Surgical Anatomy of the Skull Base

Positioning

The key for optimal patient positioning in any skull base procedure is to orient the head in a way that, while safe for the patient, allows the best final working corridor to expose the target. In many instances, both objectives are fulfilled if the position of the head takes into account the effect of gravity on the brain being exposed. The park bench, or three-quarter prone, position allows for natural retraction of the cerebellar hemisphere away from the surgical corridor. The park bench position requires positioning the patient at a 45-degree angle from a completely prone position with the head rotated 45 degrees away from the lesion and flexed laterally toward the floor. At the end of the positioning, the neurosurgeon should be able to effortlessly palpate the mastoid tip, the inion, and the spinous processes of the upper vertebrae while standing above the patient.

Skin Incision

An optimal skin incision for the far lateral approach takes into consideration the location of the lesion, which determines the surgical exposure, surgical time (longer skin incisions may generate more bleeding and increased surgical time), and aesthetics. Two valid skin incisions are widely used for the far lateral approach: the inverted hockey stick and the lazy S. The inverted hockey stick incision starts 2 cm below the tip of the mastoid process and continues straight superiorly until above the superior nuchal line, where it turns medially toward the level of the inion. It then turns inferiorly until the level of the C2 or C3 vertebra. The benefit of the inverted hockey stick incision is that it provides wide exposure of the ipsilateral suboccipital musculature, allows access to the lateral aspect of the occipital condyle and transverse process of the atlas, and uses an avascular plane along the midline over the suboccipital space and spinous processes in the neck. Although it provides exceptional, wide exposure, drawbacks of the inverted hockey stick incision include a longer surgical time, potentially more blood loss, and suboptimal cosmesis compared with the lazy S. A lazy S–shaped incision uses a diagonal trajectory from the asterion to the foramen magnum and then curves medially toward the spinous process of the axis (C2). This incision provides space enough for a basic far lateral approach and its transcondylar and supracondylar variants. However, complete exposure of the transverse process of the atlas, if necessary, may be limited for a paracondylar variant of the far lateral approach.

Muscular Layer

It is generally agreed that the optimal muscular flap is that which creates the least distortion of the muscular structure while providing sufficient bone exposure. Multiple-layer dissection during the muscular phase can prompt dehiscence, loss of function, or ischemic atrophy and is typically avoided. However, the far lateral approach requires that all efforts be made to protect the vertebral artery, and this is best accomplished if the muscular phase is divided into two stages: the nuchal and the suboccipital. The nuchal muscles are anatomically contained between the superior and inferior nuchal lines. The nuchal muscular flap is obtained en bloc from 1 cm inferior to the superior nuchal line. The nuchal muscular flap contains the sternocleidomastoideus, trapezius, longissimus capitis, and splenius and semispinalis capitis muscles. The posterior belly of the digastric muscle and the occipital artery, which runs on the medial surface of the digastric muscle, may be spared if transmastoid access is not needed in the final exposure. After the nuchal muscular flap is reflected inferiorly and laterally, the suboccipital triangle is exposed (see Fig. 3.1E ). The suboccipital triangle is formed by the superior oblique (superior and lateral), the inferior oblique (inferior and lateral), and the rectus capitis posterior major muscles (the rectus capitis posterior minor sits deep to his major twin). Identifying the suboccipital triangle is key to avoid both copious venous bleeding and damage to the third segment of the vertebral artery. The dorsal ramus of the C1 cervical nerve and the posterior arch of the atlas may be exposed if the suboccipital triangle is dissected.

The vertebral venous plexus is a complex of tangled and densely anastomosed veins that fills the suboccipital triangle and connects to the sigmoid sinus and jugular bulb via the posterior condylar emissary vein. Embedded in this venous plexus are the muscular branches of the vertebral artery and the dorsal roots of the C1 cervical nerves. In some cases, the third segment of the vertebral artery loops higher than normal, being at special risk of inadvertent injury if the venous plexus is not dissected with extreme caution. Although rare, both the posterior spinal and the posterior inferior cerebellar arteries can take off at this segment of the vertebral artery and be mistaken for a muscular branch. After the muscles forming the suboccipital triangle are reflected, three additional muscles become relevant. At the superior aspect of the C1 transverse process, deep below the superior oblique muscle, is the rectus capitis lateralis. The rectus capitis lateralis is a short muscle that attaches to the jugular process at the posterior edge of the jugular tubercle. Although very small, this muscle is of extraordinary value in guiding the paracondylar dissection toward the jugular foramen. In the same axis as this muscle, but inferior to the transverse process of the atlas, is the levator scapulae muscle. Early identification of the levator scapulae muscle during the cervical dissection of the far lateral approach is advisable. This muscle provides an invaluable landmark to safely expose the second segment of the vertebral artery (medial to the muscle) and protect the carotid compartment of the parapharyngeal space (lateral to the muscle). Finally, the rectus capitis posterior minor muscle is reflected medially for a complete exposure of the atlantooccipital membrane (see Fig. 3.1F ).

