Posterolateral Approach

This is a lateral extension of the standard midline approach ( Figs. 41.5, 41.6, 41.7, and 41.8 ).2,3,5,9,23,24 It is best to start from the midline, where the anatomy is familiar. The patient’s position may be sitting, prone, or lateral, depending on the surgeon’s preference. Skin incision starts in the midline, extending vertically up from the C4–C5 level to the occipital protuberance, then curving laterally along the superior occipital line toward the mastoid process. The posterior muscles are cut together with the skin, leaving a cuffof tissue for further reattachment. The muscles are split from the bone to expose the posterior fossa and generally the posterior arch of the atlas and the lamina of C2.

Bone must be split subperiosteally, especially at the level of the arch of the atlas. Exposure progresses from medial to lateral, then from inferior to superior. The lamina of C2 is exposed up to the joint of C1–C2, the posterior arch of the atlas up to the transverse foramen. Between the arch of the atlas and the lamina of C2, the C2 nerve root is identified with venous plexuses on both sides.

Elevating the periosteum from the inferior edge of the arch of the atlas to the superior one allows the surgeon to free the groove and mobilize the vertebral artery out of it ( Fig. 41.6 ). In fact, this technique preserves the periosteal sheath surrounding the vertebral artery and therefore avoids troublesome venous bleeding. The entire length of the vertebral artery groove can be exposed, thus allowing bone resection up to the C0–C1 joint (occipital condyle–lateral mass of the atlas) ( Fig. 41.7 ). For resection of the occipital bone above the vertebral artery toward the occipital condyle, the superior aspect of the vertebral artery needs to be controlled. There is no landmark at this level, as the occipitoatlantal ligament continues with the periosteal sheath of the vertebral artery. Moreover, this ligament is sometimes calcified or even ossified. The bone opening thus obtained, without any drilling of the lateral mass of the atlas or of the occipital condyle, allows the surgeon to reach the intradural space anterior to the neuraxis.

The surgical approach is then decided according to tumor location, with consideration of essentially two parameters: whether the lesion is above or below the vertebral artery, and whether it is anterior or lateral to the neuraxis. The bony resection is extended above (toward the occipital condyle) or below (toward the lateral mass of the atlas), depending on the location of the tumor with respect to the vertebral artery. The difference between anterior and lateral tumors is the axis of displacement of the neuraxis. Lateral tumors displace the neuraxis laterally and therefore enlarge the access. Consequently, bone resection does not need to be extended at its maximum. In contrast, anterior tumors develop in front of the neur-axis, which is displaced straight posteriorly. Therefore, the space lateral to the neuraxis is not enlarged, and bone has to be drilled up to the C0–C1 joint. However, drilling of the occipital condyle must be exceptional and very limited.

Dura is opened by a curvilinear incision sometimes with a counter incision toward the vertebral artery ( Fig. 41.7d ), so that the space between the neuraxis and the dura is maximally opened.

In any case, the bone opening must be extended toward the opposite side beyond the midline so that the neuraxis cannot be compressed against the bone during tumor manipulation. It is also important to release the neuraxis as soon as possible after the bony and dural opening to give more space to the neuraxis. This is achieved by dividing the two first arches of the denticulate ligament and the first (sometimes also the second) cervical nerve root. The first arch gives passage to the vertebral artery and the first cervical nerve root. The nerve roots must be cut lateral to their connection with the accessory nerve. Indeed, sometimes proximal segments of the first and second cervical nerve roots are the main components of the accessory nerve.

In cases where the tumor is below the vertebral artery, the lower cranial nerves are pushed cranially and posteri-orly by the tumor. These nerves will be found at the superior pole of the lesion at the end of surgery. The resection must start at the caudal aspect of the meningiomas, with the goal of releasing the dural attachment and controlling the vascular supply first; then the tumor is debulked in a dry surgical field with a sucker, an ultrasound aspirator, or a laser, according to the tumor consistency. When freeing the dural insertion, it is important to keep a small part of the base, at the side of the neuraxis, undetached, to avoid free movement of the lesion, which can cause inadvertent damage to the neuraxis during the remnant resection. When hollowing the tumor, a small layer is also kept with its capsule against the neuraxis. This part will be resected as the last surgical step and under better conditions when the meningioma is completely devascularized and the surgical field is widely open.

