Middle Fossa Floor Meningiomas
Keywords: middle fossa floor meningioma, Meckel’s cave meningioma, pterional approach, subtemporal approach, fronto-temporo-orbito-zygomatic approach
Little is known regarding meningiomas that primarily arise from the floor of the middle fossa as opposed to the other middle fossa meningiomas. In this chapter, we treat this relatively new entity, including primary Meckel’s cave (MC) meningiomas because they respect similar anatomical landmarks.
Meningiomas of the middle cranial fossa can be approached by two distinct routes: an anterolateral approach or a lateral approach; in other words, via a pterional or a subtemporal approach. Both approaches can be further extended by means of additional osteotomies, such as the cranio-orbital zygomatic approach and the temporo-zygomatic approach. “Extended” approaches and adequate cerebrospinal fluid drainage, are helpful to achieve a “retractorless” surgical technique. It is also mandatory to achieve good surgical outcomes to preserve venous structures, as the vein of Labbè.
The aim of this chapter is to treat “middle fossa floor” meningiomas as a clinical entity that is distinct from meningiomas arising from the sphenoid wing and cavernous sinus, which have been already described in other chapters of this book, and to include in authors’ classification primary MC meningiomas as well.
The middle cranial fossa is the site of several tumors. These tumors are primarily intracranial, arising from the meninges or cranial nerves, such as meningiomas or schwannomas, while other tumors, such as chordomas and chondrosarcomas, may arise from the bones and cartilages. Lastly, some tumors can originate from extracranial tissues and secondarily invade the middle cranial fossa structures, for example, nasopharyngeal carcinoma, esthesioneuroblastoma, lymphoma, and systemic metastasis. The majority of middle cranial fossa tumors are meningiomas; among these, sphenoid wing meningiomas are the most common.
The first successful removal of a lateral sphenoid wing meningioma was described in 1774 by Louis A. In 1918, Heuer GJ pioneered the pterional approach as a surgical technique to approach lesions of the middle cranial fossa, although Dandy W is frequently credited with inventing this operation. As regards trigeminal tumors, Cushing in 1938 stated: “It is possible of course that a method may some day be evolved whereby a Gasserian ganglion neuroma or meningioma may be safely approached and removed. Should this come to pass, it will be another conquest for neurosurgery.”
More recently, many surgeons have modified the pterional approach to gain a more direct working distance and minimize brain retraction. The orbitozygomatic approach and its various other osteotomies have been proposed as extensions of the pterional approach to wider exposure of the middle cranial fossa and enable entry into the adjacent compartments (orbit, posterior fossa, pterygopalatine space).
Moreover, the subtemporal approach has been proposed as a safe surgical strategy for removing middle fossa tumors.
To sum up, many advanced methods have been introduced to treat tumors of the middle fossa, based on the standard pterional and subtemporal surgical routes, resulting in a better outcome.
Meningiomas arising primarily from the floor of the middle fossa are an uncommon occurrence, representing only 1.1% of all meningiomas in the series described by Sughrue et al. 1 However, many of these tumors in the past may have been grouped with other meningiomas of the convexity dura, sphenoid wing, lateral wall of the cavernous sinus, and tentorium.
In this chapter the authors emphasize that “middle fossa floor meningiomas” are a clinical entity distinct from meningiomas arising from the sphenoid wing and cavernous sinus (already described in other chapters of this book), and this denomination includes primarily MC meningiomas too.
12.2 Preoperative Definition of Lesion Features
12.2.1 Surgical Anatomy of the Middle Cranial Fossa
The middle cranial fossa is formed by the sphenoid and temporal bones. The anterior border of this fossa consists of the sphenoid wing and anterior clinoid process. Posteriorly, it is limited by the superior border of the petrous temporal bone with the sulcus for the superior petrous sinus, and the dorsum sella of the sphenoid bone. The floor is formed by the greater wing of the sphenoid anteriorly and the squamosal temporal bone posteriorly. The middle cranial fossa is divided into medial and lateral portions. The medial part is formed by the body of the sphenoid bone. The cavernous sinus occupies a significant portion of this region but its anatomy will be described elsewhere in this book. The lateral part is formed by the lesser and greater sphenoid wings, with the superior orbital fissure between them. The lesser wing is connected to the body of the sphenoid bone by an anterior root, which forms the roof of the optic canal, and by a posterior root, also called the optic strut, which forms the floor of the optic canal, and separates the optic canal from the superior orbital fissure. The greater wing forms the largest part of the middle fossa, with the squamosal and the petrosal parts of the temporal bone completing this surface. The superior orbital fissure transmits the oculomotor, trochlear, ophthalmic, and abducens nerves, a recurrent meningeal artery, and the superior and inferior ophthalmic veins. The maxillary nerve passes through the foramen rotundum of the greater sphenoid wing, which connects the middle cranial fossa to the pterygopalatine fossa. The mandibular nerve passes through the foramen ovale of the greater sphenoid wing, which connects the middle cranial fossa with the infratemporal fossa. The upper surface of the petrous bone is grooved along the course of the greater and lesser petrosal nerves. 2 The greater superficial petrosal nerve arises from the geniculate ganglion in the petrous bone, pierces the hiatus fallopii, then runs parallel to the lateral wall of the horizontal internal carotid artery in a groove called the sulcus of the greater superficial petrosal nerve, exiting the middle cranial fossa through the foramen lacerum and the pterygoid (vidian) canal. 