Craniotomy

A fundamental rule in skull base surgery is to gain maximal surgical exposure through bone removal. Therefore, there is an optimal craniotomy, or combination of craniotomy and craniectomy, for each lesion. However, for educational purposes, the far lateral approach can be standardized into a basic lateral suboccipital craniotomy with three different extensions: transcondylar, supracondylar, and paracondylar. The lateral suboccipital craniotomy opens a window in the squamous part of the occipital bone. It is limited superiorly by the transverse sinus and laterally by the sigmoid sinus and the occipital condyle (see Fig. 3.1G ). In most cases the suboccipital craniotomy includes the condylar fossa and its posterior condylar vein. The asterion is a useful landmark to design the craniotomy in relation to the sigmoid sinus. The superior nuchal line may also be used to limit the cranial extension of the craniotomy so as to avoid the transverse sinus and the torcula. The medial extent of the suboccipital craniotomy may be guided with the external occipital crest and can be extended widely beyond it if the lesion occupies the majority of the cisterna magna. Although there is no anatomic boundary, the medial margin of the suboccipital craniotomy usually does not cross the occipital crest, providing enough room for cerebellar distention.

The standard suboccipital craniotomy can be accompanied by the removal of the ipsilateral half of the posterior arch of the atlas. The first and most important step in the preparation for the removal of the posterior arch of the atlas is dissecting the vertebral artery from its connective sheath. Next, the root of the C2 spinal nerve can be identified medial to the atlantoaxial joint. Two cuts are placed to elevate the ipsilateral half of the posterior arch of the atlas in one piece. The first cut is placed in the midline, medial to the rectus capitis posterior minor muscle. The second cut divides the lateral mass from the posterior arch. After the completion of the two cuts, the posterior arch is elevated in one piece, and the vertebral artery is freed from the transverse foramen after unroofing the foramen transversarium. At this point, the vertebral artery can be displaced laterally to gain access to both the posterior fossa and the occipital condyle.

Of the three main types of craniectomy that can follow the suboccipital craniotomy, the transcondylar extension is the most used. The posterior third of the occipital condyle is removed uniformly until a change in bone consistency and color is noticed. The cancellous body of the condyle becomes the solid posterior wall of the hypoglossal canal. The hypoglossal canal contains a venous plexus, which adds a dark blue color to its wall. The hypoglossal canal should be preserved if there is no lesion involving it, yet the condylectomy can advance below and medial to the hypoglossal canal en route to the clivus. The transcondylar approach provides direct access to the intracranial segment of the vertebral artery and a wide access to the anterior medullary zone and lower clivus. However, if the lesion infiltrates the condyle or its vicinity, a complete condylectomy may be necessary.

If access to the jugular foramen or upper medulla is required, the supracondylar extension may be performed. The supracondylar craniectomy requires removing the posterior aspect of the jugular tubercle in a narrow window limited inferiorly by the hypoglossal canal, superiorly by the sigmoid sinus, and laterally by the jugular bulb. It is imperative that the drilling of the jugular tubercle be done with a diamond bur and using extreme caution at the medial (dural) limit of the drilling, because the spinal rootlets of the accessory nerve travel alongside and in contact with the dura at this region. The accessory, vagus, and glossopharyngeal nerves are exposed at the neural compartment of the jugular foramen. The supracondylar approach allows maneuvering above the vagus nerve, the petroclival junction, and the midclivus.

Lesions involving the jugular bulb, the lower sigmoid sinus, and the meatal segment of the glossopharyngeal, vagus, and accessory nerves can be accessed through the paracondylar variant of the far lateral approach. This craniectomy is directed to the superior and lateral aspect of the occipital condyle, the exocranial aspect of the jugular foramen, and the posterior aspect of the mastoid process. A more lateral muscular flap must be raised, which may also include the posterior belly of the digastric muscle. The digastric groove should be identified because it provides an excellent landmark to the position of the stylomastoid foramen, where the facial nerve exits the mastoid process. Drilling around the exocranial aspect of the jugular foramen must be performed with a diamond bur and constant electrophysiologic monitoring of the glossopharyngeal, vagus, accessory, and hypoglossal nerves. The bone removal is mainly directed to the jugular process, an osseous protuberance at the posterior aspect of the jugular foramen. At the end of the craniectomy, the lower part of the sigmoid sinus, the jugular bulb, and the jugular vein are exposed together with the neural compartment of the jugular foramen and the pharyngeal segment of the internal carotid artery.

