Endoscopic Transnasal Anatomy of the Skull Base and Adjacent Areas

1 Classification of Endoscopic Transnasal Approaches to the Skull Base and Adjacent Areas

Piero Nicolai, Marco Ferrari, Roberto Maroldi, Alperen Vural, Marco Maria Fontanella, Luigi Fabrizio Rodella, Lena Hirtler, Manfred Tschabitscher

The development of transnasal endoscopic techniques has provided immense perspectives in the field of skull base surgery. Various meticulous anatomical studies have improved the understanding of skull base anatomy from the endoscopic perspective, and endoscopic transnasal surgery has become notably valuable for accessing and treating pathologies of the skull base.1-3 This significant evolution, which started with pituitary surgery, has progressively provided a myriad of approaches extending from the posterior frontal plate to axis (C2) and laterally to the parasellar area, petrous apex, jugular foramen, infratemporal fossa, and upper parapharyngeal space.1,4-8

By using the natural surgical corridor of the sinonasal tract, transnasal approaches give access to a wide range of sites, which can harbor a proportionally wide range of lesions. The ideal approach to a specific lesion should be selected with the intent to provide an exposure that avoids complications and achieves complete surgical resection and adequate reconstruction. Another characteristic should be the potential of being expanded in case unanticipated extension of the lesion is detected, and permit identification and protection of important neurovascular structures.3,9 Consequently, selection of the surgical approach is mostly based on the type and location of the disease, its relationships with critical structures, and characteristics of the expected defect.1,6,9 Specifically, critical neurovascular structures must be located around the perimeter of the corridor. This allows for direct manipulation of the lesion, minimizing the need to cross neurovascular structures when coming from a transnasal route.10 Thus, a thorough understanding of the numerous anatomic relationships is crucial for the surgeon to determine the safest and most effective way of accessing lesions of the ventral skull base. Knowledge of anatomy allows one to minimize morbidity, maximize patient safety, and contribute to the progress of the surgeon throughout the learning curve of transnasal endoscopic surgery.2

Anatomic surgical modules, based on their relation to the internal carotid artery (ICA) in the sagittal and coronal planes, provide access to the entire ventral skull base (?Table 1.1).6,7,9 The sphenoid sinus is the center at the intersection of these planes and is the starting point for most approaches, in which significant structures such as optic nerves and ICA are identified and then followed through other areas of the skull base.11 Moreover, a number of additional "doors" can be used to get access to several regions of the skull base, including the frontal sinus, nasoethmoidal complex, maxillary sinus, nasopharynx, orbital walls, and pterygoid process. Sagittal plane modules provide exposure of median structures extending from the posterior plate of frontal sinus to C2 between the two orbits and ICAs.9,12,13 These consist of transfrontal, transcribriform, transplanum-transtuberculum, transsellar, transclival, and transodontoid approaches.11 Coronal plane modules comprise the paramedian and lateral skull base, covering areas adjacent or lateral to the ICA. Three progressively deep planes are considered to further classify coronal plane modules. The anterior one corresponds to the anterior cranial fossa and orbits, including approaches to the lamina papyracea, orbital roof, and orbital cavity. The middle one is in relation to the parasellar area, middle cranial fossa, and anterior infracranial spaces, extending from the lateral wall of sphenoid sinus and posterior nasal cavity to the cavernous sinus, superior petrous apex, Meckel's cave, and pterygopalatine and infratemporal fossa. The posterior one corresponds to the posterior fossa and posterior infracranial spaces, providing access to the inferior petrous apex, lateral craniocervical junction, and upper parapharyngeal space.9,11,13 All these modules are mutually related and partially overlapping. This atlas aims to provide a thorough and schematic knowledge of these modules, which can be variably combined and suited according to actual clinical needs.

The framework of the skull base and adjacent areas is essentially made of bony structures. As a consequence, anatomical orientation mostly relies on bony planes and landmarks, with subperiosteal dissection being probably one of the key abilities to move within this exceedingly complex area. Moreover, some neurovascular structures pass across different modules in a relatively constant fashion, thus serving as valuable guides to get oriented and judiciously pursue dissection.10,13 The complexity and numerosity of structures forming the skull base and adjacent areas force the surgeon to simplify an extraordinarily intricate geometry into a reliable schematization. Overall, the mental map of skull base anatomy should be built up based on the reciprocal relationships of key structures, namely defined anatomical landmarks (?Table 1.2, ?Table 1.3).

