Anatomy

Muscular Anatomy

Despite our belief that separation of each individual muscle layer is unnecessary for the far lateral approach, an understanding of the upper cervical muscles and their attachments is an important aspect of performing the far lateral approach safely. The key to the exposure of the far lateral approach lies in the identification and preservation of the extradural vertebral artery, which lies within a triangle of muscles referred to as the suboccipital triangle (Fig. 79-1).

FIGURE 79-1 Schematic of the suboccipital muscular triangle.

There are four superficial muscles that overlie the suboccipital triangle and three that comprise it. The former are the sternocleidomastoid, splenius capitis, longissimus capitis, and semispinalis capitis muscles in order from superficial to deep. The suboccipital triangle itself is formed by the rectus capitis posterior major medially, the superior oblique muscle superiorlaterally, and the inferior oblique muscle inferolaterally. The rectus capitis posterior major attaches to the spinous process of C2 and inserts on the occiput. The superior oblique muscle attaches to the transverse process of C1 and inserts on the occiput. The inferior oblique muscle attaches to the transverse process of C1 and inserts on the spinous process of C2. The floor of this triangle is comprised of the atlanto-occipital membrane and the posterior arch of the atlas. Practically speaking, the four muscles overlying the suboccipital triangle are often reflected as a single layer, regardless of the incision chosen.

Anesthetic Technique and Neuroprotection

Several anesthetic maneuvers including hyperventilation and administrations of a bolus dose of 20% mannitol (0.5–1 mg/kg) can be implemented to achieve adequate brain relaxation. This minimizes cerebellar trauma due to excessive retraction while maximizing aneurysm exposure. Electrophysiologic monitoring using somatosensory and motor-evoked potentials and brain stem auditory-evoked potentials allows early detection of ischemia or excessive retraction and manipulation.

Surgical Technique

Various positions can be used including supine with the head rotated, lateral or half-lateral decubitus, and a three-quarter prone or park-bench position. At our institution we prefer a straight lateral position. The patient is placed on the operating table with the side of the lesion placed upward. The dependant arm hangs off the bed and is cradled in a padded sling between the table edge and the Mayfield head holder. The head is then secured in a three-pin Mayfield head clamp. Pin placement is crucial if an occipital artery bypass is contemplated as it may hinder the procedure if placed improperly. The pins are placed so that the single pin is 2 cm superior and anterior to the ear pinna ipsilateral to the lesion. The paired pins are positioned so that the posterior pin is 2 cm above the contralateral ear pinna. The head is positioned with three or four movements:

FIGURE 79-2 A, In the three quarter prone position, the head is flexed and tilted down. B, Additionally, the head is rotated facing toward the floor. The ipsilateral shoulder is taped caudally to augment the surgical space.

The surgical site is shaved. If the occipital artery is to be harvested, a portable Doppler probe is used to identify the course of the artery over the scalp from the mastoid to approximately 4 cm above the superior nuchal line. In such cases scalp infiltration solutions containing vasoconstrictive agents should not be used. The scalp is then prepped and draped in a standard fashion. Several skin incisions may be used including a linear or curvilinear retromastoid, S-shaped, or hockey-stick incision (Fig. 79-3). The linear or curvilinear retromastoid incision starts at about the level of the top of the ear approximately three finger-breadths medial to the mastoid process and extends straight down or a as a gentle curve to just below the mastoid tip. The S-shaped incision starts from the same point described above; however after an initial extension of the incision straight down toward the mastoid, the incision is curved sharply medially to the midline, and then straight downward to the spinous process of C3.

FIGURE 79-3 Types of skin incisions.

Advantages of the linear/curvilinear or S-shaped incisions include a more direct approach to the atlanto-occipital joint and vertebral artery, and because the incision is carried through the muscle there is less muscle bulk to retract laterally. Disadvantages include increased trauma to the muscles, sectioning of the occipital nerve and artery, and difficulty identifying bony landmarks, which in our opinion increases the risk of inadvertent vertebral artery injury.

The hockey-stick incision, which is currently preferred at our institution, is carried through the midline aponeurosis with minimal injury to the muscle mass and allows early identification of the spinous processes and lamina of C1 minimizing the risk of injury to vertebral artery. Superolaterally the muscle mass is cut along its attachment to the superior nuchal line leaving a small cuff of muscle and fascia for better anatomic closure. The main disadvantage to this incision is that the bulk of the musculature is retracted laterally. Since the angle of attack is lateral to medial, this can hinder access to the atlanto-occipital joint or obstruct the surgical view from an inferior direction under the cerebellar tonsil if not adequately retracted. The hockey-stick incision starts approximately at the level of the spinous process of C3 and extends superiorly in the avascular midline plane to approximately 2 cm above the superior nuchal line. The incision is then turned laterally parallel to the superior nuchal line. If the occipital artery is to be harvested, a curved hemostat is used to dissect over and protect the artery. The incision is then continued over the occipital artery to a point immediately superior to the mastoid process. Finally, the incision is curved inferiorly to end just inferior to the mastoid tip (Fig. 79-4).

