Chapter 79 Far Lateral Approach and Transcondylar and Supracondylar Extensions for Aneurysms of the Vertebrobasilar Junction

Mohamed Samy Elhammady, Eric C. Peterson, Roberto C. Heros, Jacques J. Morcos

Lesions of the anterior and anterolateral brain stem present special challenges to the neurologic surgeon. Unlike the posterior surface of the brain stem, the anterior surface is buried deep within several vital structures precluding easy anterior access. For this reason, lateral approaches have been proposed to access the anterolateral surface of the lower brain stem from a posterior incision. Heros ¹ first described extensive removal of the lateral foramen magnum up to the condyle to approach these lesions. Later authors advocated progressive drilling of bony structures lateral to the foramen magnum such as the occipital condyle, jugular tubercle, atlanto-occipital joint, and mastoid. As each lateral structure is removed, the angle of approach to the anterolateral surface of the brain stem is increased and the surgical corridor widened. However, morbidity from damage to the vertebral artery (VA) and cranial nerves as well as frank instability of the craniocervical junction is also increased with each structure removed.

The far lateral approach refers to the lateral suboccipital craniotomy and removal of the lateral edge of the foramen magnum all the way to the condyle and lateral mass of C1. The two most common extensions of the far lateral, the transcondylar and the supracondylar, involve adding a resection of the occipital condyle and jugular tubercle, respectively. Other extensions of the far lateral such as the paracondylar and extreme lateral (ELITE) are rarely used for aneurysms and will not be discussed.

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.

Extradural Anatomy

The anatomic keys to the extradural stage of the far lateral approach are twofold. The first is the location and course of the vertebral artery, variants of which can make the exposure dangerous if not recognized. As soon as the VA exits the transverse foramen of C1, it makes a sharp turn medially, running in a groove in the superior surface of the atlas known as the sulcus arteriosus. At this turn, it is immediately medial to the rectus capitis lateralis, an important landmark for the paracondylar extension for exposure of the jugular foramen. Medially, the VA turns cranially to enter the dura. Occasionally, the sulcus arteriosus is not just a sulcus but a circumferential bony canal enclosing the VA as it courses medially above C1. This variation places the VA at higher risk of injury due to torquing or laceration when the posterior arch of the atlas is removed with either the drill or a rongeur.

Three specific points regarding the extradural VA deserve mention. First, as it courses medially in the sulcus arteriosus, it may loop cranially near the occipital bone. This must be kept in mind during the muscular stage of the exposure—it is surprisingly easy to inadvertently bovie into the artery when attempting to separate the last bit of cervical musculature off the suboccipital region. Second, while the posterior inferior cerebellar artery (PICA) usually arises from the VA intradurally, in approximately 5% to 20%² of specimens it arises extradurally and can be confused with a muscular branch off the VA. Third, the VA sits in a venous plexus as it courses in the sulcus arteriosus, which often leads to troublesome bleeding if one chooses to expose it.

The second anatomic key to the extradural stage of the far lateral approach is understanding the role that the bony protuberances of the occipital bone play in hindering access to different areas of the foramen magnum and anterolateral brain stem. The two most important of these bony protuberances are the occipital condyle and jugular tubercle.

The occipital condyle is an oval shaped structure that constitutes the occipital portion of the atlanto-occipital joint. The oval is pointed anteromedially as it is located on the anterolateral border of the foramen magnum at the level of the cervicomedullary junction. From an inferolateral point of view looking superomedially, it hinders access to the anterolateral medulla.

Running just above the occipital condyle almost perpendicular to its long axis is the hypoglossal nerve in the hypoglossal canal. This nerve represents the anterior limit to the condylar drilling, thus understanding of its course is vital to safely performing the transcondylar extension of the far lateral approach. Because the hypoglossal canal is oriented perpendicular to the long axis of the condyle, and because the canal runs in an anterolateral direction, more of the condyle can be drilled laterally before reaching the hypoglossal canal than can be drilled medially. The canal is surrounded by cortical bone, thus careful drilling of the cancellous bone of the condyle ensures its easy identification—the transcondylar extension has reached its limit when the cortical bone of the hypoglossal canal is reached. There is a prominent vein within the condyle which is often encountered in the course of the condylar drilling. This vein, the condylar emissary vein, is simply a communication between the perivertebral venous plexus and the sigmoid sinus and can be routinely sacrificed.

The second bony protuberance that hinders exposure to the anterolateral brain stem is the jugular tubercle. The jugular tubercle is a medially projecting bump of the occipital bone anterior and rostral to the occipital condyle. It thus represents to exposure of the anterolateral pontomedullary junction what the occipital condyle represents to exposure of the anterolateral medulla—an obstruction. Like the occipital condyle, it also is associated with cranial nerves, specifically the 9th, 10th, and 11th, which wrap around the posterior aspect of the tubercle on their way to the jugular foramen. The hypoglossal nerve runs caudal to the jugular tubercle, so that drilling of the jugular tubercle is directed superior to the hypoglossal canal but inferior to cranial nerves 9, 10, and 11. This drilling of the jugular tubercle is the defining step in the supracondylar extension of the far lateral approach.

An interesting anatomic study quantified the benefit of removing these two structures in terms of visualization and surgical freedom.³ The authors found that removing the occipital condyle up to the hypoglossal canal resulted in a mild improvement (21% to 28%) in visualization but a much larger improvement (18% to 40%) in surgical freedom (ability to manipulate surgical instruments within that space). Resection of the jugular tubercle on the other hand, had the opposite effect with a dramatic increase (28% to 71%) in visualization but only a modest (40% to 52%) increase in surgical freedom. This emphasizes the role the occipital condyle plays in narrowing the surgical spatial cone through which instruments are manipulated.

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:

1. Flexion in the anteroposterior plane with slight distraction until the chin is two finger breadths from the clavicle uncovers the suboccipital region.

2. Contralateral bending (approximately 30 degrees) increases the surgical space between the ipsilateral shoulder and the suboccipital region.

3. Upward translation partially and subtly subluxes the ipsilateral atlanto-occipital joint and facilitates possible condylar drilling.

4. Contralateral head rotation in order to bring the ipsilateral side uppermost in the surgical field. Although some authors have advocated this manuever,⁴ the senior author (RCH) believes that this maneuver is not advisable as it results in rotation of the ventral surface of the brain stem and thus the vertebrobasilar junction away from the surgeon. In fact, upon completion of the bony drilling and opening of the dura, we routinely rotate the operating table toward the surgeon to allow a tangential view of the ventral brain stem. The operating table is placed in slight reverse Trendelenberg so as to position the patient’s head above the level of the heart to reduce cerebral venous congestion. The patient’s superior shoulder is retracted towards the patient’s feet using adhesive tape to keep the cervical-suboccipital angle open (Fig. 79-2).