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

10.1055/b-0039-172563

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.

**Table 1.1** Overview of sagittal and coronal endoscopic transnasal approaches to the skull base and adjacent areas
Plane	Approach
Sagittal	Transfrontal (Chapter 6)
	Transcribriform (Chapter 7)
	Transplanum–transtuberculum (Chapter 8)
	Transsellar (Chapter 9)
	Transclival	Upper clivus (transdorsal) (Chapters 10 and 11)
		Middle clivus (midclivus) (Chapter 11)
		Lower clivus (Chapter 12)
	Transodontoid (Chapter 13)
Coronal	Anterior	Orbital decompression (Chapter 14)
		Supraorbital (Chapter 14)
		Transorbital (Chapter 14)
	Middle	Transcavernous	Medial (Chapter 17)
		Transcavernous	Lateral (Chapter 18)
		Optic decompression (Chapter 14)
		Medial petrous apex (Chapter 19)
		Transpterygomaxillary (Chapter 15)
		Infratemporal fossa (Chapter 16)
		Transpterygoid	Upper (Chapter 21)
		Transpterygoid	Lower (Chapter 23)
		Suprapetrous (Meckel’s cave) and transalisphenoid (Chapter 21)
	Posterior	Infrapetrous (Chapter 20)
		Transcondylar–transjugular tuberculum (Chapter 22)
		Parapharyngeal space	Medial (Chapter 23)
		Parapharyngeal space	Lateral (Chapter 24)

