Sphenoid Sinus

The sphenoid sinus is variable in shape and size and the cavities within it are almost never symmetrical and frequently are divided by small septa, which are seldom located at the midline. Even when a single major septum divides the sinus (48% of individuals), it is often off the midline. A septation inside the sphenoid sinus can lead to the carotid artery in 87% of individuals. ^{3 , 4}

The sphenoid sinus is rectangular in the coronal plane, and is usually larger laterolaterally than craniocaudally. Based on the degree of pneumatization, three types of sinuses can be distinguished: conchal, sellar, and presellar. ⁵ In the conchal type, the sphenoid sinus is a solid block of bone without an air cavity, or it does not extend beyond the sphenoid conchae. This type is most commonly encountered in children younger than 12 years old. The presellar type contains an air cavity that extends no further posteriorly than a plane perpendicular to the sellar wall. This type occurs in about 11 to 24% of individuals. In the sellar type, an air cavity is present which extends into the body of the sphenoid below the sella and goes all the way posteriorly to the clivus. This type of pneumatization is the most common, being seen in as many as 76 to 86% of cases. ³

The sphenoid sinus has an anterior wall, a floor, two lateral walls, a roof, and a posterior wall. The anterior wall of the sphenoid sinus comprises the sphenoidal concha, ostium, and rostrum. The sphenoidal crest is located at the anterior wall and gives attachment to the bony part of the nasal septum, which is formed by the vomer and the perpendicular plate of the ethmoid. ³ The sphenoid rostrum lies on the anteroinferior wall of the sphenoid sinus and represents the inferior border of the sphenoid sinus floor. Occasionally, the pneumatization of the sinus may extend anteriorly through the rostrum into the septum. As a rule, the more the rostral pneumatization is extended, the more lateral the natural ostium is located. ⁵ The sphenoid sinus ostium is a round or elliptic aperture, through which the sphenoid sinus opens into the sphenoethmoidal recess behind the superior turbinate. This recess is present in only 48.3% of individuals. ⁵

Various landmarks can be recognized on the lateral wall of the sphenoid sinus, especially in a well-pneumatized sinus. In a superior to inferior direction, three prominences are visible: the optic nerve (CN II) canal, internal carotid artery (ICA), and maxillary nerve (V2). ⁵ The bony lateral sphenoid sinus wall over the ICA and CN II is usually very thin and may be dehiscent in some areas. ³ Some recesses and grooves can be visualized between these bony prominences. The lateral opticocarotid recess is a shallow recess formed by CN II superiorly and the ICA inferiorly, and it is visualized better when the optic strut is pneumatized. The medial opticocarotid recess represents the lateral aspect of the tuberculum sellae, which is the area of contact between the point of origin of the optic canal medially and the posterior margin of the parasellar carotid artery. That is to say, the tuberculum sellae connects with the medial opticocarotid recess on both sides. ⁵

The vidian nerve (the nerve of the pterygoid canal) runs in the lateral part of the inferior wall of the sphenoid sinus toward the pterygoid fossa. Whenever the sphenoid sinus is well pneumatized, the nerve can be in direct contact with the mucosal membrane of the sinus due to absorption of the superior osseous wall of the pterygoid canal. Moreover, the nerve can protrude into the sphenoid sinus and then run in an osseous ridge.

On the posterior wall, the clival recess is seen below the pituitary prominence. The clival recess is a wide groove corresponding to the sphenoidal portion of the clivus.

On the roof of the sphenoid sinus, the tuberculum sellae, the prechiasmatic sulcus, and the planum sphenoidale are visible from posterior to anterior. The dorsum sellae and posterior clinoids comprise the posterior borders of the sella turcica and the cavernous sinuses constitute the lateral borders. ⁵

Clivus

The clivus (Latin for “slope”) separates the posterior cranial fossa from the nasopharynx. It is formed of two parts: basisphenoid and the basiocciput. The former corresponds to the posterior portion of the sphenoid body and the latter corresponds to the basilar part of the occipital bone. ⁶ The clivus lies posterior and extends inferior to the sphenoid sinus. This anatomical relationship with the sphenoid bone is unique, making it surgically accessible through transsphenoidal and nasopharyngeal corridors. ⁷

On the basis of the extracranial landmarks exposed in the transnasal approach, the clivus is classified into upper, middle, and lower parts. This classification helps facilitate segmental transclival approaches according to the location of the lesion and the particular area of interest to be exposed. Therefore, a thorough understanding of the relationships between the extra- and intracranial structures is required for an accurate endoscopic endonasal approach to the intracranial structures. ¹

From an endoscopic endonasal perspective, the clivus can be divided into three parts from rostral to caudal:

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  The upper third includes the dorsum sellae and posterior clinoid processes, and it extends down to the level of the sellar floor.

  
  
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  The middle third extends from the lower extent of the sella down to the sphenoid floor (when well pneumatized at the level of the roof of the choana).

