The Far-Lateral Approach and Its Variations

10.1055/b-0034-84458

The Far-Lateral Approach and Its Variations

Nicholas C. Bambakidis, Cliff A. Megerian, and Robert F. Spetzler

A wide variety of surgical approaches has been described for lesions of the clivus, foramen magnum, and the inferior pontomedullary surface of the brainstem. Surgeons must balance the need to visualize the lesion and its surrounding eloquent structures adequately to ensure safe resection against the potential complications each approach entails. Exposure of the skull base in the posterior fossa is gene rally classified as a posterior or lateral approach ( Fig. 27.1A ). When viewed in the sagittal plane, the options include supratentorial and infratentorial directions ( Fig. 27.1B ). In terms of accessing the anterolateral craniovertebral junction (CVJ), the far-lateral approach1,2 is the workhorse. Variations such as extreme lateral approaches3 or the addition of other approaches can then be utilized to increase visualization as needed.

Schematic representation of the surgical approaches to the skull base in the **(A)** axial and **(B)** sagittal planes. These can be broadly classified as posterior or lateral and supra- or infratentorial. (Reprinted with permission from Barrow Neurological Institute.)

Both the far-lateral transcondylar approach, as described by Heros4 and modified by Spetzler,1,2 and the extreme lateral transcondylar approach, as described by Sen and Sekhar,3 provide excellent surgical access to the lower third of the clivus, the pontomedullary junction, and the anterolateral foramen magnum. However, the far-lateral approach has several advantages over the extreme lateral approach. Although both approaches afford a wide lateral approach to the brainstem, the far-lateral approach requires significantly less bone dissection and mobilization of the vertebral artery. Therefore, it requires less time and entails a somewhat lower risk of vascular injury. Although both approaches require expertise with skull base techniques, the far-lateral approach is significantly faster to perform. In our experience, the far-lateral approach also carries a lower risk of cerebrospinal fluid (CSF) leak. Both techniques may be combined with additional cervical or cranial exposure to augment exposure of the petroclival and CVJ regions.2,5 Because it entails significantly less bone and muscle dissection, the far-lateral approach is also less likely to create instability at the CVJ. Hence, unlike the extreme lateral approach, the far-lateral approach rarely requires subsequent craniovertebral fusion.

The most common indications for the far-lateral approach to the inferior clivus and anterolateral brainstem are neo-plastic and vascular lesions. These include aneurysms of the posterior inferior cerebellar artery (PICA) and the confluence of the vertebral artery, cavernous vascular malformations of the brainstem, and tumors anterior to the lower pons and medulla. Meningiomas of the anterior foramen magnum, schwannomas of the lower cranial nerves, and occasionally intramedullary tumors at the CVJ are typical indications.

The far-lateral approach is an extremely versatile skull base technique that may be combined with subtemporal, petrous, and cervical approaches for enhanced exposure of the CVJ.2 To maximize the exposure and minimize brain retraction, these approaches can be added individually or in combination, depending on the location and size of the lesion.

Operative Technique

Patient Positioning

Perhaps to an even greater degree than in most cranial base exposures, positioning is critical to the success of the far-lateral approach. Difficulties with angle of approach, venous congestion, and inadequate exposure can be avoided with careful attention to positioning. After intubation and induction of anesthesia, the patient is placed in a modified park bench position. The head is secured with a Mayfield head holder, following three cardinal movements for optimal cranial positioning. First, the cervical spine is flexed in the anteroposterior plane, allowing at least two finger-breadths between the patient′s chin and sternum. Next, the head is rotated 45 degrees to the contralateral side, leaving the mastoid process as the highest point in the operative field. Finally, the cervical spine is flexed laterally 30 degrees toward the opposite shoulder, resulting in the cranial vertex pointing somewhat below the horizontal plane ( Fig. 27.2 ). The head in this position places the cranio tomy sufficiently inferior to provide generous exposure of the foramen magnum and provides an operating angle of approach that allows a surgical line of sight anterior to the brainstem and directly “up the clivus,” which in this position stands almost vertical and inverted, perpendicular to the horizontal plane.1 If the operative indication requires a combined subtemporal or petrosal exposure, the head may be positioned somewhat more parallel to the floor with the zygomatic arch elevated. The dependent arm should be positioned carefully and padded below the level of the body. The arm hangs independently from the end of the table, which is extended several inches with a ¾-inch plastic sheet. The arm is cradled underneath the edge of the table and attached to the Mayfield head holder with adhesive tape and copious foam padding. Care should be taken to ensure that the arm is not abducted severely. A foam roll is placed beneath the dependent axilla and between the patient′s knees. The ipsilateral shoulder is pulled toward the feet and secured, like the entire body, with adhesive tape to allow full rotation of the table ( Fig. 27.3 ). The trapezius muscle should be palpated to test its tension so that undue traction that could cause “positioning neural apraxia” can be avoided.