**Table 1.2** Main characteristics of endoscopic transnasal sagittal approaches to the skull base and adjacent areas
Approach		Anatomical boundaries	Key anatomical landmarks	Critical structures	Target(s)	Limitations	Common pathologies
Transfrontal		Nasal bones anteriorly Nasal septum and cribriform plate posteriorly Orbital cavities bilaterally	First olfactory phyla Crista galli	Superior sagittal sinus Medial orbitofrontal and frontopolar vessels Bridge veins	Posterior wall of frontal sinus	Far-lateral extension (beyond the orbital sagittal midplane) Involvement of the anterior frontal plate	Intracranial complications of rhinosinusitis (epidural abscess and subdural empyema) ¹⁴ Mucocele/mucopyocele ¹⁵ Tumors (fibro-osseous tumors and inverted papilloma) ¹⁶ ^, ¹⁷ Dermoid cyst and other nasofrontal dysembryogenic lesions ¹⁸ ^, ¹⁹ CSF leaks ²⁰ ^, ²¹
Transcribriform		Posterior wall of frontal sinus anteriorly Planum sphenoidale posteriorly Lamina papyracea bilaterally	Ethmoidal arteries Crista galli Falx cerebri Periorbit	Medial orbitofrontal vessels Anterior cerebral arteries Superior sagittal sinus Gyrus rectus Medial orbitofrontal gyrus	Midline anterior skull base above the nasoethmoidal complex	Far-lateral extension (beyond the orbital sagittal midplane) Massive cranial extension Extension to the suprasellar area Extension to nasal bones and face soft tissues	Nasoethmoidal malignancies ¹ ^, ²² – ²⁴ Meningiomas ²⁵ – ²⁷ Schwannomas ²⁸ ^, ²⁹ Dysembryogenic lesions ³⁰
Transplanum–transtuberculum		Fovea ethmoidalis and cribriform plate anteriorly Sella turcica posteriorly Optic canals and paraclinoid and intracranial ICAs bilaterally	Medial opticcarotid recesses Optic canals Posterior clinoid processes	Circle of Willis Paraclinoid and intracranial ICAs Optic nerves Optic chiasm Optic tracts Pituitary stalk Superior hypophyseal arteries Gyrus rectus Medial orbitofrontal gyrus	Midline anterior skull base above the sphenoid sinus Suprasellar area Third ventricle	Extension lateral to ICAs Extension lateral to optic nerves	Pituitary adenomas ⁶ ^, ³¹ ^, ³² Craniopharyngiomas ³³ – ³⁵ Meningiomas ³⁶ Aneurysms of the anterior cerebral circulation ³⁷ ^, ³⁸ Tumors of the third ventricle ³⁹ ^, ⁴⁰ ^, ⁴¹
Transsellar		Anterior superior intercavernous sinus superiorly Anterior inferior intercavernous sinus inferiorly Cavernous sinuses bilaterally	Tuberculum sellae Lateral and medial optic-carotid recesses Carotid prominences Sellar floor “Four blues” (anterior superior intercavernous, anterior inferior intercavernous, and cavernous sinuses)	Parasellar and paraclinoid ICAs Pituitary gland Optic chiasm	Sella turcica	Extension lateral to ICA Extension posterior to dorsum sellae Suprasellar extension	Pituitary adenomas/apoplexy ⁶ ^, ⁴² Rathke’s cleft cysts ⁴³ ^, ⁴⁴ Craniopharyngiomas ⁴⁵ Arachnoid cysts ⁴⁶
Transclival	Transdorsal	Cavernous sinus bilaterally Midclivus inferiorly Suprasellar area superiorly	Sellar floor Dorsum sellae Posterior clinoid process Parasellar ICA	Oculomotor nerve Parasellar and paraclinoid ICAs Pituitary gland and stalk Basilar artery and its branches Mesencephalon and pons	Upper clivus Inter- and parapeduncular space (retroclival area)	Extension lateral to ICA Extension lateral to the oculomotor nerve Suprasellar extension Inferior prepontine extension	Chordomas ⁴⁷ Craniopharyngiomas ⁴⁸ Meningiomas ⁴⁹ Aneurysms ⁵⁰
	Midclivus	Sellar floor superiorly Paraclival ICAs bilaterally Plane of the vidian nerves inferiorly	Vidian nerves Fibrocartilago basalis	Abducens nerve Paraclival ICAs Basilar artery and its branches Pons	Midclivus Prepontine space	Extension lateral to ICA Extension lateral to abducens nerve Superior prepontine interpeduncular extension Premedullary extension	Chordomas ⁵¹ Chondrosarcomas ⁵² ^, ⁵³ Meningiomas ⁵⁴ Aneurysms ⁵⁰
	Lower clivus	Plane of the vidian nerves superiorly Hypoglossal canals bilaterally Foramen magnum inferiorly	Basion Jugular tubercula Occipital condyles Hypoglossal canals	Abducens nerve Hypoglossal nerve Vertebral arteries and their branches Medulla oblongata	Lower clivus Superior premedullary space	Extension lateral to hypoglossal nerves Superior prepontine extension Extension to the craniocervical junction or inferior premedullary space	Chordomas ⁵⁵ Chondrosarcomas ⁵³ Meningiomas ⁵⁵ Aneurysms ⁵⁰
Transodontoid		Basion superiorly Superior border of the axis body inferiorly Occipital condyles and lateral masses of the atlas bilaterally	Lower clivus Basion Anterior arch of the atlas Odontoid process	Vertebral arteries First and second cervical nerves Suboccipital cavernous sinuses	Craniocervical junction Inferior premedullary space	Inferior extension to the body of the axis Lateral extension to occipital condyles or lateral masses of the atlas Superior premedullary extension	Inflammatory pannus ⁵⁶ Craniovertebral invagination ⁵⁶ Chordomas ⁵⁷
Abbreviations: CSF, cerebrospinal fluid; ICA, internal carotid artery.

