**3.3 Regional pathology**

Tumors affecting the skull base of the middle fossa include chordomas, meningiomas, and schwannoma.

Chordomas arise from the clivus and extend into the middle crania fossa in approximately one third of cases. They tend to grow locally but aggressively. Radical resection can be curative, but recurrence rates are high when subtotal resection is achieved, even with adjuvant proton beam therapy.

Meningiomas of the middle fossa are typically slow growing and most commonly present with headaches and visual disturbances. Tumors that invade the adjacent cavernous sinus may cause additional cranial neuropathies that affect extra-ocular movement. Meningiomas of the middle fossa typically arise from the medial sphenoid wing or the petrous ridge, and grow to span multiple cranial compartments (**Figure 4**).

Schwannomas in children are rare. Bilateral vestibular schwannomas are a hallmark of NF-2. Intracanalicular vestibular schwannomas can be resected via the middle cranial fossa approach, whereas larger lesions are typically resected via trans or retro-mastoid approaches. Observation is recommended for small or incidentally discovered trigeminal schwannomas. Small tumors show excellent control with radiosurgery in adults, but data is limited in children, and even stereotactic radiation in children may be linked to meningioma development later in life. For this reason, surgery is favored in our practice for schwannomas that show growth on surveillance imaging alongside cranial neuropathy or brainstem compression. Trigeminal schwannomas typically present with facial numbness. Trigeminal neuralgia tends to occur in lesions at and beyond the Gasserian ganglion, and involves all three divisions of the trigeminal nerve.

#### **Figure 4.**

*A 4-year-old male presented with one month of nausea and headaches with a large cystic meningioma filling the temporal fossa and extending across the tentorium (A-C). Tumor was resected through a modified orbitozygomaticapproach. Immediate post-operative MRI (DF) with gross total resection and normal swelling.*

#### **3.4 Surgical approaches**

We perform all middle cranial fossa, petrous ridge, and infratemporal fossa cases with a neurosurgery-neuro-otology skull base team. The middle fossa approach incision is a reverse question mark based at the root of the zygoma and extending to the superior temporal line. The muscle is elevated separately from the skin anteriorly and inferiorly. We use cutting and diamond burrs to drill a 5x5cm square craniotomy based 2/3rd anterior to the root of the zygoma, and 1/3rd posterior. Two points should be stressed in this exposure. First, the craniotomy should extend low, to the floor of the middle fossa, which is approximated by the root of the zygoma. Second, every attempt should be made to adequately expose the anterior petrous ridge to aid in deeper parts of the dissection. Failure to adequately expose the anterior floor of the middle fossa and petrous ridge places the neural structures (GSPN, IAC) in the exposure at risk.

The dura is elevated anteriorly until the middle meningeal artery (MMA) is identified. It is coagulated closer to the dural base than to the skull base, to prevent retraction and hemorrhage into the infratemporal fossa. Once the MMA is identified, elevation of the middle fossa dura proceeds in a posterior to anterior course, to avoid avulsion and injury to GSPN, which carries parasympathetic innervation to the lacrimal gland. Every attempt is made to avoid dividing the GSPN, as concomitant GSPN and V1 injury can lead to devastating insensate corneal abrasions and blindness. Elevation of the dura allows for identification of the arcuate eminence. This is followed medially to the petrous ridge, where the groove of the superior petrosal sinus is identified and divided. A shallow depression in the petrous ridge just above the porus acusticus can be used as a landmark for drilling.

The bone overlying the superior semicircular canal is drilled down and a House-Urban retractor is placed against the true petrous ridge. Care should be taken to avoid lacerating the dura during elevation and retraction, as seizure may occur.

Injury to the dominant temporal lobe can also result in aphasia, and is more likely when the vein of Labbe drains from an anterior position. Drilling begins at the pre-meatal petrous ridge, just anterior and medial to the IAC. Once saucerized, drilling of the post-meatal petrous ridge is done to expose a 270-degree arc of the IAC tunnel. All drilling is done with suction-irrigation to avoid thermal injury to the facial and cochlear nerves. Drilling laterally exposes the labyrinthine segment of the facial nerve, Bills bar, and the superior vestibular nerve. There is very low tolerance for a drilling misadventure in this area, as the labyrinthine segment of the facial nerve lies within 1 mm of the basal turn of the cochlea. The dura is opened along the length of the IAC, and on its posterior edge, to avoid injury to the expected location of the facial nerve anteriorly. Resection of inferior vestibular nerve tumors, the most common intracranial schwannoma, requires division of the superior vestibular nerve [23].

Access to the pre-pontine cistern, as well as the premeatal posterior cranial fossa can be accomplished through an extended middle fossa approach (eMFC). Drilling of the premeatal triangle in eMFC is generous, extending anteriorly to just under the Gasserian ganglion and V3, and laterally to the horizontal segment of the petrous internal carotid artery (C2) (**Figure 5**) [24].

Meckel's Cave, the lateral petrous apex, as well as the pterygopalatine and infratemporal fossae can be accessed through the endonasal corridor via a transmaxillary, trans-pterygoid approach [25]. Bi-nostril access is required, with working access achieved through the contralateral nostril. This should be accounted for by the operative team and referral to experienced skull base teams is recommended for this approach. The posterior maxillary wall provides access to the pterygopalatine fossa. The pterygopalatine fossa is exposed in its entire anterior–posterior extent, taking care to protect the sphenopalatine ganglion, vidian nerve, palatine and orbital

#### **Figure 5.**

*A 21 month old female presented with right facial weakness since 5 months of age. A small meningioma in the right cerebellopontine angle was found(upper row A-C). This tumor was observed at first, but her weakness progressed and the lesion had grown. A middle fossa anterior petrosectomy was performed. Post-operative images are seen on the bottom row (D-F). Pathology demonstrated meningioma.*

#### *Pediatric Skull Base Tumors DOI: http://dx.doi.org/10.5772/intechopen.95605*

nerves. The inferior orbital nerve serves as a landmark for the perpendicular plate of the palatine bone [26]. Resection of the orbital process of the palatine bone exposes V2, near the sphenopalatine foramen. Dissection of the sphenoid process of the palatine bone exposes the vidian nerve at the pterygoid canal and the maxillary nerve at the foramen rotundum. It also exposes the base of the pterygoid process, which when resected allows for access to Meckel's cave through the quadrangular space bounded by V2, V3, cranial nerve VI, the and the transition of the petrous internal carotid artery to its laceral segment, in close proximity to the petrolingual ligament [27].
