Davide Lancini, Marco Ravanelli, Marco Ferrari, Alberto Schreiber
According to the topographical classification of the petrous apex into three segments (superior, anteroinferior, and posteroinferior), the transnasal infrapetrous pathway leads to the anteroinferior portion.1A general overview on endoscopic transnasal approaches to the petrous apex is reported in Chapter 19 together with details on the topography of this skull base area.
The infrapetrous route exploits the space below the horizontal tract of the petrous internal carotid artery to expose the anteroinferior portion of the petrous apex and adjacent structures. This narrow pathway is laterally bounded by the lateral pterygoid plate, the mandibular nerve, and the parapharyngeal internal carotid artery and crosses the area of the basipterygoid and the foramen lacerum, which is composed of a dense tissue called fibrocartilago basalis. Depending on the need for exposure and freedom of movement, the corridor can be harvested by either removing (as explained in Chapters 23 and 24) or displacing the eustachian tube inferiorly.2This chapter includes the description of the infrapetrous approach both as an extension of the medial petrous apex (described in Chapter 19) or the medial parapharyngeal approach (described in Chapter 23) and as independent route to the petrous apex with eustachian tube transposition via the lower transpterygoid approach. The trajectory of the infrapetrous approach leads to the area of the hypoglossal canal and neighboring neurovascular structures, serving as midway corridor between the transcondylar/transjugular tuberculum “far medial” approach (described in Chapter 22) medially, the lateral parapharyngeal approach (described in Chapter 24) laterally, and the suprapetrous approach to Meckel’s cave (described in Chapter 21) superiorly.
The infrapetrous route has been employed either alone or in combination with other transnasal corridors to manage lesions of the inferior portion of petroclival area, mostly cholesterol granulomas3or extradural tumors such as chordomas and chondrosarcomas.4–6As mentioned in the previous chapter, the role of transnasal endoscopic routes toward the petrous apex should be seen in the wide context of other corridors, including classical transcranial and newly emerging transorbital and contralateral transmaxillary approaches.7–11
Fig. 20.1 Subtemporal lateral-to-medial view of the transnasal route toward the anteroinferior petrous apex. This cadaver picture shows with a lateral-to-medial epidural perspective left middle cranial fossa and carotid canal. V2, maxillary nerve; V3, mandibular nerve; VII, facial nerve (tympanic tract); AIPA, anteroinferior petrous apex; ET, eustachian tube; GSPN, greater superficial petrosal nerve; IJV, internal jugular vein; JuB, jugular bulb; Lab, labyrinth; Na, nasopharynx; peICA, petrous tract of the internal carotid artery; SiS, sigmoid sinus; SpS, sphenoid sinus; VN, vidian nerve.Fig. 20.2 Intracranial superolateral-to-inferomedial view of the petroclival area. This cadaver picture shows with a superolateral-to-inferomedial intracranial perspective the left petroclival area. The oculomotor (III), trochlear (IV), abducens (VI), ophthalmic (V1), maxillary (V2), and mandibular nerve (V3) have been displaced along with the gasserian ganglion (GG), trigeminal stem (V), and internal carotid artery. AIPA, anteroinferior petrous apex; BaP, basilar plexus; DPN, deep petrosal nerve; DSe, dorsum sellae; FCB, fibrocartilago basalis; GSPN, greater superficial petrosal nerve; Hyp, hypophysis; LSPN, lesser superficial petrosal nerve; ON, optic nerve; PCJ, petroclival junction; peICA, petrous tract of the internal carotid artery; SuPA, superior petrous apex; VN, vidian nerve.Fig. 20.3 (a–h) Axial CT and MRI anatomy of the foramen lacerum and the petrous apex. The panel includes four CT and four contrast-enhanced T1-weighted MRI axial images passing through the vidian nerves cranially (upper images) and the jugular foramen (lower images) caudally. The anteroinferior petrous apex (AIPA) lies posterior to the horizontal portion (h) of the petrous tract of the internal carotid artery (peICA) and is enclosed between the midclivus (MC), medially, and the internal acoustic meatus and inner ear, laterally. The basilar plexus (BP) and the abducens nerve (VI) are located in a medial position with respect to the anteroinferior petrous apex, which is adjacent to several venous vessels including the petroclival vein (PCV), anteriorly, and the inferior petrosal sinus (IPS), posteriorly. Anteromedially