FIGURE 79-4 The skin incision is shaped like a hockey stick spanning the midline and extending to the mastoid tip.

The suboccipital muscles are cut leaving a cuff as stated above. This facilitates tight muscle closure at the end of the procedure and minimizes postoperative cerebrospinal fluid leaks. This is particularly important if an occipital artery bypass is to be performed as a watertight dural closure is not possible because of the necessity of creating an opening for passage of the occipital artery. The suboccipital musculature is then swept laterally in a subperiosteal fashion to expose the occiput as far laterally as the mastoid process as well as the arch of C1. The skin and muscle flap are retracted inferolaterally and held in position by fishhooks (Fig. 79-5). Adequate retraction of the cervical musculature is important (particularly if a hockey-stick incision is used) to avoid negating the advantage of a more lateral angle of vision achieved with the far lateral approach. An alternative method of layer-by-layer muscle dissection from lateral to medial has been described to avoid this problem.⁵ The technique consists of cutting the muscles in layers leaving small cuffs of the sternomastoid, splenius capitis, and semispinalis muscle attached to the bone for later reattachment. The muscle mass is then elevated medially.

FIGURE 79-5 The muscle mass is retracted latero-caudally with fish hooks attached to the Leyla bar, leaving a 1-cm cuff on the superior nuchal line for resuturing during closure.

Basic Far Lateral Approach

The basic far lateral approach consists of a suboccipital craniotomy and lateral removal of the bony rim of the foramen magnum up to the occipital condyle, which is left intact. The suboccipital craniotomy is performed extending from the junction of the transverse and sigmoid sinuses superiorlaterally to just beyond the midline through the foramen magnum in a teardrop fashion. After the suboccipital craniotomy has been accomplished, the arch of C-1 is removed from just beyond the midline on the opposite side to the sulcus arteriosus underlying the vertebral artery (Fig. 79-6).

FIGURE 79-6 A, The right hemiarch of C1 has been removed to a point beyond the entrance of the vertebral artery through the dural (V3–V4 junction). Note the glistening white surface of the articular surface of the occipital condyle (∗), made visible by the upward subtle translation of the head during positioning. The V3 segment (#) is well-delineated and dissected in the sulcus arteriosus for proximal control. The stump of C1 arch is easily visible (^). B, The suboccipital bone flap has been removed to just beyond the midline. C, Schematic of the craniotomy.

In the past, the senior author routinely exposed the vertebral artery. Currently we believe that for a basic far lateral approach, exposure of the vertebral artery is unnecessary and risks injuring the artery. In such cases the assistant retracts and protects the vertebral artery along with the perivertebral venous plexus inferiorly as the surgeon completes the bone removal.

The critical aspect of the basic far lateral approach is the radical removal of bone in the area of the foramen magnum. The resection should be carried as far laterally as the occipital condyle to a point lateral and superior to the entry of the vertebral artery into the dura. This allows an approach to the ventral aspect of the brain stem from an inferolateral angle with minimal or no brain stem retraction. Each extra millimeter removed from the lateral rim of the foramen magnum permits several extra degrees laterally in the angle of exposure. In this respect, the lateral rim of the foramen magnum represents to the suboccipital exposure what the pterion represents to the frontotemporal exposure. As the bone removal in the region of the foramen proceeds laterally, the bone edge becomes more vertical, and it becomes impossible to reach under the bone with the footplate of a Kerrison rongeur. A high-speed air drill is very useful for the removal of the final 3 to 4 mm of the exposure. It is not uncommon to encounter a posterior condylar emissary vein that runs through the condylar canal located in the condylar fossa. The emissary vein communicates the perivertebral venous plexus with the sigmoid sinus and can be controlled with bipolar coagulation and bone wax. This marks the lateral extent of the bone drilling for a basic far lateral approach. Any opened mastoid air cells must be thoroughly sealed with bone wax to avoid postoperative cerebrospinal fluid leaks.

Transcondylar Extension

The basic far lateral approach may be extended more laterally by resection of posterior aspect of the occipital condyle. This transcondylar approach allows a more lateral direction of view than that offered by the basic far lateral approach, thus providing improved exposure of the lower clivus and anterolateral medulla (Fig. 79-7).

FIGURE 79-7 A, Schematic of the partial condylar resection. B, Schematic of the dural opening. C, Following partial drilling of the medial posterior portion of the occipital condyle, the dura was opened and retracted laterally. This provides a flatter view of the lateral intradural space at the cerebellomedullary cistern.

