Chordomas and Chondrosarcomas of the Skull Base: Transpetrosal Approaches

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Indications and Preoperative Planning

Chordomas and chondrosarcomas, while distinct entities, are both slowly growing and locally aggressive bony neoplasms that can develop in similar regions including the skull base. Treatment for both tumor types can involve surgical resection with postoperative adjuvant radiation. Since extensive resection improves long-term outcomes, planning out the most appropriate surgical approach is a key to optimal treatment. Most operations for these lesions involve an anterior approach. Under specific circumstances, posterolateral (transpetrous) approaches may improve the extent of resection.

Skull base lesions present particular access challenges because of their location and relationship with adjacent neural and cerebrovascular structures. Important features include proximity or attachment to significant arteries, cranial nerves, and the brainstem (and the thickness of tumor compressing it), the presence of these structures along the path of surgical access, and the required degree of brain retraction (particularly of the temporal lobe). Typical posterolateral approaches, such as the retrosigmoid and suboccipital approaches, have limited access to the anterior midline and position the tumor on the other side of the cerebellum, brainstem, and cranial nerves from the surgeon’s perspective. Multiple approaches can be combined in single or multiple stages in an effort to safely resect more of the tumor. Careful consideration of preoperative CT and MR imaging, which are complementary in treatment planning, as well as angiography (conventional, CT, or MRI) should be undertaken by a team including neurosurgery, rhinology, neurotology, and radiology to determine the optimal approach(es) to maximize tumor resection, while minimizing morbidity and mortality.

Skull base chordomas usually occur in the midline clivus and are typically approached anteriorly, for which endoscopic endonasal techniques have gradually supplanted microsurgical anterior approaches. Posterolateral approaches are indicated primarily when tumor extension posterolateral to the intrapetrous carotid artery blocks the surgical view from the anterior perspective ( Fig. 17.1 ). A second indication is extensive spread into the posterior fossa, especially accompanied by intradural extension. A more lateral perspective affords direct visualization of the tumor–brainstem interface and the critical vertebrobasilar system. When tumors extend intradurally, anterior approaches through nasal mucosa carry a higher risk of meningitis and controlling cerebrospinal fluid (CSF) leaks becomes more difficult. The third indication for a posterolateral approach is tumor penetration of the jugular foramen. They afford proximal and distal control of the sigmoid–jugular complex and visibility of the tumor interface with the lower cranial nerves. Finally, transpetrous approaches are sometimes necessitated when the tumor has extended laterally within the skull base to involve the otic structures.

Chondrosarcomas of the skull base typically arise at the petroclival junction from fibrocartilage of the foramen lacerum. Again, a posterolateral approach is indicated when retrocarotid disease is not readily accessible via an anterior approach. A posterolateral approach is more common in chondrosarcomas than with chordomas given the typical paramedian location of chondrosarcomas ( Fig. 17.2 ).

The posterolateral approaches to skull base chordomas and chondrosarcomas include: (1) transpetrosal (retrolabyrinthine, translabyrinthine, or transcochlear) ( Fig. 17.3 ), (2) transjugular, (3) far lateral approach to foramen magnum, and (4) combined approaches. As foramen magnum approaches are considered in Chapter 19 , our focus will be on transpetrosal, transjugular, and combined approaches. The combined transpetrosal approaches are: (1) middle fossa and transpetrosal , (2) combined transpetrosal and far lateral approach to foramen magnum , and (3) subtemporal transpetrous apex . A transcochlear or transjugular approach with facial nerve rerouting increases the risk of facial nerve palsy or paresis, an important morbidity to weigh against access; if access is sufficient with a fallopian bridge, the risk of facial weakness can be reduced. Posterolateral approaches can allow for more complete removal of tumor adjacent to the brainstem, thus reducing the postoperative radiation dose to the brainstem. Combined approaches may also allow further tumor extirpation with less morbidity than with either a limited single approach or staged surgeries. The access of single approaches such as the translabyrinthine or transcochlear that leads to ipsilateral anacusis must be weighed against an approach like the retrolabyrinthine approach that can be combined for additional access with a subtemporal approach while still allowing hearing preservation.

