Chapter 22 The “Far Medial” (Transcondylar/Transtubercular) Approach to the Inferior Third of the Clivus
Introduction
The expanded endonasal approach (EEA) has been described as a minimally invasive approach to access lesions of the anterior portion of the foramen magnum and inferior one-third of the clivus.1,2 These lesions can include clival tumors such as chordomas and chondrosarcomas, and tumors in the anterior foramen magnum (usually meningiomas). In addition, access via EEA to the odontoid process (for lesions such as basilar invagination, rheumatoid pannus, odontoid fractures, and tumors) has been a tremendously successful approach.3,4 One of the main benefits of EEA in this setting is that the lateral or posterolateral open approaches (e.g., far lateral, retrolabyrinthine presigmoid) require manipulation of the lower cranial nerves or the internal carotid artery (ICA) to gain access to the lesion; the direct medial-to-lateral approach of EEA obviates the need for this manipulation of critical neurovascular structures and avoids crossing the plane of the respective cranial nerves that are located posterolateral.
One significant limitation of the standard EEA approach to the inferior third of the clivus is in the presence of lateral extension of the tumor along the occipital condyle beyond the hypoglossal canal. However, if the lesion is located medial to the hypoglossal canal, then the “far medial” approach affords access to these more medially placed tumors at the pontomedullary and cervicomedullary junction.5,6 Anatomically, the lateral inferior clivus is divided into two compartments by the hypoglossal canal: (1) superiorly, the jugular tubercle, and (2) inferiorly, the occipital condyle. The transtubercular approach can be used to gain access to the jugular foramen, to biopsy or resect lesions such as schwannomas, paragangliomas, and meningiomas. Resection of the occipital condyle is more limited, as significant resection of the condyle can result in spinal instability. As a general rule, if one respects the plane of the hypoglossal canal, that is, does not cross this as the lateral limit, then it is much less likely to create instability as the majority of the condyle and atlanto-occipital (AO) joint capsule will be preserved. In addition, tumors in the hypoglossal canal proper can be biopsied or excised through this “far medial” approach; in some cases, the etiology of these lesions may be unknown until they are exposed through EEA. Lesions such as schwannomas, hemangiomas, extramedullary plasmacytomas, and lymphomas may occur within the hypoglossal canal, requiring exposure for tissue diagnosis.
22.1 Surgical Technique
22.1.1 Step 1: Standard Exposure of the Nasopharynx
The transtubercular approach begins with the removal of the ipsilateral inferior turbinate. Typically, in endonasal skull base surgery, the right middle turbinate is removed to allow room for the endoscope, but in cases involving the nasopharynx or clivus, the inferior turbinate is removed instead. A nasoseptal flap, if desired, is raised on the side contralateral to the tumor. A posterior septectomy is performed. The nasopharynx, torus tubarius, and Eustachian tube (ET) orifice are then visualized.
22.1.2 Step 2: Division of the Ipsilateral Eustachian Tube
The next maneuver is to enter the ipsilateral parapharyngeal space and to mobilize the ET orifice medially (it will be resected en bloc with the nasopharyngeal mucosa). The endoscopic scissors are inserted into the mucosa just lateral to the ET orifice, and oriented vertically at first and spread to dissect the parapharyngeal space and creating a window between the carotid and ET. A cut in this vertical direction at the level of the foramen lacerum would lacerate the petrous carotid artery. Therefore, the scissors are turned from a vertical to a horizontal plane, making them parallel to the plane of the horizontal carotid, and only then is the ET transected. This rotation from a vertical to horizontal cannot be overemphasized to protect the ICA ( Fig. 22.1 ).
22.1.3 Step 3: Nasopharyngectomy/Resection of the Basopharyngeal Fascia
Next, a nasopharyngectomy is performed. The mucosa at the superior aspect of the nasopharynx is transected with electrocautery, and the mucosa of the nasopharynx is removed. The nasopharyngeal mucosa is extremely adherent to the underlying muscle, especially in the midline. As such, it is difficult to remove and a variety of strategies can be employed to remove this mucosa, including the use of cautery, Kerrison forceps, and/or the microdebrider. Once the nasopharyngeal mucosa is removed, the longus capitis muscle is exposed.
At this point, the basopharyngeal fascia needs to be divided. A wide sphenoidotomy is performed, and the palatovaginal canal is identified. The basopharyngeal fascia is continuous with the palatovaginal canal, and its attachment there is the most adherent. This attachment is divided with a Colorado tip electrocautery. It is critical to observe here that the basopharyngeal fascia is also continuous with the cartilaginous ring around the carotid artery at the foramen lacerum, so division of this fascia requires attention to avoid injuring the artery. The division of the basopharyngeal fascia exposes the carotid artery and foramen lacerum ( Fig. 22.2 ).
22.1.4 Step 4: Completion of Resection of the Vomer
The next step is to complete the removal of the vomer. The caudal aspect of the vomer has been removed with the posterior septectomy, and the cranial aspect can now be removed en bloc by passing the drill through the palatovaginal canal bilaterally, then drilling inferior and superior to the vomer, and fracturing it off laterally. The removal of the vomer further exposes the longus capitis, which is removed in a fashion similar to that of the nasopharyngeal mucosa removal, with a variety of instruments including cautery, Kerrison forceps, and the microdebrider ( Fig. 22.3 ).
22.1.5 Step 5: Identification of the Ipsilateral Medial Pterygoid Plate
The identification of the medial pterygoid is the next step. An endoscopic medial maxillectomy is performed, the extent of which depends on the nature of the lesion being exposed (more lateral extension requires more caudal resection of the lateral nasal wall). The mucosa over the medial pterygoid plate is then stripped away; the inferior turbinate artery will often bleed and require cautery during this step. The descending palatine and greater palatine arteries are identified at this time and mobilized laterally. The medial pterygoid plate, once exposed, is removed with high-speed drill; this step is critical to achieve lateral visualization ( Fig. 22.4 ). The removal of the medial pterygoid allows identification of the medial pterygoid muscle.
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