These variants of the far lateral approach could be combined with different levels of mastoidectomy (retrolabyrinthine or translabyrinthine approach) and a supratentorial craniotomy for full access to the lateral and anterolateral zones of the brainstem.

Dural Opening

The dura exposed after a suboccipital craniotomy is limited superiorly by the transverse sinus, laterally by the sigmoid sinus, and inferiorly by the marginal sinus—all potential sources of bleeding (see Fig. 3.1G ). Other potential sources of bleeding include the posterior meningeal artery, which in some rare cases originates from the intradural segment of the vertebral artery; the posterior spinal artery, which may take off from the vertebral artery at its dural cuff; the posterior meningeal artery; and the meningeal branch of the ascending pharyngeal artery in cases in which the dural incision extends to the lateral margin of the bone opening (especially after a supracondylar extension). Each dural flap may be customized to adapt to the features of each patient. However, many, if not all, include long midline and superior incisions to reflect the dural flap laterally. The dural opening should always be designed according to the lesion location and size and should create a surgical window that allows all anticipated surgical trajectories. In the case of the far lateral approach, a wide dural flap eases cerebellar retraction and access to the superior spinal region. In addition, if the lesion extends to the cerebellopontine angle, the dura should be opened close to the transition between the transverse and sigmoid sinuses, leaving a margin for safe closure. After the dura is opened, the cisterna magna is incised and the cerebrospinal fluid (CSF) is evacuated to allow atraumatic retraction of the cerebellar hemispheres and maximize the effect of gravity.

Intradural Anatomy

The intradural phase of the far lateral approach provides wide access to the dorsolateral compartment of the posterior fossa and limited access to the ventromedial compartment, including the petroclival region and the lateral aspect of the middle and lower thirds of the clivus (see Fig. 3.1D ). Like most skull base approaches, the number and exposure of structures accessed through the far lateral approach increases proportionally to the progressive bone removal of its three variants (i.e., transcondylar, supracondylar, and paracondylar craniectomies). The suboccipital craniotomy allows complete access to the cisterna magna and the ipsilateral cerebellar hemisphere (see Fig. 3.1H ). In the cisterna magna, the inferomedial aspect of the cerebellar hemisphere, the cerebellar tonsil, and the lower medulla coexist. The obex (inferior angle of the fourth ventricle) is also viewed on arachnoidal dissection and points toward the fourth ventricle in the foramen of Magendie. Opening the cisterna magna also exposes the vertebral artery. The vertebral artery transitions from its third segment (V3, over the posterior arch of the atlas) to its intradural segment around the inferomedial aspect of the occipital condyle. At this point, it is anchored to the craniocervical junction by the denticulate ligament.

After the vertebral artery becomes intradural, it travels through the cerebellomedullary and premedullary cisterns in an anteromedial trajectory toward the clivus, where it joins the contralateral vertebral artery to form the vertebrobasilar junction. In the majority of individuals, the posterior inferior cerebellar artery (PICA) takes off from the vertebral artery at the premedullary cistern close to the inferior olive (anterior medullary segment). After a short course, the PICA crosses the rootlets of the vagus and accessory nerves (CNs X and XI) toward the cerebellomedullary cistern below the cerebellar hemisphere (lateral medullary segment). It then turns medially and upward around the cerebellar tonsil (tonsillomedullary segment). Following this, the PICA runs between the cerebellum and the posterior wall of the fourth ventricle (telovelomedullary segment), away from the surgical exposure obtained with the far lateral approach. It finally turns superficial (cortical segment) to feed the posterior aspect of the cerebellar hemisphere. Another critical branch of the vertebral artery is the posterior spinal artery. The neurosurgeon dissecting the arachnoid space in the cisterna magna should identify this artery early and protect it.

