and the olfactory nerve (arrow on right).

D, The tumor is exposed through a transsylvian route. Two M3 branches have already been identified in the distal sylvian fissure (arrows).

Arrowhead: optic nerve. E, The tumor capsule is coagulated followed by internal debulking. Arrowhead: optic nerve.

F, Further debulking allows to mobilize and resect the temporal tumor component. Arrow: dura of the middle fossa.

G, Beginning in the distal sylvian fissure, the tumor is dissected away from the branches of the middle cerebral and finally the carotid artery (arrow). Note the preservation of an arachnoidal plane between the tumor and the vessels.

H, Residual tumor in the region of the anterior clinoid process (arrow) with mild compression of the optic nerve (arrowhead). Similar to the anterior circulation arteries, the optic nerve is separated from the tumor by an arachnoidal layer. Short arrow: Oculomotor nerve.

I, After decompression of the optic nerve, the dural tumor origin in the region of the anterior clinoid process is removed. There was no tumor growth extending medially or into the optic foramen. Much of the dural sign in this region (compare Fig. 32-3A, upper panel) was found to correspond to neovascularization and not tumor during surgery.

J, The tumor-infiltrated outer layer of the lateral wall of the cavernous sinus is removed, completing the resection of the dural origin of the tumor.

K, After tumor resection.

L, The craniotomy is closed using standard techniques. Bone cement has been used to fill some burr holes and part of the bony defect resulting from drilling of the sphenoid wing to obtain a good cosmetic result.

FIGURE 32-5 Bilateral spheno-orbital meningioma.

A, CT scans showing the bony involvement.

B, T1-weighted contrast-enhanced MRI scans depicting bilateral bone and soft tissue involvement.

(From Ringel F, Cedzich C, Schramm J. Microsurgical technique and results of a series of 63 spheno-orbital meningiomas. Neurosurgery 2007;60(4 Suppl 2):214-21.)

FIGURE 32-6 Orbital involvement in spheno-orbital meningiomas.

A, Exophthalmos, proptosis, and downward displacement of the globe (left; permission to reproduce the photograph was granted by the patient) through a combination of tumoral thickening of the lateral orbital wall (right, arrowhead) and intraorbital tumor growth and in a patient with a right-sided spheno-orbital meningioma.

B, Invasion of the (supero) lateral orbita with intraorbital tumor mass and submuscular tumor growth (arrows) depicted by axial (upper panel) and coronal (lower panel) T1-weighted MR images.

(From Ringel F, Cedzich C, Schramm J. Microsurgical technique and results of a series of 63 spheno-orbital meningiomas. Neurosurgery 2007;60(4 Suppl 2):214-21.)

FIGURE 32-7 Intraosseous tumor growth and secondary bone formation in spheno-orbital meningiomas.

A, AP and lateral x-ray film showing hyperostosis (arrowheads) of the sphenoid ridge and pterion. B, CT scans (bone windows) show intraosseous tumor in the lateral orbital wall, the orbital roof, the sphenoid ridge, and pterion. There is secondary bone formation in the soft tissue component of the tumor (arrows).

C, Typical middle fossa involvement seen in many cases of spheno-orbital meningiomas. Intraosseous growth in the floor of the middle cranial fossa often extends up to the level of the foramen rotundum and ovale and may involve the walls of the sphenoid sinus and the ethmoidal cells (arrow).

(From Ringel F, Cedzich C, Schramm J. Microsurgical technique and results of a series of 63 spheno-orbital meningiomas. Neurosurgery 2007;60(4 Suppl 2):214-21.)

FIGURE 32-8 Meningeal spread in spheno-orbital meningiomas. Extensive carpet-like dural involvement, including the convexity dura, the dura of the anterior and middle fossa (arrows), and the lateral wall of the cavernous sinus (arrowhead) in a left-sided spheno-orbital meningioma.

(From Ringel F, Cedzich C, Schramm J. Microsurgical technique and results of a series of 63 spheno-orbital meningiomas. Neurosurgery 2007;60(4 Suppl 2):214-21.)

A simple classification of sphenoid meningiomas will distinguish between medial and lateral sphenoid wing meningiomas and spheno-orbital (hyperostosing or en plaque) meningiomas.^16–22 The first classification of sphenoid meningiomas was proposed by Cushing.²³ Based on different growth patterns, site-dependent symptomatology, and site-specific surgical issues, Cushing and Eisenhardt distinguished between meningiomas of the deep or clinoidal third, those of the middle, and those of the outer (pterional) third of the sphenoid ridge (i.e., the ala magna). Cushing felt that early symptomatic involvement of the optic apparatus, oculomotor palsies, and exophthalmos, and the surgical challenges posed by the vicinity of the carotid artery, the superior orbital fissure, and the pituitary stalk, serve to distinguish tumors of the deep from tumors of the middle sphenoid ridge. The latter may often become sizable growths before they are diagnosed. Pterional meningiomas are thought to either grow en plaque or as sometimes rather large spherical (globular) tumors.

A somewhat more refined classification has been proposed by Bonnal and colleagues and Brotchi and Pirotte.^24,25 These authors distinguish between five groups of sphenoidal meningiomas. Cushing’s clinoidal tumors are referred to as group A tumors. Invasion of the cavernous sinus is listed as one characteristic of these growths. Meningiomas en plaque with hyperostosis of the sphenoid bone often invade large parts of the sphenoid bone, the optic foramen and orbit, and even the infratemporal fossa and fossa pterygopalatina (see Figs. 32-6, 32-7, and 32-8). Hence their designation as pterional tumors as proposed by Cushing may not be fully appropriate. These tumors are termed group B meningiomas by Bonnal and colleagues and Brotchi and Pirotte. Large invasive tumors of the sphenoid ridge (“invasion en masse”) are classified as group C tumors. They are thought to combine features of both group A and B tumors: globular and invasive growth en plaque. Group D and E meningiomas correspond to Cushing’s middle sphenoid ridge and globular pterional tumors.

Basso and colleagues subdivide meningiomas of the inner third of the sphenoid wing into clinoidal and sphenocavernous tumors.²¹ Nakamura and co-workers and Russel and Benjamin also classify medial sphenoid wing meningiomas into tumors with or without cavernous sinus involvement.^22,26 Among sphenoid wing meningiomas with growth into the cavernous sinus, Abdel-Aziz and colleagues further distinguish among sphenocavernous, clinoidocavernous, and sphenoclinoidocavernous meningiomas.²⁷ Clinoidal meningiomas may also be subclassified based on the presence (or absence) of interfacing arachnoid membranes between the tumor and cerebral vessels²⁸ and, in addition, tumor size.²⁹

Roser and colleagues described a subset of 86 cases of sphenoid wing meningiomas with osseous involvement among a total of 256 sphenoid wing meningiomas. They distinguish between globular tumors of the lateral, middle, and medial sphenoid wing with osseous infiltration, en plaque meningiomas with and without cavernous sinus invasion, and finally purely intraosseous tumors.³⁰ Honeybul and co-workers also differentiate between invasive en plaque and purely intraosseous sphenoid meningiomas.³¹

Irrespective of the classification one chooses to subdivide one’s case series, when planning an operation it is important to distinguish among the types outlined by Roser and colleagues,³⁰