**Table 1.3** Main characteristics of endoscopic transnasal coronal approaches to the skull base and adjacent areas
Approach		Anatomical boundary	Key anatomical landmark	Critical structures	Limitations	Common pathologies
Anterior	Orbital decompression Supraorbital Transorbital	Orbital roof and related dura superiorly Orbital floor inferiorly Lacrimal system and eyeball anteriorly Orbital apex posteriorly	Lamina papyracea Ethmoidal arteries Superior oblique, medial rectus, and inferior rectus muscles	Optic nerve Ophthalmic artery Extrinsic ocular muscles and their nerve supply	Lateral and/or superior extension with respect to the optic nerve and ophthalmic artery Extension to lacrimal system Preseptal extension Extension to orbital apex	Graves orbitopathy ⁵⁸ ^, ⁵⁹ Hemangiomas ⁶⁰ Schwannomas ⁶¹ Meningiomas ⁶¹
Middle	Medial transcavernous	Sella turcica medially Parasellar ICA laterally Petroclival junction inferiorly Posterior clinoid process and oculomotor triangle posteriorly and superiorly (of note, the corridor can be extended posteriorly and superiorly via interdural hypophysiopexy ⁶² and transoculomotor approach) ⁶³	Medial optic-carotid recess Carotid prominence Pituitary ligaments Medial wall of the cavernous sinus Posterior clinoid process Oculomotor triangle	Parasellar ICA Meningohypophyseal trunk Oculomotor nerve Abducens nerve	Extension to the lateral and posterior compartments of the cavernous sinus Extension to the sella turcica	Pituitary adenomas ⁶⁴ ^, ⁶⁵ Meningiomas ⁶⁶
	Lateral transcavernous	Parasellar and paraclival ICA medially Meckel’s cave and dura of the middle cranial fossa laterally Anterior clinoid process and roof of the cavernous sinus superiorly Petrolingual ligament inferiorly	Lateral optic-carotid recess Carotid prominence Carotid sulcus Maxillary strut	Parasellar and paraclival ICA Oculomotor nerve Trochlear nerve Abducens nerve Ophthalmic nerve Maxillary nerve Inferolateral trunk	Lateral extension to the Meckel’s cave Extension to the superior compartment of the cavernous sinus Extension to the petrous apex	Meningiomas ⁶⁶ ^, ⁶⁷ Schwannomas ⁶⁶ ^, ⁶⁸ Hemangiomas ⁶⁶
	Optic decompression	Medial optic-carotid recess and tuberculum sellae posteromedially Lateral optic-carotid recess inferiorly Planum sphenoidale superiorly Lamina papyracea anterolaterally	Medial optic-carotid recess Lateral optic-carotid recess Annulus of Zinn	Optic nerve Ophthalmic artery Paraclinoid ICA Extrinsic ocular muscles	Extension to the orbital cavity Extension lateral and/or superior to the optic nerve Extension to cavernous sinus/suprasellar space	Traumatic optic neuropathy ⁶⁹ Nontraumatic optic neuropathy ⁷⁰
	Medial petrous apex	Midclivus medially Paraclival ICA anterolaterally Abducens nerve superiorly	Vidian canal and vidian nerve Carotid sulcus Fibrocartilago basalis	Paraclival ICA Abducens nerve	Extension to the inferior petrous apex or lateral portion of the superior petrous apex (lateral to the abducens nerve) Extension to cavernous sinus	Chordomas ⁷¹ Chondrosarcomas ⁷² ^, ⁷³ Cholesterol granulomas ⁷⁴ Meningiomas ⁵⁴ ^, ⁷⁵
	Transpterygomaxillary	Tails of turbinates medially Pterygomaxillary fissure laterally Inferior orbital fissure superiorly Pterygomaxillary junction inferiorly	Posterior maxillary wall Infraorbital canal Sphenopalatine foramen Pterygoid process Pterygomaxillary junction	Internal maxillary artery and its branches Maxillary nerve	Lateral extension to the infratemporal fossa Superior extension to the orbital cavity Posterior and/or inferior extension to the masticatory space	Schwannomas ⁷⁶ Angiofibromas ⁷⁷ Malignant tumors ⁷⁷
	Infratemporal fossa	Lateral pterygoid plate posteromedially Orbital floor superiorly Alveolar process inferiorly Coronoid process anterolaterally	Infraorbital canal Zygomatic recess Coronoid process Lateral pterygoid muscle Temporal muscle	Internal maxillary artery and its branches Mandibular nerve	Extension to the temporal fossa Extension to the buccal space Extension to the parapharyngeal space Extension to the adjacent skull base	Angiofibromas ⁷⁸ ^, ⁷⁹ Schwannomas ⁸⁰ ^, ⁸¹ Meningiomas ⁸² Sinonasal or nasopharyngeal tumors ⁸⁰ ^, ⁸³
