1.1.1 Anterior Cranial Base

The anterior cranial base is formed by three different bones: frontal, ethmoid, and sphenoid (Fig. 1.1). The lateral aspect of the floor of the anterior cranial fossa comprises the orbital process of the frontal bone, whereas the deepened central anterior skull base is formed by the ethmoid bone. The cribriform plate lies in the center and is perforated by multiple small dural invaginations containing the olfactory filia, whereas the crista galli protrudes along the midline as the main anterior bony attachment of the falx. The horizontal cribriform plate is bounded laterally by the vertical lateral lamella of the ethmoid bone. The lateral lamella joins the cribriform plate to the fovea ethmoidalis of the frontal bone, which covers the roof of the ethmoid sinus.1

The sphenoid bone forms the posterior part of the anterior cranial base (Fig. 1.2). Posterior to the cribriform plate is the planum sphenoidale, which forms the roof of the sphenoid sinus on the exocranial side of the skull base. It is bordered laterally by the lesser wings of the sphenoid bone, which medially and posteriorly form the anterior clinoid processes, which are landmarks for the clinoidal internal carotid artery (ICA) as well as the optic nerves. Above the cribriform plate and the planum sphenoidale lie the olfactory bulb and tract, respectively, which run in the olfactory sulcus of the frontal lobe. Posteriorly, the planum is separated from the prechiasmatic sulcus by the limbus of the sphenoid, a surgical landmark for localization of the optic canal and falciform ligament.2

The anterior skull base is intimately related to the paranasal sinuses as well as the orbit. The ethmoid sinuses lie in between the orbits. Directly posterior we find the sphenoid sinus. The frontal sinus lies anteriorly between the thin posterior plate and thicker anterior plate of the frontal bone. The orbit is roughly pyramidal and is related superiorly to the frontal sinus, medially to the ethmoid sinus, and inferiorly to the maxillary sinus. The medial wall is formed from anterior to posterior by the orbital process of the frontal bone and the lacrimal, ethmoid, and sphenoid bones. The floor of the orbit is formed by the maxilla and by the zygoma anterolaterally. The infraorbital groove transmitting the infraorbital nerve is found on the orbital floor. The lateral wall is formed by the frontal and zygomatic bones anteriorly and by the greater and lesser wings of the sphenoid bone posteriorly, separated by the superior orbital fissure. It is essential to realize that although the anterior lateral wall is related to the temporalis muscle, the posterior lateral wall is related to the middle fossa and the temporal lobe. The orbital roof is formed mainly by the orbital process of the frontal bone, although the lesser wing of the sphenoid contributes to some extent posteriorly. The orbital apex comprises three main portals: the superior and inferior orbital fissures and the optic canal. The annulus of Zinn, the common tendinous attachment of the orbital muscles, separates the orbital apex into lateral and medial compartments. The lateral superior orbital fissure transmits the lacrimal, frontal, and trochlear nerves as well as the superior ophthalmic vein. The superior and inferior divisions of the oculomotor nerve as well as the nasociliary and abducens nerves are transmitted within the annulus of Zinn in the medial compartment of the superior orbital fissure. Superomedially within the annulus of Zinn, we find the optic canal transmitting the optic nerve and ophthalmic artery. The ophthalmic artery arises from the supraclinoidal ICA just medial to the anterior clinoid process. After running inferiorly and laterally to the optic nerve, the ophthalmic artery within the orbital apex crosses superiorly and gives off the supratrochlear and frontal arteries (important for vascularized pericranial flaps) as well as the anterior and posterior ethmoidal arteries. Notably, in 7 to 13% of cases, the optic canal can be completely encompassed within posterior ethmoidal cells (Onodi cells), and if this is not acknowledged on review of preoperative imaging, the optic nerves can be injured during endonasal approaches.3 The inferior ophthalmic vein joins the superior ophthalmic vein at the orbital apex to drain into the cavernous sinus.

1.1.2 Middle Cranial Base

A detailed understanding of the anatomy of the sphenoid bone is crucial for both open and endoscopic endonasal approaches. The body of the sphenoid bone forms the central portion of the middle skull base. The sphenoidal body is cuboidal in shape and houses the sphenoidal sinus, which is the portal for most endoscopic approaches. The sides of the sphenoid body are grooved by the ICA as it transitions from the petrous segment over the foramen lacerum to the cavernous segment. The lateral aspect of the middle skull base is roughly triangular, limited by the sphenoid ridge anteriorly and by the petrous ridge posteriorly. The floor consists of the greater wing of the sphenoid anteriorly, the petrous apex of the temporal bone medially, and the squamous temporal bone laterally.

