Читать книгу Anterior Skull Base Tumors - Группа авторов - Страница 12

The midline segment of the ASB serves as a watershed between the sinonasal tract and intracranial space. On the sinonasal side, it lies above the olfactory fissure, anterior and posterior ethmoid air cells, and sphenoid sinus (Fig. 2, 3).

Fig. 2. Paramedian sagittal section of a specimen (right side, seen from lateral to medial). A1, precommunicating tract of the anterior cerebral artery; AE, anterior ethmoid; ER, ethmoidal roof; FS, frontal sinus; IT, inferior turbinate; MOG, medial orbital gyrus; MT, middle turbinate; ON, optic nerve; PE, posterior ethmoid; PG, pituitary gland; PS, planum sphenoidale; PSt, pituitary stalk; SpS, sphenoid sinus; TS, tuberculum sellae.

Fig. 3. Axial section at 1 cm below the ASB (seen from below). The anterior (AE), posterior ethmoid, and olfactory fissure (OlF) lie just below the midline ASB, namely the ethmoidal roof (ER) and cribriform plate, respectively. The lateral ASB is located just above the orbital content. Ey, eyeball; FOV, fronto-orbital vein; GR, gyrus rectus; MCA, middle cerebral artery; MOFA, medial orbitofrontal artery; MOG, medial orbital gyrus; MRM, medial rectus muscle; ON, optic nerve; OpA, ophthalmic artery; SOV, superior ophthalmic vein.

Fig. 4. The dural lining of the anterior cranial fossa is slightly regular except for the area of the olfactory groove (OlG), where it is depressed and thinner. At the level of the crista galli (CGa), the falx cerebri (FaC) inserts on the ASB dividing the anterior cranial fossa into two compartments that communicate through the area of the planum sphenoidale. III, oculomotor nerve; DoS, dorsum sellae; ICA, internal carotid artery; ON, optic nerve.

Fig. 5. Scheme showing the trans-sinus (A), supraorbital (B), and pterional (C) trajectories of Figures 6, 10, and 11, respectively.

The cribriform plate, a complex and delicate part of the ethmoid that constitutes the central segment of the ASB, is perforated by several olfactory phyla and some terminal branches of the ethmoidal arteries and nerves. The cribriform plate is composed of two symmetric subunits, separated in the midline by the crista galli. When viewed intracranially, each subunit creates a niche, called the olfactory groove, which is about 21 mm long and 5 mm wide and “houses” the olfactory bulb (Fig. 4–6) [1]. In a coronal section view, each subunit is formed by a horizontal and a vertical lamella: the former is traversed by the olfactory phyla, while the latter is crossed by the ethmoidal bundles entering the intracranial compartment. The junction between the horizontal and vertical lamella corresponds to the area where the middle and superior turbinates insert onto the skull base (Fig. 7). This area is remarkably delicate due to several factors: the bone is thin and pierced by the olfactory phyla and ethmoidal bundles, making the area likely to be injured if handled roughly; its shape is angulate and the surrounding anatomy can be extremely variable and complex, making this area a high-risk source of cerebrospinal fluid leak during endoscopic endonasal surgery [2]. Keros [3] proposed a classification of the depth of the olfactory fossa into three grades in terms of cranial to caudal length: type 1, 1–3 mm; type 2, 4–7 mm, and type 3, 8 mm or higher, with type 2 being the most common. In addition, the inclination of the vertical lamella can vary remarkably and may also differ from side to side in the same patient. From an endonasal perspective, in case of great depth of the olfactory groove (higher Keros type) and a steep inclination of the vertical lamella, when exposing this area patients are at a higher risk of injury to the skull base during endoscopic procedures (Fig. 7). Similar to the bone, the dura mater that lines the anterior cranial fossa becomes thinner the closer it gets to the olfactory groove, where a dural envelope follows the phyla towards the olfactory mucosa.