  
  
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  The lower third extends from the sphenoid floor/roof of choana down to the foramen magnum. ⁸

The posterior fossa is approached via a transsphenoidal corridor through the upper two thirds of the clivus. On the other hand, a nasopharyngeal corridor is used to access the posterior fossa through the lower clivus. In this case, drilling the bone below the sphenoid rostrum is usually adequate. ²

The intracranial surface of the upper third of the clivus is related to the sella and posteriorly leads us to the mesencephalon. The middle third of the clivus faces the pons. The lower third lies in front of the medulla. The extracranial surface of the clivus gives rise to the pharyngeal tubercle at the junction of the middle and lower clivus. ⁶ The petroclival fissure separates the upper and middle clivus from the petrous portion of the temporal bone bilaterally. ²

The foramen lacerum and the paraclival portion of the cavernous ICA lie lateral to the middle third of the clivus, whereas inferiorly, the clivus is bordered laterally by the petrooccipital fissure, which is grooved by the inferior petrosal sinus. The ICAs are farther lateral at this level; thus, the occipital condyles and the hypoglossal canals represent the lateral limits of the dissection. ^{5 , 6} The abducens nerve (CN VI) runs superiorly and laterally just above the vertebrobasilar junction (VBJ) along the ventral aspect of the clivus before it enters Dorello’s canal and cavernous sinus, which makes it vulnerable to injury. ^{5 , 6}

Each third of the clivus has a respective nasal, bony, dural, cisternal subarachnoid anatomy, as well as relevant arteries, nerves, and a portion of the brainstem that can be analyzed in three different modules. ⁸

Upper Clivus

The rostral extension of the superior third of the clivus is bordered by the dorsum sellae in the midline and the posterior clinoids in the paramedian region. This approach is designed to reach the interpeduncular fossa and the pituitary gland lies in front of it. There are surgical settings in which the pituitary gland may be sacrificed because of prior established panhypopituitarysm. However, when the pituitary gland function is normal, a pituitary transposition is performed. In this scenario, it is important to understand that two layers of dura cover the inner side of the sella: the periosteal and the meningeal layers. These layers are found only where there is bone; otherwise, only a single (meningeal) layer is found. Accordingly, the sella has two layers in the face, floor, and posterior wall, between which run the venous channels that communicate with both cavernous sinuses, such as the superior, inferior, and posterior intercavernous sinuses, respectively. On the lateral walls, however, the sella has only a single meningeal layer separating it from the medial wall of the cavernous sinus. This is a very important concept when pituitary transposition is performed, in which the capsule of the gland should not be violated and the pituitary ligaments should be detached carefully from the medial cavernous sinus wall. In this way, the pituitary gland can be transposed superiorly. ⁸

The posterior intercavernous sinus is located posterior to the pituitary gland and is kept attached to the gland while it is elevated. The clival dura harboring the basilar venous plexus is posterior to the dorsum sellae, which lies posterior to the posterior intercavernous sinus. ⁸

While performing the pituitary transposition, it is very important to understand the anatomy of the superior and inferior hypophyseal arteries and their relationships with other sellar structures. The inferior hypophyseal artery arises from the meningohypophyseal trunk at the cavernous segment of the ICA and courses medially toward the pituitary gland. It travels within the cavernous sinus and then penetrates the medial wall posteriorly to supply mainly the posterior pituitary gland. The superior hypophyseal artery emerges from the medial aspect of the paraclinoid segment of the ICA. It travels from the carotid cave into the subarachnoid space in the direction of the pituitary stalk and chiasm. ⁹

The left and right ICAs are closest to each other just below the tuberculum sellae with an average distance of 13.9 mm (range, 10–17 mm). ³

Once the dorsum sellae is removed, the dura with the basilar plexus is exposed. Once the dura is open, the interpeduncular fossa is exposed. The mesencephalon is exposed posteriorly and can be directly visualized once the Liliequist’s membrane is removed. The basilar apex with all branches is also visualized in front of the brainstem. The oculomotor nerves (CNs III) form the lateral limit of the approach as they run lateral to Liliequist’s membrane in the crural cistern ( Fig. 11.1 ).

Middle Clivus

The middle segment of the clivus is limited by the sellar floor superiorly and by the sphenoid rostrum inferiorly, which is located at the level of the sphenoid floor in well-pneumatized sphenoid sinuses. The paraclival protuberances of the ICA limit the mid-clivus laterally. ⁸ The paraclival portion of the cavernous ICA are separated at this region by an average distance of 17 mm. ³ After the bone of the middle clivus is drilled, the clival dura harboring the basilar venous plexus is exposed. ⁸

The basilar plexus (also called clival plexus) is a large inter-cavernous venous connection located between the layers of dura posterior to the clivus that extends across the posterior aspect of the dorsum sellae. It communicates with the cavernous sinuses superiorly, the inferior petrosal sinuses laterally, and the marginal sinus and epidural venous plexus inferiorly, creating a large venous confluence along the cavernous sinus posterior wall. It is the largest communicating channel between the two cavernous sinuses. CN VI often runs through the basilar plexus at the level of the confluence with the inferior petrosal sinus to enter the posterior part of the cavernous sinus. This anatomical relationship is of great importance and should be considered while performing a clivectomy, to avoid injury to CN VI. ^{2 , 5}

Once the dura is open and the basilar plexus is controlled, the prepontine cistern is entered and the pons can be visualized posteriorly with the basilar artery in front of it. This approach is limited laterally by CNs VI ( Fig. 11.2 ).

Inferior Clivus

The inferior third of the clivus borders the anterior aspect of the foramen magnum inferiorly. The superior border is located at the sphenoid rostrum junction at the level of the sphenoid sinus floor. In contrast to the middle third, the inferior third of the clivus is not limited directly by the ICA on both sides, therefore, further lateral dissection can be performed safely. The petroclival synchondrosis is found laterally and can be traced all the way to the jugular foramen. The occipital condyles are located in the anterior portion of the foramen magnum and are considered the lateral limits while drilling the inferior part of the clivus. When the lateral exposure needs to be augmented to include the subarachnoid origin of the vertebral artery, a medial condylectomy is performed. In this scenario, the hypoglossal nerve (CN XII) tracking inside the hypoglossal canal is considered the lateral limit.

Once the dura is opened, the premedullary cistern is reached, with the medulla posteriorly and the vertebral arteries laterally. CN XII runs lateral to the vertebral arteries in the cistern and also represents the limit of this approach intracranially ( Fig. 11.3 ).

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