The medial and lateral pterygoid plates project down from the pterygoid body on either side of the sphenoid sinus and articulate with the palatine bone anteriorly. The greater wings extending from the inferior sphenoid body and the lesser wings extending from the superolateral aspect of the body have been described as resembling bat’s wings from a superior view. The anterior clinoid process has two attachments to the sphenoid bone: the superior one forms the roof of the optic canal as it joins the planum, whereas the inferior one, the optic strut, forms the floor of the optic canal. The optic strut, when it is well pneumatized, forms a recess that corresponds to the lateral opticocarotid recess endoscopically. The prechiasmatic sulcus is a groove that extends between the two optic canals. This sulcus is bound posteriorly by the tuberculum sella and anteriorly by the limbus of the sphenoid bone. Between the tuberculum sellae anteriorly, the dorsum sellae and posterior clinoids posteriorly, and the cavernous sinuses and cavernous carotids on either side lies the sella turcica with the pituitary gland (Fig. 1.3).

Occasionally a middle clinoid process, a bony prominence that extends from the superolateral aspect of the sella toward the tip of the anterior clinoid process, can be developed enough to connect to the anterior clinoid process, completely encasing the clinoidal ICA in a caroticoclinoidal ligament.4 Usually, however, the anterior genu of the parasellar carotid is only partially encased. The middle clinoid is an endonasal landmark, for it marks the roof of the cavernous sinus as well as the transition between the cavernous and paraclinoidal segments of the ICA.4 The diaphragma is a dural fold that extends from the tuberculum to the dorsum sellae, forming the roof of the sellar turcica. The pituitary stalk traverses the plane of the diaphragma through a central aperture—the pituitary aperture. Notably, the arachnoid does not usually follow the stalk inferiorly through the aperture, and thus the contents of the pituitary fossa are normally not bathed in cerebrospinal fluid.5 Laterally the diaphragma is continuous with the dura of the cavernous sinus.

Several important foramina are found in the middle cranial fossa. The superior orbital fissure connects the middle fossa and the cavernous sinus to the orbit and its contents. The foramen rotundum lies inferior to the superior orbital fissure and transmits the maxillary division of the trigeminal nerve. The two are separated by a bony island—the maxillary strut of the sphenoid bone. The foramen rotundum has an average length of approximately 4 mm and actually presents as more of a canal than a foramen.6 Posterolateral to the foramen rotundum, the foramen ovale transmits the mandibular division of the trigeminal nerve and may also serve as the conduit for an accessory meningeal artery, the lesser superficial petrosal nerve (LSPN), and emissary veins to the pterygoid plexus. These neurovascular structures may also exit or enter the skull base using their own independent foramina. Immediately posterolateral to the foramen ovale we find the foramen spinosum transmitting the middle meningeal artery. The foramen lacerum lies medial to the foramen ovale and is formed by the confluence of the petrous, sphenoid, and occipital bones. Its inferior compartment comprises fibrocartilage, whereas the superior compartment contains the lacerum segment of the ICA. The foramen lacerum is actually the continuation of the petrous carotid canal. The inconstant foramen of Vesalius is seen within an island of bone bridging the foramen ovale and foramen lacerum, which we call the “mandibular strut.” It transmits an emissary vein from the pterygoid plexus to the cavernous sinus. Immediately posterolateral to the foramen spinosum, the innominate foramen transmits the LSPN, which courses toward the foramen ovale. Medial and parallel to the LSPN, the greater superficial petrosal nerve (GSPN) exits through its homonymous foramen and courses parallel to the petrous ridge toward the foramen lacerum, where the GSPN joins the deep petrosal nerve and enters the vidian canal to become the nerve of the vidian canal. The GSPN can be traced posteriorly to the geniculate ganglion of the facial nerve. The course of the GSPN is an important landmark for the horizontal segment of the petrous carotid, which lies just inferior and medial to this nerve.7

Other anatomical landmarks of the middle skull base, from medial to lateral, include the trigeminal impression on the petrous ridge, which accommodates cranial nerve (CN) V in its course from the posterior cranial fossa to Meckel’s cave. The ICA runs in its bony canal just under this area, which may be dehiscent. The trigeminal impression is followed posterolaterally by the trigeminal prominence and then by the meatal impression, which corresponds to the underlying internal acoustic canal. Proceeding more laterally, we find the arcuate eminence, which corresponds to an elevation created by the underlying superior semicircular canal, and then the flattened paper-thin roof of the tympanic cavity and mastoid air cells—the tegmen tympani and tegmen mastoideum, respectively.