Patient position for the right far-lateral approach. The patient is in a modified park bench position. The head has been secured in a Mayfield head holder after (1) cervical spine flexion in the anteroposterior plane; (2) head rotation 45 degrees to the contralateral side, leaving the mastoid process as the highest point in the operative field; and (3) lateral cervical flexion of 30 degrees toward the opposite shoulder, resulting in the cranial vertex pointing below the horizontal plane. The dependent arm is cradled from the Mayfield head holder and padded with a foam roll. (Reprinted with permission from Barrow Neurological Institute.)

Schematic of a patient positioned for the right lateral approach viewed from above. The right arm is pulled downward and secured with 3-inch tape. This maneuver opens up the cranial cervical angle. The patient is taped securely to the operating room table to permit axial rotation during microscopic dissection. (Modified figure from Baldwin HZ, Miller CG, van Loveren HR, Keller JT, Daspit CP, Spetzler RF. The far lateral/combined supra- and infratentorial approach. J Neurosurg 1994;81:60–68.)

Incision and Initial Soft Tissue Dissection

The opening of the far-lateral approach starts with an inverted hockey stick incision that begins just medial to the tip of the mastoid process and extends up to the superior nuchal line. The incision continues medially along the nuchal line to the midline, where it turns inferiorly down to the level of the C3 to C4 vertebrae ( Fig. 27.4 ). A myocutaneous flap is elevated and reflected laterally. A 1-cm cuff of cervicalis fascia and splenius capitis is left attached to the superior nuchal line. Preservation of the osteoligamentous and nuchal attachments permits anatomical muscle reapproximation and helps to reestablish a watertight closure at the end of the procedure. A subperiosteal dissection is performed below the nuchal ligament down to the level of the foramen magnum and over the ipsilateral laminae of the C1 and C2 vertebrae. Identification of the C2 ganglion as it exits above its lamina allows preservation of cervical sensation and upper paraspinous musculature tone. The myocutaneous flap is retracted laterally and downward with the aid of fishhooks on a Leyla bar (Aesculap, San Francisco, CA).

If a subtemporal craniotomy is required, the incision should be modified to begin at the root of the zygoma. It extends superiorly and posteriorly in circumlinear fashion over the pinna and reaches posteromedially to the inion. The myocutaneous flap is dissected in a subperiosteal plane down to the level of the zygoma and the spine of Henle and then is retracted laterally by fishhooks to a Leyla bar.6 The additional exposure permits petrosal dissection, if necessary.

Vertebral Artery Dissection

As the myocutaneous flap is turned laterally, surgical manipulation should remain in the subperiosteal plane. The vertebral artery, shrouded in its paravertebral venous plexus, is dissected in a subperiosteal plane from the superior lamina of C1 using a Key or an Adson periosteal elevator. The dissection begins at the sulcus arteriosus of the atlas and continues laterally to the foramen transversarium of the lateral mass. Once proximal control of the vertebral artery has been established, the dissection is carried medially to the entry point of the vertebral artery into the dura. It is unnecessary to skeletonize the paravertebral venous plexus off the vertebral artery. Blunt dissection usually provides adequate exposure of the vertebral artery over the atlas. The foramen magnum, the dorsal and ventral surfaces of the atlas, and the ipsilateral atlanto-occipital joint are dissected clean of soft tissue to prepare for the bony resection. The atlanto-occipital membrane may be resected if further dural exposure is desired, such as with a combined approach.