	Transpterygoid	Upper transpterygoid Greater wing of the sphenoid bone laterally Sphenoid floor medially Superior orbital fissure superiorly Scaphoid fossa inferiorly Lower transpterygoid Base of the pterygoid process superiorly Lateral pterygoid muscle laterally Pterygomaxillary junction inferiorly Nasopharynx medially	Descending palatine canal Vidian nerve and vidian canal Foramen rotundum Fibrocartilago basalis Medial and lateral pterygoid plate Foramen ovale Pterygomaxillary junction	Anterior genu of the internal carotid artery (between the petrous and paraclival tract) Pterygoid, pharyngeal, and foraminal plexuses Maxillary nerve	Depending on the targeted area (see column “Common pathologies”)	The transpterygoid approach is propaedeutic to other approaches (i.e., suprapetrous, infrapetrous, and parapharyngeal) that are characterized by idiosyncratic lesions
	Suprapetrous (Meckel’s cave) Transalisphenoid	Abducens nerve superiorly Paraclival ICA and petrolingual ligament medially Gasserian ganglion and dura of the middle cranial fossa laterally Petrous ICA inferiorly	Vidian nerve Foramen rotundum Maxillary strut Lingual process of the sphenoid and petrolingual ligament Mandibular strut	Paraclival and petrous ICA Abducens nerve Gasserian ganglion and trigeminal branches	Extension to the cavernous sinus Extension to the inferior petrous apex or medial portion of the superior petrous apex Extension to infratemporal fossa Far-lateral extension into the middle cranial fossa	Schwannomas ⁸⁴ Meningiomas ⁸⁴ Malignant tumors (nasopharyngeal carcinoma and minor salivary gland carcinomas) ⁸⁴
Posterior	Infrapetrous	Petrous ICA superiorly and laterally Jugular tuberculum and hypoglossal canal inferiorly Lower clivus medially	Medial pterygoid plate Vidian canal Fibrocartilago basalis Hypoglossal canal	Petrous ICA Hypoglossal nerve	Extension to the posteroinferior petrous apex or superior petrous apex Extension to the clivus Extension to occipital condyle	Cholesterol granulomas ⁸⁵ Chordomas ⁸⁶ Chondrosarcomas ⁸⁷
	Transcondylar–transjugular tuberculum (far-medial)	Lower clivus medially Carotid canal and jugular foramen laterally Craniocervical junction inferiorly Inferior petrosal sinus superiorly	Anterior rectus capitis muscle Jugular tuberculum Hypoglossal canal Occipital condyle	Vertebral artery Hypoglossal nerve Glossopharyngeal, vagus, and accessory nerve Acoustic-facial bundle Inferior petrosal sinus	Extension to the petrous apex Extension to the vascular compartment of the jugular foramen Extension to parapharyngeal space Extension to the atlas	Chordomas ⁸⁸ ^, ⁸⁹ Chondrosarcomas ⁸⁸ ^, ⁸⁹ Other malignant tumors ⁸⁹
	Medial parapharyngeal space	Nasopharynx medially Base of the pterygoid process and fibrocartilago basalis superiorly Lateral pterygoid plate laterally Axial plane passing through the pterygomaxillary junction inferiorly	Medial pterygoid plate Eustachian tube Tensor veli palatini muscle Levator veli palatini muscle Lateral pterygoid plate	Parapharyngeal ICA	Extension to adjacent skull base Extension to infratemporal fossa Extension to the jugular foramen Extension to middle parapharyngeal space	Malignant nasopharyngeal tumors (nasopharyngeal carcinomas, minor salivary gland carcinomas, and sarcomas) ⁸⁰ ^, ⁹⁰
	Lateral parapharyngeal space	Nasopharynx medially Base of the pterygoid process and fibrocartilago basalis superiorly Lateral pterygoid muscle laterally Axial plane passing through the pterygomaxillary junction inferiorly	Lateral pterygoid plate Foramen ovale Foramen spinosum Tensor veli palatini muscle Levator veli palatini muscle	Parapharyngeal ICA Internal jugular vein Glossopharyngeal nerve Vagus nerve Spinal accessory nerve Hypoglossal nerve	Extension to adjacent skull base Extension to infratemporal fossa Extension to middle parapharyngeal space	Malignant nasopharyngeal tumors (nasopharyngeal carcinomas, minor salivary gland carcinomas, and sarcomas) ⁸⁰ ^, ⁹⁰
Abbreviation: ICA, internal carotid artery.

