Introduction

The petrous portion of the internal carotid artery (ICA) is a critical structure when managing skull base lesions placed in areas such as the petrous apex, the Meckel′s cave, and the superior aspect of the infratemporal fossa (ITF). The continued advancements of endoscopic surgical techniques and instrumentation have progressively expanded the indications for transnasal approaches over the past decade. As interest in these approaches grows, so do the risks. Injuring the ICA during skull base procedures is a devastating complication and has been reported by multiple authors.1–4 Therefore, a thorough understanding of the relationships between the petrous ICA and nearby anatomic landmarks, as well as a rigorous training method in approaching this region, is an imperative issue.

17.1 Anatomic Background

The petrous portion of the ICA is extradural and intraosseous. It extends from the carotid foramen to the superior margin of the petrolingual ligament (PLL). The carotid foramen is anteromedial to the styloid process and posteromedial to the temporomandibular joint.1 The PLL, placed between the sphenoidal lingula and the petrous apex, represents the posteromedial border of the cavernous sinus (CS). Once it enters in the petrous bone, the ICA presents a short vertical course, and then it turns in front of the cochlea, forming the posterior genu ( Fig. 17.1 ). From here, it runs horizontally within the temporal bone in an anteromedial direction, posterior to the tensor tympanic muscle, eustachian tube (ET), and the foramina spinosum and ovale. Within the carotid canal, the horizontal portion of the petrous ICA is surrounded by a venous plexus in direct continuity with the CS.1 The petrous ICA then curves upward above the foramen lacerum (FL), thus giving the anterior genu and therefore becoming vertical (paraclival segment of the ICA). At this level, the vidian canal is always lateral to the vertical segment ( Fig. 17.2 ).

17.2 Indications

Pathology will dictate the necessary exposure of the petrous ICA. A thorough understanding of the relationships of relevant landmarks is paramount to remove the lesion successfully with the most minimal bone and soft-tissue resection. Therefore, the surgical approaches related to the petrous ICA can be divided between the ones directed only to its medial aspect (anterior genu) and the ones requiring a complete exposure of the vessel from the anterior genu back to the posterior genu.

The surgical approaches directed to the medial portion of the petrous ICA are as follows:

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  The petrous apex approach (upper petroclival fissure) is directed medial to the paraclival ICA and posterosuperior to the petrous ICA. The guiding principle here is to identify the anterior genu between the paraclival and petrous ICA segments and unroof its canal completely from proximally to distally to avoid injury to the petrous segment of the sixth cranial nerve as it ascends behind the paraclival ICA to reach the uppermost edge of the petrous apex where it enters the sphenopetroclival gulf.2

  
  
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  The inferior petroclival approach is directed medial to the petrous ICA in the interval rostral to the jugular foramen along the inferior (horizontal) segment of the petroclival fissure. The venous structures related to this approach are the inferior petrosal sinus and the inferior petroclival vein. The inferior petroclival approach allows access to the posterior cranial fossa at the level of cranial nerves VII and VIII.

  
  
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  The petrous ICA constitutes the inferomedial boundary of the front door to Meckel′s cave approach. This approach is directed lateral to the paraclival ICA and below the V2 nerve, which can be followed posteriorly toward the gasserian ganglion.3

  
  
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  The inferior CS approach is performed laterally to the paraclival ICA and superiorly to the petrous ICA. Although practically very similar to the front door to Meckel′s cave approach, the target lesion for this approach is more medial and superior and by definition localized in the CS. Hence, special care must be taken while opening the dura and resecting the tumor to protect the abducens nerve.3

The surgical approaches requiring a complete exposure of the petrous ICA are generally directed below the vessel and extended laterally as far as the parapharyngeal ICA. The petrous ICA lies at the superomedial margin of these approaches. For this reason, the inferior aspect of the petrous ICA is completely skeletonized, from the anterior genu back to the posterior one, to safely identify the carotid foramen and therefore the parapharyngeal tract of the ICA.

The main surgical procedures entailing such an exposure of the petrous ICA are as follows:

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  Endonasal nasopharyngectomy extended laterally to include the ET (type III nasopharyngeal endoscopic resection).4

  
  
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  Endoscopic approaches to the ITF and upper parapharyngeal spaces (UPPS).5

17.3 Surgical Steps

17.3.2 Approach to the Medial Aspect of the Petrous ICA

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  The dissection begins with a partial middle turbinectomy and total ethmoidectomy. An ipsilateral sphenoidotomy with enlargement of the natural ostium is performed. The mucosa overlying the inferior margin of the sphenoidotomy is elevated to preserve the vascular pedicle of the ipsilateral nasoseptal flap (so-called “rescue flap”). Generally, bilateral sphenoidotomies are performed to provide more space for instrumentation and allow a four-hand surgery by a team consisting of an otolaryngologist and a neurosurgeon. For the same reason, the posterior one-third of the nasal septum is disarticulated from the rostrum and removed. A bilateral exposure allows improving the angle for visualization and instrumentation from the side opposite to the lesion. Anatomic landmarks within the sphenoid sinus are identified: planum sphenoidale, sella, clival recess, optic canal, medial and lateral opticocarotid recesses, and the carotid canal ( Fig. 17.3 ). Septations within the sphenoid sinus are removed. Aggressive instrumentation is avoided during removal of these septations to avoid injury to the ICA, and thus drilling is preferred.

  
  
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  The floor of the sphenoid is drilled out. Vidian canal and foramen rotundum are identified along the face of the pterygoids base. The vidian canal provides a good depth gauge for the position of the ICA as it transitions from the petrous to the paraclival segment at the level of the FL ( Fig. 17.4a ). In light of this, the vidian nerve (VN) and vidian artery are cauterized and cut, and the medial aspect of the pterygoids base is addressed as the vidian canal is completely drilled out. This provides exposure of the anterior genu of the ICA at the level of FL.1 The fibrocartilago basalis is removed as far as the superior aspect of the cartilaginous ET is encountered. The paraclival and petrous tracts of the ICA are skeletonized using a drill with diamond burr ( Fig. 17.4b ).

  
  
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  Once the vertical (paraclival) and horizontal (petrous) segments of the ICA are visualized, the drilling may proceed inferiorly and posteriorly to the petrous carotid into the petrous apex ( Fig. 17.5 ) or it may be carried on above the petrous carotid toward the Meckel′s cave and the inferior portion of the CS.3

Related posts:

Section 4 Middle Cranial Fossa Chapter 16 The Anteromedial Corridor via the Expanded Endonasal Approach: The “Front Door to Meckel′s Cave” Chapter 6 Medial Maxillectomy Chapter 15 Transorbital Neuroendoscopic Approach Chapter 30 Anterior Transpetrosal Approach versus EEA Transclival Approach Chapter 28 Retrosigmoid–Transclival Approach

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