Anatomical Landmarks of Anterior Approaches: An Endoscopic View

4 Anatomical Landmarks of Anterior Approaches: An Endoscopic View

Cristian Ferrareze Nunes and Juan C. Fernandez-Miranda

Abstract

Knowledge of the anatomical landmarks of anterior approaches to the skull base is key to safely perform the expanded endoscopic endonasal approaches. Since the nasal cavity is a direct corridor to the ventral skull base, mastering its anatomy and relations to neurovascular structures is essential. The transsphenoidal, transtuberculum, and transplanum approaches, centered on the sphenoid sinus, allow access to the sella, cavernous sinus, suprasellar space, and the posterior third of the anterior skull base, thus allowing the treatment of pathologies such as pituitary adenomas, craniopharyngiomas, tuberculum sellae meningiomas, planum sphenoidale meningiomas, and Rathke cleft cysts. Moving anteriorly toward the ethmoid cells and the frontal sinus, in the sagittal plane, permits approaching olfactory groove meningiomas and esthesioneuroblastomas through a transcribriform transethmoidal approach. Further still in the sagittal plane, bellow the sella, approaches to the middle, lower clivus and craniovertebral junction give access to the ventral brainstem area, site of meningiomas and chordomas. Finally, expanding laterally, on the coronal plane, through a transpterygoid approach, the Meckel’s cave area and the infratemporal fossa may be reached.

Keywords: Keywords: anatomy, endoscopic endonasal approaches, skull base, transsphenoidal approaches

4.1 Introduction

The endoscope has been widely used as a tool for better visualization and more direct access to certain regions of the skull base. Thus, mastering the anatomy from an endoscopic perspective and relating it to the microsurgical anatomy is crucial. The nasal cavity is the starting point for all the endoscopic endonasal approaches, which makes comprehending its anatomical landmarks an essential step in performing the approaches.

4.2 Nasal Cavity

The nasal fossa begins at the limen nasi anteriorly and ends at the choana posteriorly. It extends from the hard palate inferiorly to the base of the skull superiorly. The nasal septum, which separates the two nasal cavities, is formed anteriorly by the nasal septal cartilage and continues posteriorly with the perpendicular plate of the ethmoid, superiorly, and the vomer, the crest of the maxillary bone and the crest of the palatine bone, inferiorly.1 It is the main landmark for the midline; although in most cases it is deviated on its anterior and middle portions, it always leads to the midline of the sphenoid rostrum, posteriorly, and to the anterior skull base, superiorly² ( Fig. 4.1a).

Fig. 4.1 The nasal cavity anatomy. (a) Endoscopic view of the nasal cavity after partial removal of the septum. On the left side the complete turbinates and the ethmoid sinus were completely removed, and the sphenoid and the maxillary sinus were opened. On the right side, the middle turbinate was removed, exposing the middle meatus contents. (b) Endoscopic view of the posterior wall of the nasal cavity: the middle turbinate is being laterally displaced to expose superior turbinate and the sphenoethmoidal recess. White arrow, sphenoid ostium; white asterisk, sphenoethmoidal recess; black asterisk, location of the sphenopalatine branches to the septum. CR, clival recess; PS, planum sphenoidale; TS, tuberculum sella; black arrow, ethmoidal crest; white dashed line, middle turbinate insertion site.

The turbinates are shell-shaped structures that arise laterally in the nasal cavity. The inferior turbinate originates from the lateral wall of the nose, anteriorly from the maxillary bone and posteriorly from the palatine bone. It delimitates the inferior meatus that relates to the medial wall of the maxillary sinus. The middle turbinate originates from the ethmoid bone and delimitates the middle meatus. It attaches superiorly to the skull base, at cribriform plate and, laterally, to the lateral wall of the nose, at the lamina papyracea. Its lateral attachment, the basal lamella, anatomically separates the anterior and posterior ethmoid air cells ( Fig. 4.1a). The middle turbinate is frequently removed during surgery to create more room for instruments or permit access to the middle meatus and the ethmoid cells. The middle meatus contains the uncinate process, the hiatus semilunaris, and the ethmoidal bulla. The uncinate process of the ethmoid bone is a thin curved bony structure which forms the anteroinferior border of the hiatus semilunaris, which houses the maxillary ostium. The ethmoid bulla is the largest air cell of the anterior ethmoid sinus. Its lateral limit is the lamina papyracea and it bulges into the middle meatus. The superior turbinate originates from the ethmoid bone and delimitates the superior meatus. Above and posterior to the turbinate is located the sphenoethmoidal recess, where the sphenoid ostium is located inferomedially to the turbinate tail, and about 1.5 cm superior to the upper boarder of the choana. The choana is the opening of the nasal cavity into the nasopharynx and serves as a reference point for the inferior clivus^1,2 ( Fig. 4.1a, b).