**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; MRM, medial rectus muscle; MS, maxillary sinus; MT, middle turbinate; OC, optic canal; OCo, occipital condyle; ON, optic nerve; OpA, ophthalmic artery; OR, orbital roof; P2, postcommunicating tract of the posterior cerebral artery; pcICA, paraclinoid tract of the internal carotid artery; PE, posterior ethmoidal complex; peICA, petrous tract of the internal carotid artery; phICA, parapharyngeal tract of the internal carotid artery; PhO, pharyngeal ostium of the eustachian tube; PMF, pterygomaxillary fissure; PPFS, posterior plate of the frontal sinus; PSph, planum sphenoidale; PWMS, posterior wall of the maxillary sinus; sICA, parasellar tract of the internal carotid artery; SpS, sphenoid sinus; SRM, superior rectus muscle; SuPA, superior petrous apex; TL, temporal lobe; TPAt, transverse process of the atlas; UP, uncinate process; VA, vertebral artery; ZR, zygomatic recess.

**Fig. 1.4 Mapping of endoscopic transnasal approaches on a sagittal section (part 2)**. Medial-to-lateral view of the right side (both images). IP, infrapetrous approach; ITF, infratemporal fossa approach; LPPh, lateral parapharyngeal space approach; LTC, lateral transcavernous approach; MPA, medial petrous apex approach; MPPh, medial parapharyngeal space approach; MTC, medial transcavernous approach; OD, orbital decompression; OpD, optic decompression; SO, supraorbital approach; SP, suprapetrous (Meckel’s cave) approach; TA, transalisphenoid approach; TC, transcribriform approach; TCJ, transcondylar–transjugular tuberculum approach; TOr, transorbital approach; TPM, transpterygomaxillary approach; TPT, transplanum–transtuberculum approach.

**Fig. 1.5 Coronal view of the skull base and adjacent areas (part 1)**. Anterior-to-posterior view (both images). AE, anterior ethmoidal compartment; AEA, anterior ethmoidal artery; ANC, agger nasi cell; CrP, cribriform plate; EFa, extraconal fat; Ey, eyeball; FaC, falx cerebri; FoE, fovea ethmoidalis; FS, frontal sinus; FSC, frontal septal cell; GR, gyrus rectus; h, horizontal lamella of the cribriform plate; IFa, intraconal fat; IOCa, infraorbital canal; IOF, inferior orbital fissure; IOM, inferior oblique muscle; IRM, inferior rectus muscle; IT, inferior turbinate; ITF, infratemporal fossa; LMW, lateral maxillary wall; LP, lamina papyracea; LRM, lateral rectus muscle; LS, lacrimal sac; MMW, medial maxillary wall; MOFA, medial orbitofrontal artery; MOG, medial orbital gyrus; MRM, medial rectus muscle; MS, maxillary sinus; MT, middle turbinate; NLD, nasolacrimal duct; NS, nasal septum; OBu, olfactory bulb; OFi, olfactory fissure (or cleft); ON, optic nerve; OrF, orbital floor; PE, posterior ethmoidal compartment; SAC, supra-agger cell; SAFC, supra-agger frontal cell; SOM, superior oblique muscle; SOpV, superior ophthalmic vein; SRM, superior rectus muscle; ST, superior turbinate; TM, temporal muscle; v, vertical lamella of the cribriform plate; ZR, zygomatic recess.

**Fig. 1.6 Mapping of endoscopic transnasal approaches on a coronal section (part 1)**. Anterior-to-posterior view (both images). ITF, infratemporal fossa approach; OD, orbital decompression; SO, supraorbital approach; TC, transcribriform approach; TF, transfrontal approach; TOr, transorbital approach; TPM, transpterygomaxillary approach.