Intraoperative image guidance is particularly helpful for skull base chordomas and chondrosarcomas as resection may need to be performed in deep and narrow fields, surrounded by vital structures, and with few orienting landmarks. Intraoperative imaging, although not widely available, is also useful to establish anatomical localization and to assess the extent of resection. Intraoperative cranial nerve monitoring assists in identifying nerves in distorted fields and may help limit cranial neuropathies. With incomplete resection, or high-grade tumors, postoperative radiotherapy is often indicated.

Surgical Techniques Common to All Transpetrosal Approaches

In transpetrosal skull base cases, the patient is generally positioned in supine position with a shoulder bump ipsilateral to the side of the incision and a Mayfield head holder for stabilization. A malleable retractor system is used and, as hemorrhage is often encountered during these procedures, bone wax, Flowseal, Surgicel, thrombin-soaked Gelfoam and ¼ and ½ cottonoids are prepared for hemostasis. When conducting a transpetrosal approach, the sigmoid sinus and jugular bulb are exposed and then can be protected with a strip of Telfa or other suitable material.

After resection of tumor, the defect is narrowed as much as possible by apposition of remaining dura. When a large defect remains, it is usually repaired with a free fat graft harvested from the left lower abdominal quadrant, below the umbilicus or below the iliac crest. The fat is cut into strips and laid within the bony defect to the level of the dura. Connections to the middle ear on the floor of the mastoid cavity are sealed with temporalis fascia. When the ear canal and middle ear have been removed as part of the procedure, the external auditory meatus is closed and the Eustachian tube is obliterated with bone wax.

Transpetrosal (Retrolabyrinthine, Translabyrinthine, Transcochlear)

Retrolabyrinthine

These approaches can be used alone or combined with others for additional exposure. The incision starts posterior-inferior to the mastoid tip and extends in curvilinear fashion postauricularly until superior to the external auditory canal. The incision is carried directly down to bone and periosteum, and the flap is elevated subperiosteally and retracted forward. A standard cortical mastoidectomy is performed with a large cutting bur and then 1–2 cm of retrosigmoid dura and the sigmoid sinus are skeletonized with a large diamond bur. The thin eggshell of bone can be removed gently with an elevator. After decompression of the sigmoid sinus, the antrum is opened and the lateral semicircular canal is identified. The fossa incudis should not be widely opened as it is a potential route for cerebrospinal fluid otorhinorrhea. Next, the remaining bone over the posterior fossa dura between the sigmoid sinus and the labyrinth is removed along with the tegmen to expose the middle fossa dura. The endolymphatic sac is left intact. For additional exposure, the facial nerve can be found at the second genu and skeletonized down toward the stylomastoid foramen to allow the remainder of the sigmoid sinus to be exposed to the level of the jugular bulb. All three semicircular canals should be visualized but not entered. If one is accidently entered, the lumen should be occluded immediately with bone wax. The dural flap is designed to preserve the endolymphatic sac and its aqueduct ( Fig. 17.4 ). This opening alone offers limited exposure, but combined with the middle fossa subtemporal craniotomy, it affords much greater exposure for clival lesions while still preserving hearing.

Translabyrinthine

The translabyrinthine approach provides more access than the retrolabyrinthine approach at the cost of hearing. The approach is the same as the retrolabyrinthine until the semicircular canals have been reached. Removal of the semicircular canals begins at the sinodural angle using a cutting bur followed by drilling of deep troughs parallel to the posterior and middle fossa. The labyrinthectomy is completed with a diamond bur and the facial nerve is identified at the second genu. The vestibule is then opened widely. The endolymphatic duct is divided at the operculum to allow further elevation of posterior fossa dura from the petrous bone ( Fig. 17.5 ).

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