Fig. 6. Transfrontal sinus endoscopic view of the median ASB (the trajectory is shown in Fig. 5) before (a) and after (b) section of the outer arachnoid (OAr). Each olfactory bulb (OBu) is enclosed between the crista galli (CGa) medially, cribriform plate (CrP) inferiorly and inferior-laterally, and ethmoidal roof (ER) superior-laterally. Superiorly, each olfactory bulb is connected to the respective gyrus rectus (GR) and medial orbital gyrus (MOG) by a layer of outer arachnoid. The gyrus rectus and medial orbital gyrus of each side are divided by the olfactory sulcus. The medial orbitofrontal artery (MOFA) runs on the inferior surface of the frontal lobe, while the frontopolar artery (FPA) is located within the interhemispheric fissure. OlT, olfactory tract; OR, orbital roof; PS, planum sphenoidale.

Fig. 7. Anatomical variability of the cribriform plate. a Tilted (right side) and high (left side) vertical portions of the cribriform plate. b Non-tilted and low vertical portions of the cribriform plate. Note the asymmetry between the vertical lamellas of different sides in both cases. The white dashed line shows the vertical lamella, and the white dotted line depicts the horizontal lamella. CG, crista galli; NS, nasal septum; T, common lamella of the middle and superior turbinate.

The anatomy of the arachnoid of this region has been described in two different ways. Seeger [4] reported that the olfactory bulbs lie in a subdural area because the outer arachnoid overlies them as a tent; consequently, they are not directly in contact with cerebrospinal fluid. Conversely, Key and Retzius [5] proposed that the arachnoid envelops the olfactory bulbs and phyla following the dura. Thus, the olfactory bulbs are in contact with cerebrospinal fluid. In all likelihood, the truth probably lies somewhere in between: the anatomy of the arachnoid of the olfactory region is variable and shaped by the physiology and hydrodynamics of cerebrospinal fluid, as observed in other skull base areas (i.e., sella turcica).

Fig. 8. Transorbital macroscopic view of the ASB. The orbital roof is formed by the orbital portion of the frontal bone (OPFB) anteriorly and lesser wing of the sphenoid bone (LWSB) posteriorly. The anterior (AEF) and posterior (PEF) ethmoidal foramina lie on the frontoethmoidal suture, between the lamina papyracea (LP) and frontal bone. The white asterisk identifies the optic canal. FR, foramen rotondum; GWSB, greater wing of the sphenoid bone; MT, middle turbinate; NS, nasal septum (perpendicular process of the ethmoid bone); SOF, superior orbital fissure; UP, uncinate process; VC, Vidian canal; Vo, vomer.

The crista galli is a triangular, median process of the ethmoid bone where the falx cerebri inserts anteriorly and inferiorly. It is usually composed of compact bone, but can be pneumatized by surrounding paranasal sinuses in <13% of cases [6]. The foramen caecum lies anterior to the crista galli and can harbor a small vein connected to the superior sagittal sinus. In this area, some nasofrontal dysembriogenic lesions such as dermal sinuses, dermoid cysts, meningoencephaloceles, or nasal gliomas may develop [7]. The anterior portion of the falx cerebri, which extends from the cranial vault to the corpus callosum, divides the anterior cranial fossa into two compartments, which “houses” the frontal lobes and related vessels.

