**3.1 Introduction**

The middle fossa and petrous apex are at the center of the skull base and are surrounded by several critical structures. Tumors in this area grow from or envelope multiple cranial nerves, as well as deep venous and arterial structures with little collateral flow. The risks posed in the pre-pontine cistern are formidable, and surgical resection in this anatomically complex region should be performed by an experienced, multi-disciplinary skull base team. Most pathologies occurring in this area are benign, and gross total resection confers a benefit to the prognosis and quality of life of a child. Regardless of histology, maximum safe resection should be carried out. Outcomes are best in centers with experienced pediatric skull base, neuro-anesthesiology, critical care, and rehabilitation teams.

### **3.2 Regional anatomy**

In our practice, resection of such tumors in the middle and infratemporal fossae, as well as the petrous apex is carried out by a combined neuro-otology and neurosurgery skull base team in all cases. A thorough understanding of the anatomy of the petrous temporal bone, anterior and posterior clinoid processes, and sphenoid bone is essential to safe removal of tumors in this area. Landmarks are neither constant nor readily apparent, and laboratory dissection is essential to familiarizing the surgeon with anatomy in this region.

The floor of the middle fossa is delimited by several key structures. Anteriorly, the middle meningeal artery can be identified as it enters foramen spinosum. An accessory meningeal artery is inconsistently seen in foramen ovale, anterior and medial to the middle meningeal artery [20]. A ridge of bone frequently obscures visualization of the foramen spinosum and can be drilled away for better exposure. The greater superficial petrosal nerve (GSPN) runs in a groove medial to the middle meningeal artery and enters the vidian canal under the mandibular

nerve (V3). The GSPN is frequently dehiscent through the middle fossa floor in its course. The maxillary nerve (V2) enters the foramen rotundum superior and medial to foramen ovale. The area between V2 and V3 forms the lateral loop [21]. The sphenoid sinus and its invested vidian canal can be accessed through the lateral loop. The vidian canal is encountered laterally in patients with an over-developed sphenoid sinus, and medially when the sinus has conchal anatomy. The infratemporal fossa is accessed lateral to V3 at the floor of the middle fossa.

The arcuate eminence demarcates the location of the superior semicircular canal as well as the geniculate ganglion, which typically lies anterior to its lateral border [22]. The relationship of the arcuate eminence to both these landmarks is inconsistent, and drilling of the arcuate eminence is often necessary to clarify the anatomy of the middle fossa floor. Drilling to better visualize the anatomy of the middle fossa floor should be balanced against "blue-lining" the membranous labyrinth of the superior semi-circular canal, which may lead to inadvertent injury and hearing loss. Injury of the membranous labyrinth should be controlled with bone wax packing instead suction. The face of the superior semicircular canal invariably lies perpendicular to the two ridges of the petrous apex. These ridges cradle the superior petrosal sinus. Two important working windows are defined in this area. Kawase's rhomboid is bounded by the middle meningeal artery, mandibular nerve, GSPN, and petrous ridge. Glasscock's triangle (the posterolateral triangle of the cavernous sinus) is bounded V3, GSPN, and a line between the arcuate eminence and foramen spinosum. Drilling the bone in either of these spaces reveals the carotid artery, running deep to GSPN and medial to the eustachian tube. The cochlea has no external landmarks in this region, and its constant location medial to the genu of the carotid artery and the geniculate ganglion must be kept in mind during exposure of the internal auditory canal (IAC).
