A precise knowledge of the anatomy of the nose, paranasal sinuses, sellar and parasellar region, as well as clival, paraclival, and CCJ areas with all associated crucial neurovascular structure is mandatory. Skull base neurosurgeons are used to the transcranial perspective of anatomical structures such as the ICA, but the endonasal view of it can be confusing for beginners. It definitely requires some effort, some practice in an anatomical laboratory, and the assistance of an ENT surgeon to safely identify crucial anatomical structures while performing such approaches. The use of neuronavigation system cannot replace a strong knowledge of the anatomy.

Landmarks are mandatory to navigate in the nasal cavities. One crucial landmark to begin with is the choanal arch. The choanal arch should be considered as a light in the dark for the beginners, and every time the surgeon get lost into the nasal cavities, going posteriorly, along the nasal fossa floor to the choanal arch, is particularly helpful to confirm that the line of sight of the approach is not too high toward the anterior skull base. The next major landmark is the middle turbinate. When the middle turbinate is mobilize medially, the uncinate process and, immediately behind, the bulla ethmoidalis come into view. One should keep in mind that the lateral wall of the bulla ethmoidalis is the thin medial wall of the orbit. The maxillary sinus will be reached behind and lateral to the inferior part of the uncinate process.

The sphenopalatine foramen (SPF) is a notch between the orbital and sphenoidal processes of the ascending process of the palatine bone. Following backward the ethmoid crest, which is a small spur of the bone of the medial surface of the ascending process of the palatine bone, the SPF with the sphenopalatine artery (SPA) running through is identified.

Following backward the nasal septum, the ostium of the sphenoid is located about 1.5 cm above the roof of the choanal arch. Below the level of the ostium runs the artery that vascularizes the mucosa of the nasal septum: the posterior septal artery (terminal branch of the sphenopalatine artery).

Once into the sphenoid sinus, some major landmarks may be recognized. Below the sella is the clival recess that is bordered laterally by the prominences created by the paraclival segment of the ICA. Those two prominences appear in case of a well pneumatized sphenoid sinus as two pillars on both side of the clival recess. The paraclival segment of the ICA corresponds to the posterior ascending segment of the cavernous segment of the ICA (C4 segment) that follows the lacerum segment of the ICA (C3 segment) [29]. The junction between the lacerum segment and the posterior segment of the ICA is also called the anterior or second genu of the ICA. The sellar floor limits superiorly the clival depression. On both sides of the anterior wall of the sella can be identified the slight prominence of the horizontal segment of the cavernous ICA followed by the recess formed by the middle clinoid process and, above it, the prominence of the clinoidal segment of the ICA. Above the prominence of the sella is the depression formed by the tuberculum sellae. Lateral to it is the medial opticocarotid recess that corresponds on the intracranial surface to the lateral tubercular crest. The opticocarotid recess that corresponds to the pneumatization of the optic strut (Inferior root of the anterior clinoid process) lies between the prominence of the clinoidal segment of the ICA and the prominence of the optic canal.

One should keep in mind that the shape of the posterior wall of the sphenoid sinus is highly variable from one patient to another. Two factors contribute to such variations: Firstly, the degree of pneumatization is variable, and in case of a conchal form of sphenoid sinus, none of those landmarks may be identified clearly. Secondly, the shape of the cavernous ICA is extremely variable [29], and the paraclival segment of the ICA can either run straightly vertical, slightly medially, or laterally or can be, rarely, tortuous. Those variations dictate the presence and shape of the clival depression and are important to consider preoperatively when planning an endoscopic transclival approach.

While resecting a tumor located into the clivus, the structure at high risk of damage is the abducens nerve (CN VI). From its emergence from the brain stem at the level of the bulbopontine sulcus (approximately at the level of the vertebral convergence which is a straightforward landmark after opening the dura mater in the midline), CN VI runs upward and slightly laterally. Following the same direction, it then runs into a dural canal in which CN VI is followed by a sleeve of arachnoid for a variable distance (abducens nerve cistern). Into Dorello’s canal, below Gruber ligament, CN VI changes direction and became more horizontal and moves lateral to the posterior ascending segment of the ICA. Then, it courses slightly upward along the medial and inferior surface of the horizontal segment of the cavernous sinus ICA (Fig. 26.4). When looking at the CN VI from an endoscopic endonasal 2D perspective with a 0° scope, the CN VI in its cisternal segment is medial to the paraclival ICA until it reaches Dorello’s canal. It then runs lateral to the paraclival segment of the ICA. The region at most risk for the CN VI nerve in case of clival chordomas is located immediately before and into Dorello’s canal. CN VI nerve monitoring is therefore critical and helpful to identify CN VI. However, the efficiency of CN VI nerve monitoring is often diminished by a preoperative CN VI paresis, which is one of the most frequent presenting symptoms of clival chordomas.