The accessory nerve comes into view at the lateral aspect of the cisterna magna. The accessory nerve has a long cisternal segment, which receives rootlets from both the upper spine and the lower aspect of the posterolateral sulcus in the medulla. It has a superolateral trajectory from its spinal origin toward the jugular foramen. At the jugular foramen, the accessory nerve joins the vagus nerve, which has a straight pathway from its origin at the posterolateral sulcus of the medulla. The accessory and vagus nerves form the vagoaccessory triangle, the main surgical corridor to access the ventromedial compartment of the posterior fossa (see Fig. 3.1H , yellow triangle ). The superior aspect of the vagoaccessory triangle is formed by the vagus and medullary rootlets of the accessory nerve; it is limited laterally by the body of the accessory nerve and medially by the medulla oblongata. The vagoaccessory triangle is further divided by the hypoglossal nerve into supra-hypoglossal and infra-hypoglossal windows (see Fig. 3.1H , green line ). As it transitions to the intracranial space, the accessory nerve is intimately related to the denticulate ligament and the vertebral artery. The relationship between the accessory nerve and the denticulate ligament is surgically relevant. At the spine, the denticulate ligament is anterior to the rootlets of the accessory nerve. However, as these structures ascend, they cross each other as the denticulate ligament anchors to the dural cuff of the vertebral artery, and the accessory nerve runs in an anterosuperior trajectory toward the posterior aspect of the jugular foramen (in the neural compartment). Therefore, any mobilization or manipulation of the denticulate ligament (e.g., transposition of the vertebral artery) should be done with continuous visual contact with the accessory nerve. The vertebral artery, after piercing the dura and passing the denticulate ligament, runs anterior to the accessory nerve. This leaves an unobstructed space between the accessory nerve and the lateral aspect of the medulla that is of surgical relevance. The suboccipital craniotomy also provides direct access to the inferior vermian and hemispheric veins draining the ipsilateral cerebellum to the torcula and the transverse and tentorial sinuses.

The transcondylar craniectomy allows access to the cisternal, canalicular, and cervical segments of the hypoglossal nerve. In addition, this extra window allows for an increased angle of attack and surgical exposure of both the intradural portion of the vertebral artery and the inferior half of the cerebellomedullary and premedullary cisterns. The exposure of the cerebellomedullary cistern allows for manipulation of the medullary rootlets of CN IX to XII, the lateral aspect of the medulla, and the lateral medullary segment of the PICA. The transcondylar variant of the far lateral approach maximizes the infra-hypoglossal window of the vagoaccessory triangle.

As the craniectomy is progressed through the jugular tubercle via a supracondylar extension, access to the superior aspect of the premedullary and cerebellomedullary cisterns increases. These cisterns can be widely accessed after a combined transcondylar and supracondylar craniectomy. The supracondylar approach provides the optimal bone opening for maximal access through the vagoaccessory triangle. Working through its supra- and infra-hypoglossal windows, the vertebral artery, vertebrobasilar junction, and lower basilar artery may be accessed. If the drilling of the jugular tubercle is continued anteriorly, the petroclival junction and medial aspect of the clivus can be accessed. The number and magnitude of the neurovascular structures and lesions accessible through the far lateral approach are highly dependent on the angulation of the microscope and the positioning of the retraction blades. If the jugular tubercle is removed and retraction is applied at the midportion of the cerebellar hemisphere, the hypoglossal canal, jugular foramen, internal acoustic meatus, and inferior surface of the tentorium may be accessible. This brings a broad spectrum of both anatomic landmarks and surgical lesions that can be explored from a single approach. If the microscope is angled toward the petrous bone, the distal anterior inferior cerebellar artery (AICA), the facial and vestibulocochlear nerves (CNs VII and VIII), the foramen of Lushka, and the trigeminal nerve may be explored. The three working corridors from this perspective are (1) the space formed between the vestibulocochlear and glossopharyngeal nerves (CNs VIII and IX), (2) the space between the trigeminal nerve and the facial-vestibulocochlear nerve (CN VII–CN VIII) bundle, and (3) the space between the trigeminal nerve and the tentorium. However, if the microscope is angled toward the brainstem, access to the pons in the cerebellopontine cistern, the lateral aspect of the pontomedullary sulcus, and the medulla in the premedullary and cerebellomedullary cisterns may be possible.

When used during a far lateral approach, the endoscope allows further exploration of the ventromedial compartment of the posterior fossa. The main advantage of the endoscope, used in combination with the microscope, is that the point of view and light can be brought beyond the limits of microsurgery. This is especially relevant when angled scopes are used. A 30-degree endoscope passed through the vagoaccessory triangle allows exploration of the medial portion of the medulla and the abducens nerve (CN VI) at the pontomedullary sulcus medially and the floor and medial aspect of the jugular foramen laterally. In addition, if the angled endoscope is advanced through the space between the vestibulocochlear and glossopharyngeal nerves, the origin of the trigeminal nerve, the prepontine cistern, the proximal segment of the AICA, and the cisternal segment of the abducens nerve can be explored medially and the internal acoustic meatus can be completely explored in the cerebellopontine angle laterally. However, the expanded view provided by the endoscope carries significant surgical limitations; the instrumentation is limited, the corridors are lengthy, and there is no visual control over the external surface of the endoscope, potentially making it difficult to be aware of retraction damage caused by the rod of the endoscope.