An essential component of the middle skull base is the cavernous sinus, a large venous compartment encased within two layers of dura, which surrounds the sella turcica and is continuous anteriorly with the superior orbital fissure. It is surrounded by five walls of dura. The anterior wall is formed by the periosteal layer of dura overlying the sella anteriorly as it separates from its meningeal layer laterally. The meningeal layer continues posteriorly to form the medial wall as it surrounds the pituitary gland. The posterior wall faces the posterior cranial fossa and is formed by an extension of the periosteal layer between petrous apex and dorsum sella. The roof is continuous with the diaphragma and lateral wall and is formed by the oculomotor triangle posteriorly and the clinoidal triangle anteriorly. The lateral and medial walls meet at the inferior limit or floor of the cavernous sinus, which corresponds to the maxillary division of the trigeminal nerve and Meckel’s cave posteriorly. The lateral wall is essentially a continuation of the outer or periosteal dural layer of the middle fossa. The meningeal layer of the middle fossa and the periosteal layers come together at the anterior petroclinoidal fold, which corresponds to the transition of the lateral wall to the roof.2 Multiple venous structures drain into the cavernous sinus, such as the basilar plexus, ophthalmic veins, foramen rotundum, foramen ovale, foramen of Vesalius, and sphenoparietal sinus. The cavernous sinus then drains into the superior and inferior petrosal sinuses, which then drain into the transverse and sigmoid sinuses, respectively. The cavernous carotid has a short ascending segment, a posterior genu, a horizontal segment, and an anterior genu that continues as the paraclinoidal carotid. Two major branches come off the cavernous carotid: (1) the meningohypophyseal trunk, which arises from the posterior genu and gives off the inferior hypophyseal arteries, the tentorial artery of Bernasconi and Cassinary, and the dorsal meningeal artery, and (2) the inferolateral trunk, which arises from the horizontal segment and supplies the lateral wall and the first two divisions of the trigeminal nerve. These branches may also arise independently from the ICA. The inferolateral trunk courses between the abducens medially and V1 laterally to the lateral wall of the cavernous sinus. An inconstant third branch is McConnell’s capsular artery, which has inferior and superior branches; the inferior branch arises medially from the horizontal segment, just anterior to the inferolateral trunk, to supply the anterior and inferior sellar dura, whereas the superior branch supplies the dura of the tuberculum sella and prechiasmatic sulcus.2

There are multiple important anatomical relationships within the cavernous sinus. The abducens nerve and the carotid sympathetic plexus are the only nerves that course within the sinus itself; all others are related to the lateral cavernous sinus wall. The abducens nerve enters the cavernous sinus just behind the posterior ascending cavernous carotid, then courses inferior and laterally to the horizontal segment of the carotid, lying just medial to V1 on its way to the superior orbital fissure. The oculomotor nerve, coursing from the interpeduncular fossa, enters the roof of the cavernous sinus within the oculomotor triangle, which is formed by the anterior petroclinoidal ligament laterally (extending from petrous apex to anterior clinoid), the posterior petroclinoidal ligament posteromedially (extending from petrous apex to posterior clinoid), and the interclinoidal ligament anteromedially (extending between the anterior and posterior clinoids). The dural membrane between these three dural folds forms the posterior roof of the cavernous sinus. The oculomotor nerve courses just lateral and parallel to the interclinoidal ligament and is surrounded by a thin invagination of the oculomotor triangle dura containing arachnoid and CSF (also known as the oculomotor cistern or interdural segment of the oculomotor nerve) before being incorporated into the lateral wall of the cavernous sinus under the clinoidal triangle, which forms the anterior roof of the cavernous sinus. The trochlear nerve enters the cavernous sinus in the posterolateral corner of the oculomotor triangle, just posterior to the junction of the anterior and posterior petroclinoidal ligaments. Here it lies posterior and lateral to CN III. It is then incorporated into the lateral wall of the cavernous sinus between the oculomotor nerve superiorly and V1 inferiorly, with which it forms the supra- and infratrochlear triangles, respectively.

1.1.3 Posterior Skull Base

The anterior wall of the posterior fossa is formed by the posterior surface of the petrous bone laterally and the clivus in the midline. The posterior and lateral confines are formed by the occipital bone, whereas the roof is formed by the tentorium cerebelli. The floor ends at the level of the foramen magnum.