If the tumor is above the vertebral artery, two special points must be taken into consideration: the displacement of the lower cranial nerves and the dissection of the vertebral artery branches. Indeed, in this location, the displacement of the lower cranial nerves cannot be anticipated. To prevent damage, the rootlets must be under control on the side of the jugular foramen, then followed along their courses more or less adherent to the meningioma. With the lesion being progressively debulked, the nerve rootlets can be more easily mobilized, often inferiorly, to allow a more confident tumor resection at some distance from fragile nerve structures. The tumor dissection from the vertebral artery branches, especially the PICA, is another difficulty encountered with tumors located above the vertebral artery. Precise knowledge of the anatomy based on preoperative investigation is mandatory.

If the meningioma encases the vertebral artery, the technique is as described above. Special consideration is nevertheless required if the meningioma has its insertion on the dura surrounding the vertebral artery penetration. The dural resection is better achieved by progressing from the extradural side toward the intradural aspect, along the vertebral artery, because the vertebral artery invaginates into the dura with its periosteal sheath. This furrow can be resected as long as the vertebral artery adventitia is not invaded.

A watertight dural closure is required to prevent postoperative cerebrospinal fluid (CSF) leakage. The closure is generally easy with a curvilinear incision. If necessary, a dural patch using the suboccipital aponeurosis achieves a perfect closure. In any case, the muscular and aponeurotic layers must be tightly closed.

Anterolateral Approach

This is the standard technique used to expose the vertebral artery all along the neck but applied on the suboccipital (V₃) segment ( Figs. 41.9, 41.10, 41.11, 41.12, and 41.13 ). The general principles are the same whatever the segment but with some modifications in relation to the anatomy at this level.3,5

The patient is placed in the supine position with the head extended and more or less rotated toward the opposite side ( Fig. 41.9 ). It must be kept in mind that the more the head is rotated, the more of the anterior arch of the atlas is projected away and the more the posterior arch is brought into view. Therefore, rotation must be chosen according to the part of the CVJ that has to be reached.

Skin incision follows the medial edge of the stern-mastoid muscle for 10 cm, its upper part going up to the tip of the mastoid process and then along the mastoid process and the superior occipital crest toward the inion. The sternomastoid and posterior cervical muscles are detached from the occipital bone and mastoid process. The digastric muscle is separated from its groove and reflected anteriorly. Next, the internal jugular vein is exposed at the C3 level, and the space between the internal jugular vein and the sternomastoid is opened. This space is filled with fat and lymphatic elements. The accessory nerve (CN XI) must be identified and dissected free from its junction with the sternomastoid up to its crossing with the internal jugular vein and to the skull base. For easier identification, the patient must not receive any curare-like drugs so that working close to the nerve does not induce contraction of the sternomastoid. The fad pad surrounding the nerve is freed from the deep plane of muscles and rolled around the nerve for protection. A stitch is used to retract this fat pad as well as the nerve, both inferiorly and medially ( Fig. 41.10 ).

Next, the small muscles inserted on the transverse process of the atlas are divided flush with the tip of this process. This transverse process is easily recognized, as it is projected anteriorly by rotation of the head. It can be felt with the finger 10 to 15 mm anterior and inferior to the tip of the mastoid process. Dividing all the muscles attached on the transverse process of the atlas brings into view the two segments of the vertebral artery (the C1–C2 segment and the part above C1) on both sides of the posterior arch of the atlas. Care must be taken to keep the periosteal sheath surrounding the vertebral artery intact to avoid any troublesome venous bleeding ( Fig. 41.11 ).

Still working subperiosteally, the transverse foramen of the atlas can be unroofed with resection of the trans-verse process. This permits mobilization and, whenever necessary, transposition of the vertebral artery out of the transverse foramen. When the vertebral artery is exposed from C2 to the end of the groove in the posterior arch of the atlas, any part of the CVJ on one side can be reached. The surgeon can work on the posterior part or move in front of the anterior arch of the atlas in the retropharyngeal space or work through the bone elements ( Fig. 41.12 ). In fact, when instability already exists due to tumoral invasion or inflammatory process destroying the ligaments, drilling the lateral wall of the CVJ does not worsen the patient’s status and is an acceptable technique. Drilling the occipital condyle gives access to the tip of the clivus; drilling the lateral mass of the atlas gives access to the odontoid process and the contralateral mass of the atlas;25 and drilling the jugular tubercle leads to the jugular foramen ( Figs. 41.13 and 41.14 ).8,26 The strategy must be adjusted according to the location and extent of tumor. The shortest way to reach a tumor is often the best, as it does not destroy important structures. As a rule, CVJ stability may be compromised by tumor development but not by surgical work.

Through the anterolateral approach, the surgeon can follow intradural extensions of a tumor which has its main bulk in the bone and the extradural space. However, this approach is certainly not well designed for intradural lesions.