Close-up view of the skin incision for the right far-lateral approach. Incision begins just medial to the tip of the mastoid process, extends up to the superior nuchal line, medial to the inion, and inferiorly to the C3 or C4 spinous process. (Reprinted with permission from Barrow Neurological Institute.)

Care should be taken to avoid undue cauterization of the venous plexus around the second portion of the vertebral artery. Venous bleeding should be remedied easily by gentle tamponade with Surgicel strips (Ethicon, Somerville, NJ). Venous hypertension may cause excessive venous bleeding, so care must be taken during positioning to avoid exaggerated neck flexion and rotation. Repetitive use of bipolar cauterization and excessive packing with oxidized cellulose over the paravertebral venous plexus increase the risk of vertebral artery occlusion and usually can be avoided by attention to signs of venous congestion during the initial setup and positioning of the patient.

Craniotomy and Bony Exposure

Using a high-speed drill with a footplate, a hemilaminectomy of C1 is performed from the midline to just lateral to the sulcus arteriosus. The lamina may be replaced at the conclusion of the procedure. Any remaining atlantal bone is removed with the aid of a high-speed drill and a small bone rongeur. The foramen transversarium can then be opened over its dorsal aspect to allow mobilization of the vertebral artery, if necessary. Again, care is taken not to cause undue venous bleeding that would require packing with surgical gauze.

Attention is now turned to the retrosigmoid suboccipital craniotomy. With the same high-speed drill and foot-plate, the lip of the foramen magnum is used as the initial seating point for the drill bit to perform the craniotomy. With a Penfield no. 1 or Adson periosteal dissector, the condylar vein is carefully dissected away from the foramen magnum before the bone flap is turned. Alternatively, a single burr hole may be placed at the asterion, just lateral and inferior to the junction of the sigmoid and transverse sinuses. Ideally, the craniotomy begins from the foramen magnum lateral to the midline, continues medially up to the sigmoid sinus and jugular tubercle, and terminates at the contralateral aspect of the foramen magnum. If more lateral exposure is required, it can be achieved by use of rongeurs or high-speed drill. If irrigation is constant, the lateral foramen magnum, the posterior half of the occipital condyle, and the superolateral mass and facet of C1 can be removed.

The extracranial cervical vertebral artery (V2) is identified in its surrounding venous plexus and is carefully protected with a small dissector when bone is drilled in this region. Further lateral and ventral exposure may be obtained by drilling the lateral aspect of the foramen magnum to the jugular tubercle ( Fig. 27.5 ). Cranial nerve XII lies in the anterior third of the occipital condyle; therefore, the posterior two-thirds of the occipital condyle and a medial portion of the superior lateral mass and facet of C1 may be removed with impunity.7

Intraoperative view after soft tissue dissection. The craniotomy is performed initially and incorporates the foramen magnum and removal of the arch of C1 **(A)**. The occipital condyle is then partially removed while the vertebral artery is carefully protected **(B)**. Approximately one-third of the condyle can be safely resected. Landmarks during resection include the condylar vein and hypoglossal canal. (Reprinted with permission from Barrow Neurological Institute.)

As the occipital condyle is removed, brisk venous bleeding heralds entry into the occipital condyle emissary vein. This bleeding can be controlled with judicious application of bone wax and Surgicel surgical gauze. If still greater bony exposure is required for a presigmoid approach, the mastoid process may be removed and the bony drilling carried to the occipital atlantal facet joint. If this joint is entered, however, bony fusion may be required to maintain postoperative stability of the CVJ. Nonetheless, it is this extreme lateral removal of bone from the occipital condyle, jugular tubercle, and the lateral mass of C1 that minimizes brain retraction and maximizes exposure of the anterolateral and inferior brainstem.

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