INTRODUCTION

The middle fossa is a common localization for intracranial meningiomas. The floor of the middle cranial fossa is formed by the greater wing of the sphenoid bone joining the squamosal part of the temporal bone.¹ Medially, it articulates with the clival portion of the occipital bone at the petroclival fissure. The foramen lacerum is located at the junction of the temporal, sphenoid, and occipital bones. The floor of the middle cranial fossa is bound anteriorly by the greater wing of the sphenoid (sphenoid ridge), posteriorly by the superior petrosal sinus (petrous ridge), and medially by the lateral wall of the cavernous sinus and Meckel’s cave (Fig. 35-1A, B). The bone and the dura mater of the middle fossa floor are supplied by branches of the middle meningeal artery and by caroticocavernous branches.

FIGURE 35-1 A, Superior view of the skull base. SR, sphenoid ridge; PR, petrous ridge; FR, foramen rotundum; FO, foramen ovale; FL, foramen lacerum; AE, arcuate eminence.

B, Superior view of the right middle cranial fossa with dura. Ant., anterior extension to the sphenoid wing and frontal fossa; Med., medial extension to the cavernous sinus and Meckel’s cave; Post., posterior extension to the cerebellopontine angle. CS, cavernous sinus; MC, Meckel’s cave.

Meningiomas can involve the middle fossa primarily or secondarily. Pure middle fossa meningiomas originate anterolaterally and posterolaterally at the floor of the middle cranial fossa and extend to the vicinity of the superior petrosal sinus and the exit of the vein of Labbé.² However, the majority of meningiomas encountered at the middle fossa are secondary extensions and involve the sphenoid wing, superior orbital fissure, cavernous sinus, Meckel’s cave, petroclival area, and the cerebellopontine angle. Some extend extradurally to the pterygopalatine fossa and infratemporal fossa through the foramen rotundum and ovale. Other authors reported unusual cases of the middle fossa meningiomas arising from the geniculate ganglion,³ the middle ear,⁴ and other parts of the temporal bone.⁵ Meningiomas of the middle fossa can also occasionally invade along the temporal base in en plaque fashion. Therefore, knowledge of microsurgical anatomy and variations of lateral skull-base approaches are essential for planning a surgical treatment in patients with middle fossa meningiomas.

SURGICAL APPROACHES FOR MIDDLE FOSSA MENINGIOMAS

For resection of most pure middle fossa meningiomas, a subtemporal and/or middle fossa approach or suprapetrosal approaches are adequate.⁶ In contrast, for meningiomas of the middle fossa that extend into surrounding skull-base structures (Fig. 35-1B), appropriate skull-base approaches should be tailored based on the preoperative imaging studies. Zygomatic osteotomies facilitate these approaches by widening the surgical corridor in exposure of the middle fossa, and enable access to the infratemporal fossa. In case of anterior extension to the frontal fossa, sphenoid wing, and superior orbital fissure (SOF), orbitozygomatic approaches may be planned.^7,8 In case of posterior extension to the cerebellopontine angle, anterior and/or posterior petrosal approaches may be appropriate.^9,10 In case of inferior extension to the infratemporal fossa, the zygomatic infratemporal approach may be chosen.¹¹ Depending on the tumor location and extent, combinations of these approaches may be used to achieve safe and total removal of widely invasive tumors.

Epidural or Subdural Subtemporal Approach (Middle Fossa Approach)

Most meningiomas of the middle fossa can be resected with the subtemporal approach. The lowest point of the middle cranial fossa is roughly at the level with the upper border of the zygomatic arch and is slightly posterior to the articular tubercle of the temporomandibular joint.¹² Zygomatic osteotomies therefore facilitate the exposure of the anterior part of the middle cranial fossa. The surgical landmark of the base of the middle cranial fossa is the line between just above the root of the zygoma and the supramastoid crest (Fig. 35-2).

FIGURE 35-2 Lateral view of the skull (left side). The asterisk indicates root of zygomatic arch. The dotted line indicates the supramastoid crest. Both are surgical landmarks of the lowest line of the middle fossa. The curvilinear line shows the extent of temporal craniotomy.

Knowledge of the detailed epidural anatomy of the middle fossa is important to plan epidural approaches. After temporal craniotomy, the dura is carefully elevated from the floor of the middle fossa to expose the middle meningeal artery (MMA), which may be coagulated and divided at the foramen spinosum to control bleeding from the tumor. The foramen spinosum lies 1.5 cm anterolateral to the trigeminal ganglion. Further elevation of the dura along the petrous ridge exposes the arcuate eminence, which roughly corresponds to the superior semicircular canal. The foramen ovale is located anteromedial to foramen spinosum. Therefore, division of the MMA also makes the exposure of the medial part of the middle fossa and identification of the mandibular division of the trigeminal nerve (V3) and greater superficial petrosal nerve (GSPN) easier. The medial limit for bone resection is the trigeminal nerve in Meckel’s cave. Meckel’s cave is located just ventral to the petrous apex. It is a dural fold that envelopes the trigeminal ganglion in the middle fossa.

The petrous portion of the internal carotid artery (ICA) courses posterolaterally toward Meckel’s cave, and the transition between petrous and cavernous portion of ICA occurs below the trigeminal ganglion (Fig. 35-3). The Glasscock triangle is limited by the posterior rim of the foramen ovale, foramen spinosum, and the posterior border of V3. The GSPN usually runs just above this triangle, which is an important landmark of the lateral border for preserving the petrous ICA while drilling the petrous apex.⁹ When the GSPN is adherent to the dura of the middle fossa, it can be dissected together with the middle fossa dura.

FIGURE 35-3 Lateral view of the Gasserian ganglion and posterior cavernous sinus. Peeling off the dura exposes the cavernous sinus and Meckel’s cave. The petrosal portion of the internal carotid artery (ICA) can be seen after drilling the Glasscock triangle. III, oculomotor nerve; IV, trochlear nerve; V, trigeminal nerve; V1, ophthalmic division; V2, maxillary division; V3, mandibular division; VI, abducent nerve; GSPN, greater superficial petrosal nerve; PA, petrous apex.

During epidural dissection of the divisions of the trigeminal nerve, it is quite important to respect the microsurgical anatomy of the cavernous sinus and Meckel’s cave. The dura of the middle fossa has two layers: the outer periosteal layer and the inner meningeal layer. At its exit from the dura, the meningeal layer of the dura envelopes the epineurium of the nerve and after its entry into the bony foramens of the middle fossa the nerve is covered by the periosteal layer. Therefore, by cutting the periosteal layer of the dura and dissecting between the meningeal layer of the dura and the epineurium in the lateral wall of the cavernous sinus and Meckel’s cave, the divisions of the trigeminal nerve can be exposed extradurally. Peeling off the meningeal layer of the V3 and Meckel’s cave will also facilitate the exposure of the petrous apex for the anterior petrosal approach.⁹

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