The occipital-transcondylar variant involves removal of the occipital condyle without violating the atlanto-occipital joint. Initially the medial part of the posterior third of the occipital condyle is carefully drilled. After removal of the superficial cortical bone, the underlying cancellous bone is drilled until the cortical bone surrounding the hypoglossal canal is encountered. This marks the lateral aspect of the intracranial end of the hypoglossal canal. Because the hypoglossal canal is directed anterolaterally, further drilling of the lateral part of the posterior two thirds of the condyle may be performed without entering the hypoglossal canal. However, even though the hypoglossal nerve is intact, the further drilling of the condyle increases the risk of postoperative atlanto-occipital instability. From the practical point of view the senior author routinely adds this additional step of removing about one third of the condyle medially and posteriorly to the basic far lateral exposure. This suffices for management of most aneurysms of the vertebral artery and the vertebrobasilar junction. Incidentally the senior author also uses this exposure for a variety of tumors of this region including almost all meningiomas of the foramen magnum.

The altanto-occipital transarticular variant involves resection of the posterior occipital condyle as above, but adds resection of the adjoining superior articular facet of C1 and mobilization of the vertebral artery. The first step in this variant is removal of the posterior root of the C1 transverse foramen. The vertebral artery segment extending from the transverse foramen of C2 to its dural entrance is exposed by carefully coagulating the overlying perivertebral venous plexus. Brisk venous bleeding is frequently encountered, but with patience this can be controlled with bipolar coagulation and packing with gel foam powder or Surgicel. The artery is then displaced medially and downward away from the atlanto-occipital joint. The occipital condyle along with the adjoining portion of the superior articular facet of C1 is then removed.

Complications

There are several potential complications inherent to the far-lateral approach and its extensions. These include arterial, cranial nerve, and brain stem injuries, craniocervical instability, and postoperative cerebrospinal fluid leaks.

The vertebral artery is at risk for injury during exposure or mobilization of the artery. Injuries may be prevented by careful sharp muscle dissection, stripping of the cervical musculature in a subperiosteal fashion, and avoiding the use of the bovie in the vicinity of the artery. In the event of an inadvertent vertebral artery injury it is much easier to repair a sharply made arteriotomy than that produced by a thermal injury. Preoperative knowledge of the anatomy and caliber of the contralateral vertebral artery is also helpful in guiding decisions regarding whether to repair or sacrifice the artery. The PICA is also vulnerable to injury at several stages of the procedure. Injuries may occur during the exposure if it originates extradurally as it may be mistaken for a muscular branch or for the posterior meningeal artery. The PICA and its perforators can also be injured or occluded intradurally during aneurysm dissection or clipping.

Inadvertent injury to the lower cranial nerves is a major cause of morbidity. Injuries usually result from manipulating the nerve rootlets during dissection and clipping of the aneurysm. These cranial nerves are very sensitive to manipulation, necessitating very gentle retraction and sharp dissection. Lower cranial nerve injury may result in dysphagia, dysarthria, dysphonia, and inadequate airway protection, thus patients should be extubated and started on an oral diet only after a formal evaluation of lower cranial nerve function has been performed. In addition to the lower cranial nerves, injury to the seventh–eighth nerve complex may also occur during dissection of a high vertebrobasilar junction. Brain stem injury may result from either excessive retraction or vascular injury. Compromise of the PICA or its perforators may result in postoperative lateral medullary (Wallenberg) syndrome.

Extensive removal of the occipital condyle or C1 lateral mass may potentially result in atlanto-occipital instability. In a biomechanical study, it was found that occipito-cervical mobility increased significantly as compared to baseline after removal of half of the occipital condyle.⁶ A craniocervical fusion should therefore be strongly considered if greater than 50% of the occipital condyle is resected. As mentioned above, this extensive condylar resection is rarely indicated for vascular lesions.

Postoperative cerebrospinal fluid leaks and pseudomeningioceles can be prevented by thoroughly waxing any open mastoid air cells, closing the dura in a water-tight fashion, and by performing a careful multilayer closure of the muscles and fascia.

Conclusion

The basic far lateral suboccipital approach that we routinely extend by removing the posteromedial one fourth to one third of the occipital condyle provides an excellent surgical exposure that is adequate for managing almost all aneurysms of the intracranial portion of the vertebral artery, the proximal PICA, and the vertebral basilar junction. This approach, of course, is also excellent for many extra-axial tumors of this region, particularly meningiomas of the anterolateral rim of the foramen magnum. Intra-axial lesions of the anterolateral cervical medullary region can also be approached comfortably with this exposure. More complex exposures such as the transcondylar and supracondylar extensions are rarely required for vascular lesions, but can be quite useful for some complex extra-axial tumors of this region. The main complication specifically related to the cervical approach is injury to the vertebral artery or to the PICA when it has an extradural origin. This can be avoided by detailed knowledge of the anatomy and its variances and careful sharp dissection in the region of the vertebral artery. Intradural complications, which include not only vascular injuries but also injury to the lower cranial nerves, are usually related to the specific pathology, rather than to the surgical approach.

The senior author considers the far lateral approach with minimal removal of the posteromedial condyle as described above, together with the frontal or bifrontal approach with orbitotomy, the orbital zygomatic approach, and the retrolabryinthine presigmoid approach not to be specialized exposures within the exclusive domain of “skull base surgeons,” but rather routine surgical exposures that should be part of the training of every competent neurosurgeon.