The posterior surface of the petrous bone has two important foramina: (1) the internal auditory canal transmitting the seventh and eighth nerve complexes, as well as the labyrinthine branches of the anteroinferior cerebellar artery, to the inner ear and (2) the vestibular aqueduct, which transmits the endolymphatic duct, located lateral to the internal auditory canal. The jugular foramen is found below these two foramina and is formed by the junction of the jugular process of the petrous bone with the occipital bone. The jugular foramen has a posterolateral compartment (the pars venosa, which contains the jugular bulb and CNs X and XI) and an anteromedial compartment (the pars nervosa, which contains the glossopharyngeal nerve, the inferior petrosal sinus as it courses toward the jugular bulb, and usually a posterior meningeal branch from the ascending pharyngeal artery). Inferior and medial to the jugular foramen lies the hypoglossal canal with the hypoglossal nerve.

The vertebral arteries give off the posterior inferior cerebellar arteries ventral to the lower CNs before joining to form the basilar artery at the pontomedullary junction. The basilar artery gives off the anteroinferior cerebellar artery (AICA) just distal to the vertebrobasilar junction (Fig. 1.4). The AICAs loop close to the internal acoustic canal and are intimately related to the VII–VIII complex. The superior cerebellar arteries arise from the distal basilar artery just before it bifurcates into the two posterior cerebral arteries.

From the external surface, such as that encountered when performing a far lateral craniotomy, the posterior skull base is protected by the mastoid tip and the suboccipital musculature. The sternocleidomastoid muscle arises from the mastoid tip, whereas the digastric muscle arises just medial, from the mastoid groove. The occipital artery usually courses medial to the mastoid tip. Deep to the sternocleidomastoid lie the splenius capitis and longissimus capitis muscles, whereas posteriorly in the midline lays the trapezius, with the semispinalis capitis arising deep to it. A very important anatomical landmark is the suboccipital triangle containing the vertebral artery with its venous plexus. The suboccipital triangle is exposed when the muscles superficial to it are reflected off the superior nuchal line and occipital squama. The muscles forming this triangle are the superior and inferior oblique and the rectus capitis posterior major. The superior oblique and the rectus major attach at the inferior nuchal line, a landmark for identifying the suboccipital triangle. The C1 lateral process, the site of attachment for both oblique muscles, serves as a key landmark for identifying the location of the transverse foramen and of the vertebral artery as it exits the foramen and grooves the C1 arch. Identification of the vertebral artery within this triangle is needed to safely access the occipital condyle and C1 mass, which can be drilled to provide ventral access to the foramen magnum, lower clivus, and craniocervical junction (Fig. 1.5; Fig. 1.6).

1.2.1 The Sphenoid Sinus

Central to almost all endoscopic endonasal approaches to the skull base is the anatomy of the sphenoid sinus. The sphenoid sinus can range in pneumatization from the conchal type, in which there is almost no pneumatization, to the sellar type (most common), in which pneumatization is sufficient to expose variable amounts of the sellar floor. The presellar pneumatization pattern has some pneumatization but does not extend posteriorly beyond the plane of the tuberculum sellae. Pneumatization of the sinus increases after birth and is more or less finalized by late adolescence.8 In the center of the sinus, a round bulge corresponds to the anterior surface of the sella turcica. Below the sella, pneumatization of the clivus creates a depression called the clival recess in between the paraclival segments of the ICA.9 The paraclival carotids superiorly enter the cavernous sinus to turn anteriorly in a horizontal plane before looping again posteriorly, with the apex of the anterior loop corresponding to the middle clinoid process.4 Notably, the middle clinoid forms a small depression just medial to the carotid and lateral to the sella. This anterior loop borders the sella on either side and is endoscopically referred to as the “parasellar carotid.” This anterior loop or anterior genu of the parasellar carotid is prominent on either side of the sella and corresponds to the carotid prominence. One to multiple septations usually exist within the sphenoid sinus; their anatomy is extremely variable and almost never corresponds to the midline. Notably, in approximately 80% of cases, the septations lead to the carotid prominences posteriorly.10 Above the carotid prominence on either side is another prominence that corresponds to the precanalicular segment of the optic nerve and then the optic canal as it courses from posterior and medial to anterior and lateral toward the orbit. The optic and carotid prominences are separated laterally by a very important endoscopic landmark, the lateral opticocarotid recess, which is the endoscopic correlate of the optic strut. The depth of this recess essentially corresponds to the degree of pneumatization of the optic strut.9 Medially, the medial opticocarotid recess is another variable depression bordering the carotid and optic prominences as well as the superolateral aspect of the sellar prominence. The two medial opticocarotid recesses are the lateral extensions of the tubercular recess, a depression that borders the superior aspect of the sellar prominence and that corresponds to the tuberculum intracranially.9 The depressions corresponding to the middle clinoids are slightly inferior to the medial opticocarotid recesses.