**Fig. 1.7 Coronal view of the skull base and adjacent areas (part 2)**. Anterior-to-posterior view (both images). III, oculomotor nerve; IV, trochlear nerve; VI, abducens nerve; V1, ophthalmic nerve; V2, maxillary nerve; V3, mandibular nerve; A2, postcommunicating tract of the anterior cerebral artery; ACP, anterior clinoid process; ArD, articular disk (temporomandibular joint); ArtT, articular tuberculum; ASIS, anterior superior intercavernous sinus; BP, base of the pterygoid process (or basipterygoid); ConP, condylar process of the mandible; ET, eustachian tube; FoPl, foraminal plexus; GW, greater wing of the sphenoid bone; iICA, intracranial tract of the internal carotid artery; IJV, internal jugular vein; IMA, internal maxillary artery; LoC, lower clivus; LoCM, longus capitis muscle; LPM, lateral pterygoid plexus; LVPM, levator veli palatini muscle; MMA, middle meningeal artery; MPM, medial pterygoid plexus; NaP, nasopharyngeal posterior wall; ON, optic nerve; OpA, ophthalmic artery; OSt, optic strut; pcICA, paraclinoid tract of the internal carotid artery; PG, parotid gland; phICA, parapharyngeal tract of the internal carotid artery; PhPl, pharyngeal plexus; PSph, planum sphenoidale; PtPl, pterygoid plexus; sICA, parasellar tract of the internal carotid artery; SOF, superior orbital fissure; SOpV, superior ophthalmic vein; SpF, sphenoidal floor; SpS, sphenoid sinus; StyP, styloid process; TSe, tuberculum sellae; TVPM, tensor veli palatini muscle; VN, vidian nerve.

**Fig. 1.8 Mapping of endoscopic transnasal approaches on a coronal section (part 2)**. Anterior-to-posterior view (both images). IP, infrapetrous approach; LPPh, lateral parapharyngeal space approach; LTC, lateral transcavernous approach; MPA, medial petrous apex approach; MPPh, medial parapharyngeal space approach; MTC, medial transcavernous approach; OpD, optic decompression; SP, suprapetrous (Meckel’s cave) approach; TA, transalisphenoid approach; TD, transdorsal approach; TLC, transclival (lower clivus) approach; TMC, transclival (middle clivus) approach; TPT, transplanum–transtuberculum approach; TO, transodontoid approach; TS, transsellar approach.

**Fig. 1.9 Coronal view of the skull base and adjacent areas (part 3)**. Posterior-to-anterior view (upper image) and anterior-to-posterior view (lower image). III, oculomotor nerve; IX, glossopharyngeal nerve; X, vagus nerve; XI, spinal accessory nerve; XII, hypoglossal nerve; A2, postcommunicating tract of the anterior cerebral artery; AAOM, anterior atlanto-occipital membrane; AIPA, anterior inferior petrous apex; Ar, anterior arch of the atlas; ARCM, anterior rectus capitis muscle; ArD, articular disk (temporomandibular joint); ArtT, articular tuberculum; C2, axis (body); CS, cavernous sinus; EAC, external auditory canal; GG, gasserian ganglion; h, horizontal portion of the petrous tract of the internal carotid artery; Hyp, hypophysis; iICA, intracranial tract of the internal carotid artery; IJV, internal jugular vein; JF, jugular foramen; LMAt, lateral mass of the atlas; LMAx, lateral mass of the axis; LoC, lower clivus; MC, midclivus; MCA, middle cerebral artery; ME, middle ear; MeC, Meckel’s cave; OCh, optic chiasm; OP, odontoid process; OTr, optic tract; PCJ, petroclival junction; PCP, posterior clinoid process; peICA, petrous tract of the internal carotid artery; PG, parotid gland; phICA, parapharyngeal tract of the internal carotid artery; pICA, paraclival tract of the internal carotid artery; PLLi, petrolingual ligament; PSt, pituitary stalk; SpS, sphenoid sinus; StyP, styloid process; SuPA, superior petrous apex; TPAt, transverse process of the atlas; v, vertical portion of the petrous tract of the internal carotid artery; VA, vertebral artery.

**Fig. 1.10 Mapping of endoscopic transnasal approaches on a coronal section (part 3)**. Posterior-to-anterior view (upper image) and anterior-to-posterior view (lower image). IP, infrapetrous approach; LTC, lateral transcavernous approach; MPA, medial petrous apex approach; MTC, medial transcavernous approach; SP, suprapetrous approach; TCJ, transcondylar–transjugular tuberculum approach; TD, transdorsal approach; TLC, transclival (lower clivus) approach; TMC, transclival (midclivus) approach; TO, transodontoid approach; TPT, transplanum–transtuberculum approach; TS, transsellar approach.

