*4.3.1. Anatomical and surgical considerations*

In general terms, the skull base consists of five bones: ethmoid, sphenoid, occipital, paired frontal and paired temporal bones. The anterior limit of the anterior fossa is the posterior wall of the frontal sinus. The anterior clinoid processes and the planum sphenoidale, which forms the roof of the sphenoid sinus, mark the posterior limit. The frontal bone forms the lateral boundaries.

The posterior aspect includes the optic canal, the superior orbital fissure (SOF), and the inferior orbital fissure (IOF). The SOF conveys the oculomotor (CN III), trochlear (CN IV), abducens (CN VI), ophthalmic nerves and V1, as well as the ophthalmic veins. An understanding of the relationship of the tumour to vital structures is essential to preserve these structures and to minimize morbidity associated with treatment.

The greater wing ofthe sphenoid forms the anteriorlimit ofthe middle skull base. The posterior limit is the clivus. The SOF, foramen rotundum, foramen ovale, and foramen spinosum lie in an anteroposterior and medio-lateral plane [58].

A significant structure that can be found lateral to the sphenoid sinus is the cavernous sinus. Along it lateral wall runs the internal carotid artery (ICA), CNs III, IV, V, and VI and the maxillary nerve (CN V2).The facial nerve (CN VII) and vestibule cochlear nerve (CN VIII) originate from the caudal pons. They course through the subarachnoid space and enter the porus acusticus and IAC.

With a suitable understanding of skull base anatomy and surgical access, diverse IONM techniques has been steadily improving the outcome of surgery, both in terms of disease-free survival and morbidity associated with treatment.