The sphenoid rostrum corresponds to the anterior wall of the sphenoid sinus (SS) which occupies the body of the sphenoid bone. Between the sphenoid ostium and the choana, toward the posterior end of nasal septum, runs the posterior septal artery originating from the sphenopalatine artery, which provides the blood supply to the septum and consequently to the nasoseptal flap.3

4.3 Anatomy of the Transshpenoidal, Transtuberculum, and Transplanum Approaches

The SS is the main structure related to the transsphenoidal, transtuberculum, and transplanum approaches. It varies in degree of pneumatization and is generally categorized from the least to the most pneumatized type as: conchal, presellar, sellar, or postsellar.⁴ The sinus is usually divided into one or multiple septa, which, if displaced from the midline posteriorly, leads to the carotid prominence in up to 87% of the cases.⁵

In a well-pneumatized SS the sella turcica is located centrally, above the clival recess, which is limited laterally by the paraclival segment of the internal carotid artery (ICA). The sella is bounded laterally by the carotid prominence, which is the anterior-most projection of the ICA. Superior to the sella and adjacent to the tuberculum sellae (TS) is located the optic prominence, a projection of the optic canal, containing the optic nerve and the ophthalmic artery, which runs inferior to the nerve.⁶ Laterally to the junction of the carotid prominence and the optic prominence lies a depression in the bone termed lateral opticocarotid recess (LOCR), it represents the pneumatization of the optic strut and its medial projection is related to the clinoid segment of the ICA. Approximately 5.6 mm medial to the LOCR is located the medial opticocarotid recess (MOCR), a teardrop-shaped osseous indentation formed at the medial junction of the paraclinoid ICA and the optic nerve. Inferior to the MOCR is located the middle clinoid process, which can be removed when not forming a carotid ring.^7,8 Pneumatization of the lateral recess of the SS may expand the sinus laterally allowing the identification of the superior orbital fissure (SOF), located inferiorly to the LOCR and superiorly to the maxillary strut. The maxillary strut is the bone that separates the SOF from the mandibular nerve (V2) at the foramen rotundum⁹ ( Fig. 4.2a).

Fig. 4.2 Transsphenoidal, transtuberculum, and transplanum approaches anatomy. (a) Endoscopic view of the sphenoid sinus with a 30-degree endoscope after the sphenoid rostrum was drilled out. LOCR, lateral opticocarotid recess; MOCR, medial opticocarotid recess; black asterisk, middle clinoid; black arrow, pointing to the lateral recess of the sphenoid sinus. (b) Endoscopic view of the sellar and suprasellar space with a 30-degree endoscope after the bone over the sella, tuberculum sellae, and planum sphenoidale was removed, and the dura over the sella and tuberculum sellae was opened. (c) Endoscopic view of the intracranial space of the sellar and parasellar spaces with a 30-degrees angled endoscope. The dura over the upper clivus, and anterior, middle, and posterior cavernous sinus was removed. (d) Endoscopic view of the carvernous sinus with a 30-degree endoscope. The cavernous internal carotid artery (ICA) was displaced laterally to allow visualization of the lateral wall of the cavernous sinus. III, oculomotor nerve; IV, trochlear nerve; V, trigeminal nerve; VI, abducens nerve; VII/VIII, facial/auditory nerves complex; XII, hypoglossal nerve roots; V1, ophthalmic division of trigeminal nerve; V2, maxillary division of trigeminal nerve; V3, mandibular division of trigeminal nerve; BA, basilar artery; Cav. ICA, cavernous internal carotid artery; PCA, posterior cerebral artery; SCA, superior cerebellar artery; white arrowhead, superior hypophyseal artery; white asterisk, trochlear nerve; black arrowhead, anterior communicating artery complex branches.