**Fig. 1.11 Axial view of the skull base and adjacent areas (part 1).** Inferior-to-superior view (upper image) and superior-to-inferior view (lower image). VI, abducens nerve; A1, precommunicating tract of the anterior cerebral artery; ACP, anterior clinoid process; AE, anterior ethmoidal compartment; CGa, crista galli; ChoP, conchal plate; EFa, extraconal fat; Ey, eyeball; FaC, falx cerebri; FS, frontal sinus; GR, gyrus rectus; IFa, intraconal fat; iICA, intracranial tract of the internal carotid artery; LRM, lateral rectus muscle; MCA, middle cerebral artery; MOG, medial orbital gyrus; MRM, medial rectus muscle; NS, nasal septum; OCh, optic chiasm; OFi, olfactory fissure; ON, optic nerve; OpA, ophthalmic artery; OSt, optic strut; OTr, optic tract; PE, posterior ethmoidal compartment; PPFS, posterior plate of the frontal sinus; SON, supraorbital nerve; SOpV, superior ophthalmic vein; SpS, sphenoid sinus; STN, supratrochlear nerve.

**Fig. 1.12 Mapping of endoscopic transnasal approaches on an axial section (part 1)**. Inferior-to-superior view (upper image) and superior-to-inferior view (lower image). OD, orbital decompression; OpD, optic decompression; SO, supraorbital approach; TC, transcribriform approach; TF, transfrontal approach; TOr, transorbital approach; TPT, transplanum–transtuberculum approach.

**Fig. 1.13 Axial view of the skull base and adjacent areas (part 2)**. Superior-to-inferior view (both images). III, oculomotor nerve; V2, maxillary nerve; AE, anterior ethmoidal artery; AMW, anterior maxillary wall; ASIS, anterior superior intercavernous sinus; BA, basilar artery; BaP, basilar plexus; BP, base of the pterygoid process; CPr, carotid prominence; CS, cavernous sinus; EFa, extraconal fat; Ey, eyeball; GG, gasserian ganglion; GW, greater wing of the sphenoid sinus; ION, infraorbital nerve; IRM, inferior rectus muscle; IT, inferior turbinate; ITF, infratemporal fossa; LMW, lateral maxillary wall; LP, lamina papyracea; LR, lateral recess of the sphenoid bone; MC, midclivus; MeC, Meckel’s cave; Mes, mesencephalon; MMA, middle meningeal artery; MMW, medial maxillary wall; MS, maxillary sinus; MT, middle turbinate; NLD, nasolacrimal duct; NS, nasal septum; P2, postcommunicating tract of the posterior cerebral artery; PCJ, petroclival junction; PE, posterior ethmoidal compartment; pICA, paraclival tract of the internal carotid artery; Po, pons; PPF, pterygopalatine fossa; PSIS, posterior superior intercavernous sinus; PWMS, posterior wall of the maxillary sinus; SCA, superior cerebellar artery; SER, sphenoethmoidal recess; sICA, parasellar tract of the internal carotid artery; SOpV, superior ophthalmic vein; SPA, sphenopalatine artery; SPr, sellar prominence; SpS, sphenoid sinus; ST, superior turbinate; SuPA, superior petrous apex; Te, tentorium cerebri; VC, vidian canal; ZR, zygomatic recess of the maxillary sinus.

**Fig. 1.14 Mapping of endoscopic transnasal approaches on an axial section (part 2)**. Superior-to-inferior view (both images). ITF, infratemporal fossa approach; LTC, lateral transcavernous approach; MPA, medial petrous apex approach; MTC, medial transcavernous approach; OD, orbital decompression; SP, suprapetrous (Meckel’s cave) approach; TA, transalisphenoid approach; TC, transcribriform approach; TD, transdorsal approach; TMC, transclival (midclivus) approach; TOr, transorbital approach; TPM, transpterygomaxillary approach; TS, transsellar approach.