Lateral to the cribriform plate, the midline ASB is formed by the ethmoidal roof, a thick portion of the frontal bone that joins with the ethmoidal box. The basal lamellae of the middle and superior turbinates inserts onto the ethmoidal roof and separates the anterior ethmoid from the posterior ethmoid, and the posterior ethmoid from the sphenoethmoidal recess, respectively. In the area where the lamina papyracea joins with the ethmoidal roof, the skull base tilts superiorly as a consequence of the convex shape of the orbit. The dihedral angle where the skull base turns from horizontal (ethmoidal roof) to convex (orbital roof), called the orbital beak, can be pneumatized by the superior-lateral extension of a suprabullar cell (supraorbital ethmoid cell) or the posterior extension of the frontal sinus (supraorbital recess of the frontal sinus). The ethmoidal arteries are collateral branches of the ophthalmic artery and enter the ethmoidal foramina located along the suture between the lamina papyracea and ethmoidal roof (Fig. 8, 9). In the majority of cases two arteries are found, one per ethmoidal compartment (anterior and posterior). In addition, a middle ethmoidal artery has been reported in 29–38% of cases [8–10]. Anterior ethmoidal arteries usually follow a caudal course with respect to the ethmoidal roof, while the middle and posterior ethmoidal arteries usually run into the skull base. The ethmoidal bony canals can show focal or wide dehiscent areas. The ethmoidal arteries divide into several small arteries for the nasal septum, middle and superior turbinate, and external nose, while the terminal branch perforates the vertical portion of the cribriform plate, forming a bony defect called ethmoidal sulcus, and contributes to the vascularization of the dura of the anterior cranial fossa (anterior meningeal arteries). Because of their relevance in ASB surgery, the anterior meningeal arteries running within the falx cerebri along the anterior surface of the crista galli have been specifically called anterior falcine arteries [1]. The skull base anterior to the ethmoidal roofs and cribriform plates is composed of the posterior plates of the frontal sinuses, which tilt from coronal to axial above the frontal recesses, constituting the transition area from cranial vault and cranial base.

Fig. 9. Transorbital endoscopic view of the ASB via a superior eyelid approach. The endoscope is placed in a posterior-medial (a), posterior (b), and posterior-lateral (c) direction through the superior orbital quadrant, between the orbital roof and periorbit (Per). d The yellow, green, and red circles show the areas targeted in a, b, and c, respectively. Several landmarks can be identified from anterior-medial to posterior-lateral: the anterior (AEF) and posterior (PEF) ethmoidal foramen along the frontoethmoidal suture (FES), the optic canal (OC), the superior orbital fissure (SOF), and Hyrtl’s foramen (HF), when present. GWSB, greater wing of the sphenoid bone; OPFB, orbital plate of the frontal bone; LWSB, lesser wing of the sphenoid bone.

Posterior to the ethmoidal roof, the midline ASB is formed by the planum sphenoidale, which corresponds to the area limited by the cranial insertion of the anterior walls of the sphenoid sinuses anteriorly, the tuberculum sellae posteriorly, and the optic canals laterally forming the cranial portion of the body of the sphenoid bone. The planum sphenoidale is a flat bony lamina that separates the sphenoid sinuses from the intracranial space. At the junction between the planum sphenoidale and the anterior sellar wall, the bone thickens forming the tuberculum sellae, which lies anteroinferiorly to the optic chiasm. Being the anterior insertion of the diaphragma sellae (i.e., the dural roof of the sellar region), the tuberculum sellae can be used as a landmark between the sella, below, and the suprasellar region, above. Within the suprasellar region, properly called chiasmatic cistern, the optic nerves merge together into the optic chiasm and exit as optic tracts running medially to the intracranial tract of the internal carotid artery. The pituitary stalk lies posterior to the optic chiasm and joins the pituitary gland piercing the diaphragma sellae. In this area, the superior hypophyseal arteries arise from the internal carotid arteries and supply the optic apparatus, dura, and pituitary stalk (Fig. 10).

Fig. 10. Transcranial endoscopic view of the ASB via a supraorbital subfrontal approach (the trajectory is shown in Fig. 5). The point of view is neurosurgical (upside down). a The endoscope is placed above and anterior to the anterior clinoid process (ACP) and below the posterior (POG) and medial (MOG) orbital gyri. The optic nerve (ON), medially, and fronto-orbital vein (FOV), middle cerebral artery (MCA), and lateral orbitofrontal artery (LOFA), laterally, are visible. b, c The endoscope is placed pointing at the planum sphenoidale (PS). The internal carotid artery (ICA), ophthalmic artery (OpA), and the optic nerves can be identified. More anteriorly, the olfactory tract (OlT) is identified while entering the outer arachnoid membrane parallel to the medial orbitofrontal artery (MOFA). d Turning the scope upwards towards the suprasellar region, the optic chiasm (OC), pituitary stalk (PS), and diaphragma sellae (DS) can be identified. All these structures are vascularized by the superior hypophyseal arteries (SHA).