to the petrous apex, the foramen lacerum (FL), which is filled by the fibrocartilago basalis (FCB), serves as a bed for the anterior genu of the internal carotid artery, namely, the passage between the petrous and paraclival tracts of the vessel. V3, mandibular nerve; BA, basilar artery; BaP, basipterygoid; bET, bony portion of the eustachian tube; cET, cartilaginous portion of the Eustachian tube; FOv, foramen ovale; FoPl, foraminal plexus; FSp, foramen spinosum; JuB, jugular bulb; JuT, jugular tubercle; LoC, lower clivus; LPP, lateral pterygoid plate; MMA, middle meningeal artery; MPP, medial pterygoid plate; nJuF, nervous compartment of the jugular foramen; PCJ, petroclival junction; phICA, parapharyngeal tract of the internal carotid artery; PPF, pterygopalatine fossa; PVC, palatovaginal canal; RoF, Rosemüller’s fossa; SSp, spina sphenoidalis; TuL, tubal lumen; v, vertical portion of the petrous tract of the internal carotid artery; VA, vertebral artery; VC, vidian canal; vJuF, vascular compartment of the jugular foramen.Fig. 20.4 (a–c) Sagittal and paracoronal anatomy of the petrous apex. The panel includes two sagittal contrast-enhanced T1-weighted images (a, b) and one paracoronal contrast-enhanced CT image (c) passing through the petrous apex. The orientation of the CT image is parallel to the petrous tract of the internal carotid artery. The horizontal tract (h) of the petrous segment of the internal carotid artery (peICA) lies posterior to the mandibular nerve (V3) and the cartilaginous portion of the eustachian tube (cET) and anterior to the anteroinferior petrous apex (AIPA), whereas the vertical tract (v) is located posterior to the middle meningeal artery (MMA) and the bony portion of the tube (bET) and anterior to the posteroinferior petrous apex (PIPA). The internal acoustic meatus (IAC) passes nearby both the anteroinferior and the posteroinferior portions of the petrous apex with a horizontal path, which is orthogonally cut in sagittal images. As seen in the paracoronal CT image, the anteroinferior petrous apex lies posterolaterally to the foramen lacerum (FL). IJV, internal jugular vein; JuB, jugular bulb; phICA, parapharyngeal tract of the internal carotid artery; pICA, paraclival tract of the internal carotid artery; sICA, parasellar tract of the internal carotid artery; SpS, sphenoid sinus; SuPA, superior portion of the petrous apex; ToT, torus tubarius.Fig. 20.5 (a–f) Sagittal MRI anatomy of the petrous apex. The panel contains six sagittal CISS (constructive interference in steady state) MRI images passing through the medial portion of the petrous apex, from lateral (a) to medial (f). The anteroinferior petrous apex (AIPA) can be exposed by passing through the pterygopalatine fossa (PPF) and the basipterygoid (BaP), below the petrous tract of the internal carotid artery (peICA). The inferior petrosal sinus (IPS) and the hypoglossal canal (HyC) are, respectively, located posterior and posteroinferior to the anteroinferior petrous apex. V2, maxillary nerve; VI, abducens nerve; XII, hypoglossal nerve; BP, basilar plexus; ET, eustachian tube; FRo, foramen rotundum; GG, gasserian ganglion; LoC, lower clivus; MeC, Meckel’s cave; PICA, posterior inferior cerebellar artery; pICA, paraclival tract of the internal carotid artery; SpS, sphenoid sinus; VA, vertebral artery; VN, vidian nerve.
Endoscopic Dissection
Nasal Phase
Anterior and posterior ethmoidectomy.
Transethmoidal sphenoidotomy.
Removal of the sphenoid sinus floor (only for the infrapetrous extension of the medial parapharyngeal and medial petrous apex approach).
Type B, C, or D endoscopic medial maxillectomy.
Skull Base Phase
Infrapetrous Extension of the Medial Parapharyngeal and Medial Petrous Apex Approach
Medial parapharyngeal approach.
Facultative: Medial petrous apex approach.
Step 1: Exposure of the anteroinferior petrous apex.
Step 2: Drilling of the anteroinferior petrous apex.
Step 3: Drilling of the petroclival junction.
Infrapetrous Approach with Eustachian Tube Transposition
Transpterygomaxillary approach.
Step 1: Removal of the medial pterygoid plate.
Step 2: Exposure of the fibrocartilago basalis.
Step 3: Incision of the nasopharynx.
Step 4: Inferior transposition of the eustachian tube.
Step 5: Dissection of the fibrocartilago basalis.
Step 6: Removal of the fibrocartilago basalis.
Step 7: Posteroinferior septectomy and drilling of the lower clivus.
Step 8: Removal of the cortical bone of the lower clivus.
Step 9: Incision of the dura of the lower clivus.
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