Hence, at the level of the sphenoethmoid recess, the nasal septum is detached from the prow of the anterior wall of the sphenoid bone; this latter is opened circumferentially and thus far the endoscope enters into the sphenoid cavity, often divided by one or more septa. The degree of pneumatization of the sphenoid bone is an important factor for the identification of the bony protuberances and depressions inside it. Depending on the degree of its pneumatization, a series of protuberances and depressions molded on its posterior and lateral walls can be identified. The sellar floor is at the center, the sphenoid planum is above, and the clival indentation is below; lateral to the sellar floor, the bony prominences of the intracavernous carotid artery and the optic nerve can be observed. Between them, the lateral optocarotid recess lies; it is molded by the pneumatization of the optic strut of the anterior clinoid process. The intracranial aspect of the upper border of the lateral optocarotid recess is covered by a thickening of the dura and periosteum that forms the distal dural ring, which separates the optic nerve from the clinoidal segment of the internal carotid artery (ICA). The inferior border of the lateral optocarotid recess also presents a thickening of the dura and periosteum, which forms the proximal dural ring, which separates the intra-cavernous portion of carotid artery from the clinoidal segment. The lateral aspect of tuberculum sellae represents the point where the bony prominences of the carotid artery and the optic nerve join medially (medial optocarotid recess). This recess is less evident than the lateral one; rather it represents the lateral limit to be opened to unlock the suprasellar area (Fig. 2.2).

The sella turcica is limited superiorly by the diaphragma sellae, a fold of dura with a central opening through which is pierced by the pituitary stalk and its blood supply. The diaphragma sellae separates the anterior lobe of the pituitary gland from the optic chiasm and the suprasellar cistern. The intercavernous sinus (anterior and posterior) lies in the anterior and posterior borders of the diaphragma sellae. Additional small venous sinuses in the base of the pituitary fossa drain into the intercavernous sinuses [53].

If the surgeon wants to explore the cavernous sinus, the bone that covers the intracavernous carotid artery (carotid protuberance) must be removed in order for both the medial and lateral compartments of the cavernous sinus to be exposed [15]. The space between the ICA and the pituitary gland varies depending on the anatomy of both structures. The medial wall may consist of tenacious connective tissue or may be fenestrated, incomplete, or inexistent, offering little or no anatomical resistance against tumor invasion. Surgical access to the medial and posterior wall of the cavernous sinus is possible by elevating or retracting the pituitary gland medially and by retracting the C4 (intracavernous segment) or C4–C5 bend of the ICA, laterally [21, 56]. The carotid artery itself is located in the major portion of the cavernous sinus so that the entire cavernous sinus will be disclosed only when the carotid artery is mobilized medially. Upon opening of the medial wall, the posterior and upper parts of the cavernous sinus are entered. The inferior hypophyseal artery is identified inferolaterally to the pituitary gland, arising from the meningohypophyseal trunk together with the dorsal meningeal and tentorial artery. Anteriorly to it, the inferolateral trunk, with its collaterals to the cavernous sinus cranial nerves, is detected, branching off the lateral aspect of the ICA; its origin can be seen by medial dislocation of the intracavernous segment of the ICA. Inferior hypophyseal artery divides into a medial and a lateral branch, which anastomose with the corresponding vessels of the opposite side, forming an arterial ring around the hypophysis [16, 28, 54] (Figs. 2.3, 2.4, and 2.5). The inferolateral trunk supplies all the cavernous sinus nerves, with the exception of the proximal segment of the VI cranial nerve, which receives blood from the tentorial artery. The oculomotor and trochlear nerves can be visualized from the sella through the C-shaped portion of the intracavernous sinus ICA. The oculomotor nerve enters the cavernous sinus under the posterior clinoid and then runs along the middle of the C-shaped ICA to enter into the cavernous sinus apex. At the apex, it runs along the inferior margin of the optic strut triangle until it reaches the superior orbital fissure. The trochlear nerve runs parallel and just inferior to the oculomotor nerve. When the ICA is displaced medially, the oculomotor nerve, the trochlear nerve, and the proximal and distal dural ring can be seen. The ophthalmic division of the trigeminal nerve runs obliquely in a rostral and anterior direction toward the cavernous sinus apex reaching the oculomotor and trochlear nerves at the superior orbital fissure. The abducens nerve passes through Dorello’s canal approximately 5–10 mm inferior to the sellar floor at the medial aspect of the ICA and a few millimeters below the sellar floor at the lateral aspect of the ICA. The abducens nerve heads toward the orbital apex running inferiorly to the medial aspect of the ophthalmic branch of the trigeminal nerve [21, 31, 33, 34, 43] (Fig. 2.3).

The exposure of the suprasellar region requires a more anterior trajectory: the posterior ethmoid cells and the anterior wall of the sphenoid sinus have to be widely removed. Above the sella, the angle formed by the convergence of the sphenoid planum with the sellar floor corresponds to the tuberculum sellae. This structure named after the classic Latin word “tuber,” which etymologically means “small swelling, pimple, protuberance,” appears to fit such a description when observed from above via a transcranial route, but it does not as seen from below through an endoscopic endonasal corridor. As a matter of fact, through direct visualization via an endoscopic endonasal approach, it has recently renamed “suprasellar notch” (SSN), that means “angular or V-shaped cut indentation” [29]. Indeed, in the majority of cases, the inferior point of view coincides with a sort of indentation between the superior aspect of the sella turcica and the declining part of the planum sphenoidale. Moving anteriorly, we can recognize the sphenoid planum, laterally delimited by the protuberances of the optic nerves. At this point, the bone of the suprasellar notch and the planum sphenoidale can be removed 1.5–2 cm in a posteroanterior direction and laterally up to the optic protuberances. The sellar and suprasellar dura are then opened to permit the exploration of the neurovascular structures localized above the diaphragma sellae. In the suprachiasmatic region, the chiasmatic and the lamina terminalis cisterns with relative contents are accessible. The anterior margin of the chiasm and the medial portion of the optic nerves, the anterior cerebral arteries, the anterior communicating artery, and the recurrent Heubner arteries, together with the gyri recti of the frontal lobes, can be identified. In the subchiasmatic space, the pituitary stalk is at the center of the field below the chiasm, with the superior hypophyseal artery and its perforating branches, supplying the inferior surface of the chiasm and the optic nerves. The superior aspect of the pituitary gland and the dorsum sellae are also visible. The superior hypophyseal arteries supply the optic chiasm, the floor of the hypothalamus, and the median eminence.