Finally, it is worth remembering that each module is associated with idiosyncratic lesions. This concept is of utmost importance when considering that the nature of the lesion substantially dictates the type of resection that is required. Therefore, tumor characteristics should affect the choice of a specific surgical route along with patient comorbidities, general status, and skill and experience of the operating team. Indeed, each case should be evaluated thoroughly, balancing the purpose to be elegant and minimally invasive with the probability of complications.1,5-7

The following figures summarize the anatomical structures of skull base and adjacent areas while mapping the approaches described in the atlas on the sagittal.

Fig. 1.1 Sagittal view of the skull base and adjacent areas (part 1). Medial-to-lateral view of the right side (upper image) and lateral-to-medial view of the left side (lower image). A1, precommunicating tract of the anterior cerebral artery; AE, anterior ethmoidal compartment; AICA, anterior inferior cerebellar artery; Ar, anterior arch of the atlas; AWSS, anterior wall of the sphenoid sinus; BA, basilar artery; BLMT, basal lamella of the middle turbinate; BLST, basal lamella of the superior turbinate; C2, axis (body); C3, third cervical vertebra (body); CPe, cerebral peduncle; CrP, cribriform plate; DoS, dorsum sellae; EB, ethmoidal bulla; FoE, fovea ethmoidalis; FPA, frontopolar artery; FPMB, frontal process of the maxillary bone; FS, frontal sinus; GR, gyrus rectus; Hyp, hypophysis (pituitary gland); IT, inferior turbinate; JuT, jugular tuberculum; LoC, lower clivus; LoCM, longus capitis muscle; MC, midclivus; MOG, medial orbital gyrus; MOb, medulla oblongata; MT, middle turbinate; NaF, nasal floor; NB, nasal bone; OBu, olfactory bulb; OCo, occipital condyle; ON, optic nerve; OP, odontoid process; OTr, optic tract; P1, precommunicating tract of the posterior cerebral artery; PE, posterior ethmoidal compartment; PICA, posterior inferior cerebellar artery; Po, pons; PPFS, posterior plate of the frontal sinus; PSph, planum sphenoidale; SCA, superior cerebellar artery; SER, sphenoethmoidal recess; SoP, soft palate; SpF, sphenoidal floor; SPr, sellar prominence; SpS, sphenoid sinus; ST, superior turbinate; ToT, torus tubarius; TSe, tuberculum sellae; UP, uncinate process; VA, vertebral artery; Vo, vomer.

Fig. 1.2 Mapping of endoscopic transnasal approaches on a sagittal section (part 1). Medial-to-medial view of the right side (upper image) and lateral-to-medial view of the left side (lower image). TC, transcribriform approach; TCJ, transcondylar-transjugular tuberculum approach; TD, transdorsal approach; TF, transfrontal approach; TLC, transclival (lower clivus) approach; TMC, transclival (midclivus) approach; TO, transodontoid approach; TPT, transplanum-transtuberculum approach; TS, transsellar approach.

Fig. 1.3 Sagittal view of the skull base and adjacent areas (part 2). Medial-to-lateral view of the right side (both images). V, trigeminal stem; V3, mandibular nerve; XII, hypoglossal nerve; A1, precommunicating tract of the anterior cerebral artery; AE, anterior ethmoidal complex; AIPA, anterior inferior petrous apex; AMOs, accessory maxillary ostium; ARCM, anterior rectus capitis muscle; BaP, basilar plexus; CPr, carotid prominence; CS, cavernous sinus; CSu, carotid sulcus; EB, ethmoidal bulla; EFa, extraconal fat; ET, eustachian tube; Ey, eyeball; FoE, fovea ethmoidalis; FS, frontal sinus; GW, greater wing of the sphenoid bone; IFa, intraconal fat; iICA, intracranial tract of the internal carotid artery; IMA, internal maxillary artery; ION, infraorbital nerve; IRM, inferior rectus muscle; IT, inferior turbinate; JF, jugular foramen; JuT, jugular tuberculum; LMAt, lateral mass of the atlas; LMAx, lateral mass of the axis; LoC, lower clivus; LoCM, longus capitis muscle; LOG, lateral orbital gyrus; LPM, lateral pterygoid muscle; LS, lacrimal sac; MCA, middle cerebral artery; MOG, medial orbital gyrus; MPM, medial pterygoid muscle;...