3 The lesser petrosal nerve arises from the tympanic branch of the glossopharyngeal nerve (Jacobson’s nerve), the nervus intermedius of the facial nerve and the auricular branch of the vagus nerve (Arnold’s nerve), pierces the hiatus accessorius and then runs above the tensor tympani muscle anterior and parallel to the greater superficial petrosal nerve in a groove called the sulcus of the lesser petrosal nerve: it leaves the middle cranial fossa passing through the canaliculus innominatus (foramen of Vesalius), the foramen spinosum or the sphenopetrosal suture. 4 The carotid canal extends upward and medially and allows a passage to the internal carotid artery and carotid sympathetic nerves along their courses to the cavernous sinus. The roof of the carotid canal is formed by the petrous bone. However, this bony roof is frequently dehiscent for a variable degree. In many cases, the internal carotid artery in the carotid canal is covered only by the dura of the middle cranial fossa. The internal carotid artery reaches the middle cranial fossa passing through the inner opening of the carotid canal, located near the foramen lacerum at the petrous apex. The foramen lacerum may be covered by a cartilaginous sheet and is laterally delimited by the lingula, a protrusion of the sphenoid bone located at the junction of body and greater wing which provides an attachment to the petrolingual ligament, that divides the horizontal petrous carotid from vertical cavernous carotid segment. 5 The arcuate eminence indicates the position of the superior semicircular canal. A thin lamina of bone, the tegmen tympani, roofs the area above the middle ear and auditory ossicles on the anterolateral side of the arcuate eminence. The internal auditory canal can be identified below the floor of the middle fossa by drilling along a line approximately 60° medial to the arcuate eminence, near the middle portion of the angle between the greater superficial petrosal nerve and arcuate eminence. The petrous apex, anteromedial to the internal acoustic meatus, is free of important structures. The middle cranial fossa is covered by dura and is fed by the middle meningeal and accessory meningeal arteries; it is innervated by the trigeminal nerve. The middle meningeal artery, branch of the maxillary artery of the external carotid artery circle, reaches the middle cranial fossa from the infratemporal fossa passing through the foramen spinosum, located in the greater wing of the sphenoid, posterior and lateral to the foramen ovale. The accessory meningeal artery rises from the middle meningeal artery or directly from the maxillary artery and reaches the middle cranial fossa passing through the foramen ovale. 2
MC is a 10-mm-long, tunnel-shaped subdural cavity; it contains the plexiform segment of the roots of the trigeminal nerve and the Gasserian ganglion. The walls of the cave are composed of thin meningeal dura. The lateral wall is formed by the tentorium, the superior half of the medial wall is formed by the inner reticular layer of the cavernous sinus, the inferior half of the medial wall is formed by the petrolingual ligament and by the trigeminal impression on the petrous apex, which separate the MC from the internal carotid artery. The Gasserian ganglion lies in the anterior part of MC, while the posterior part of MC communicates with the prepontine cistern. Before the trigeminal nerve enters MC, it forms an angle of 60° as it crosses over the petrous apex. The nerve fibers are loose within the cave. 6
12.2.2 Middle Cranial Fossa Floor Meningiomas
The middle cranial fossa is a common site of origin for meningiomas. This region can be represented as a rectangular open bowl rising to terminate in three distinct “ridges” and an open back. 1 The three ridges include the sphenoid wing anteriorly, the cavernous sinus medially, and the convexity dura laterally; the open back is the portion posterior to the petrous ridge that includes the tentorium. In this analogy, the concavity of the bowl consists of the floor of the middle fossa. The clinical behavior of meningiomas originating from each of these “ridges” and the tentorium is well-described, but very little is known regarding meningiomas that arise primarily from the floor of the middle fossa. Prior to the work of Sughrue et al., only 5 reports documenting the clinical outcome of a total of 19 cases of primarily middle fossa floor meningiomas could be identified in the literature. 7, 8, 9, 10, 11 In 2010, Sughrue et al. defined a “middle fossa floor” meningioma as a meningioma with more than 75% of its radiographic attachment on the floor of the middle fossa and with less than 25% of attachment on either the sphenoid wing, cavernous sinus, petrous ridge, or lateral convexity dura, which form the four anatomic boundaries of the middle fossa concavity as determined by MRI (▶ Fig. 12.1 a, b and ▶ Fig. 12.2 a, b). As reported in this paper, between 1991 and 2006, 1,228 patients were treated at the University of California, San Francisco for meningiomas: of these, 17 (1.1%) patients met their criteria for a “middle fossa floor” meningioma. Two of these patients had had previous surgery and were excluded because it was unclear where the initial site of their tumor was located. Thus, only 15 patients were included in that series, bringing the total number of middle fossa floor meningiomas cases described in the literature to 34.
Fig. 12.1 (a) Middle fossa floor meningioma. An example of preoperative MRI showing a middle cranial fossa floor meningioma, defined as meningioma with more than 75% of its attachment on the floor of the middle fossa and with less than 25% of attachment on either the sphenoid wing, cavernous sinus, petrous ridge, or lateral convexity dura. (b) Postoperative MRI of a middle cranial fossa floor meningioma. Postoperative MRI showing total resection of a middle fossa floor meningioma.