**Fig. 1.15 Axial view of the skull base and adjacent areas (part 3)**. Superior-to-inferior view (both images). V3, mandibular nerve; AIPA, anterior inferior petrous apex; AMW, anterior maxillary wall; BA, basilar artery; BaP, basilar plexus; BP, base of the pterygoid process; Co, cochlea; ConP, condylar process of the mandible; ET, eustachian tube; FoPl, foraminal plexus; GW, greater wing of the sphenoid bone; h, horizontal portion of the petrous tract of the internal carotid artery; IAC, internal auditory canal; IMA, internal maxillary artery; IPS, inferior petrosal sinus; IT, inferior turbinate; ITF, infratemporal fossa; JuT, jugular tuberculum; LoC, lower clivus; LMW, lateral maxillary wall; LPM, lateral pterygoid muscle; ME, middle ear; MMA, middle meningeal artery; MMW, medial maxillary wall; NaP, nasopharyngeal posterior wall; NS, nasal septum; PCJ, petroclival junction; peICA, petrous tract of the internal carotid artery; Po, pons; PPF, pterygopalatine fossa; PtPl, pterygoid plexus; PWMS, posterior wall of the maxillary sinus; TM, temporal muscle; v, vertical portion of the petrous tract of the internal carotid artery; ZR, zygomatic recess.

**Fig. 1.16 Mapping of endoscopic transnasal approaches on an axial section (part 3).** Superior-to-inferior view (both images). IP, infrapetrous approach; ITF, infratemporal fossa approach; LPPh, lateral parapharyngeal space approach; LTP, lower transpterygoid approach; MPPh, medial parapharyngeal space approach; TA, transalisphenoid approach; TCJ, transcondylar–transjugular tuberculum approach; TLC, transclival (lower clivus) approach; TPM, transpterygomaxillary approach; UTP, upper transpterygoid approach.

**Fig. 1.17 Axial view of the skull base and adjacent areas (part 4)**. Superior-to-inferior view (both images). V3, mandibular nerve; X, vagus nerve; X*, glossopharyngeal, vagus, and accessory nerve; XI, spinal accessory nerve; XII, hypoglossal nerve; AMW, anterior maxillary wall; APA, ascending pharyngeal artery; Ar, anterior arch of the atlas; AsPA, ascending palatine artery; CoP, coronoid process; EAC, external auditory canal; ECA, external carotid artery; ET, eustachian tube; IAN, inferior alveolar nerve; IMA, internal maxillary artery; IPS, inferior petrosal sinus; IT, inferior turbinate; ITF, infratemporal fossa; IJV, internal jugular vein; JuT, jugular tuberculum; LMAt, lateral mass of the atlas; LMW, lateral maxillary wall; LN, lingual nerve; LoC, lower clivus; LoCM, longus capitis muscle; LPM, lateral pterygoid muscle; LPP, lateral pterygoid plate; ME, middle ear; MM, masseteric muscle; MMA, middle meningeal artery; MMW, medial maxillary wall; MOb, medulla oblongata; MPM, medial pterygoid muscle; MPP, medial pterygoid plate; MS, maxillary sinus; NaF, nasal floor; NS, nasal septum; OP, odontoid process; PCV, petroclival vein; peICA, petrous tract of the internal carotid artery; PG, parotid gland; phICA, parapharyngeal tract of the internal carotid artery; PhPl, pharyngeal plexus; PICA, posterior inferior cerebellar artery; PIPA, posterior inferior petrous apex; PMJ, pterygomaxillary junction; PtPl, pterygoid plexus; SCo, spinal cord; SCS, suboccipital cavernous sinus; SoP, soft palate; StyP, styloid process; TM, temporal muscle; ToT, torus tubarius; v, vertical portion of the petrous tract of the internal carotid artery; VA, vertebral artery; ZR, zygomatic recess.

**Fig. 1.18 Mapping of endoscopic transnasal approaches on an axial section (part 4)**. Superior-to-inferior view (both images). ITF, infratemporal fossa approach; LPPh, lateral parapharyngeal space approach; MPPh, medial parapharyngeal space approach; TCJ, transcondylar–transjugular tuberculum approach; TLC, transclival (lower clivus) approach; TO, transodontoid approach; TPM, transpterygomaxillary approach.

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