At the lateral borders of the planum sphenoidale are the optic canals, which “house” the optic nerves and ophthalmic arteries. Of note, the ophthalmic artery usually runs inferior in the optic canal, and the safest area to open the optic sheath endoscopically is via the superomedial quadrant. The trajectory of the optic canals is from posteromedial to anterolateral and their orbital aperture lies about 7 mm posterior to the posterior ethmoidal foramen [11]. On its lateral side, the optic canal is separated from the superior orbital fissure by a bony structure, the optic strut, which serves as one of the three roots of the anterior clinoid process. The other roots derive from the posterolateral portion of the planum sphenoidale and from the posteromedial portion of the lesser wing of the sphenoid (Fig. 11). From an endonasal perspective, the optic strut corresponds to the lateral optic-carotid recess, while the medial optic-carotid recess serves as a landmark for the lateral edge of the tuberculum sellae. Within the optic canal and orbital cavity, the optic nerve is completely surrounded by dura, subarachnoid space, and arachnoid up to a few millimeters before joining the eyeball (globe).

Fig. 11. Transcranial microscopic view of the anterior clinoid process through a pterional approach (the trajectory is shown in Fig. 5). The point of view is neurosurgical (upside down). a The meningo-orbital fold (MOF) is identified between the lesser (LWSB) and greater (GWSB) wings of the sphenoid bone. b The fold is cut, paying attention not to damage the neurovascular structures that are located in the medial portion of the superior orbital fissure. c, d The anterior clinoid process (ACP) is exposed and drilled to expose the internal carotid artery (ICA). PS, planum sphenoidale.

The inferior surface of the frontal lobes and anterior portion of the inter-hemispheric fissure lie above the ASB. In particular, the gyrus rectus and medial orbital gyrus rest on the midline ASB and are separated by the olfactory sulcus, where the olfactory tracts run (Fig. 12). The medial orbitofrontal artery is a branch of the post-communicating tract of the anterior cerebral artery and provides blood supply to the gyrus rectus and medial orbital gyrus. The frontopolar artery arises a few millimeters after the medial orbitofrontal artery and runs on the medial surface of the frontal lobe to reach the frontal pole. These arteries and especially their related veins are frequently connected to the falx cerebri and dura of the anterior cranial fossa via some small bridge vessels that cross the subarachnoid space. During transnasal endoscopic approaches, special attention should be given to avoid injury to these vessels, which may be in contact with the cranial portion of the lesion that is being targeted for removal.

Fig. 12. Transnasal endoscopic view of the ASB via a transcribriform approach. a, b The olfactory fissures (OlF) and ethmoidal roofs (ER) have been exposed by removing the ethmoid complex, nasal septum (NS), middle (MT), and superior (ST) turbinates. c The bone of the median ASB has been removed exposing the dura of the crista galli (CGD), ethmoidal roof (ERD), and planum sphenoidale (PSD) together with the olfactory phyla (OPh). d The dura has been incised and displaced medially to identify the olfactory groove (OGr), olfactory bulb (OBu), and the outer arachnoid (OAr) that is attached to the gyrus rectus (GR) and medial orbital gyrus (MOG). e The falx cerebri (FaC) has been progressively sectioned towards the corpus callosum (white asterisk). f Scheme of the trajectory of the transnasal corridor towards the ASB. AEA, anterior ethmoidal artery; AFA, anterior falcine artery; FPA, frontopolar artery; FS, frontal sinus; LP, lamina papyracea; MOFA, medial orbitofrontal artery; PEA, posterior ethmoidal artery; SpR, sphenoid rostrum; SpS, sphenoid sinus.