Chapter 29 Far Lateral-Craniovertebral Approach

10.1055/b-0037-143535

Chapter 29 Far Lateral-Craniovertebral Approach

Giuseppe Catapano, Matteo de Notaris, Giuseppe Di Nuzzo, Nelido Gonzalez Fernandez, Iacopo Dallan, Alberto Prats-Galino

29.1 Far Lateral Approach

29.1.1 Introduction

Lesions located in the lower clivus and extending until the anterior margin of foramen magnum have always presented as a challenge to the neurosurgeon. Chordomas, chondrosarcomas, and meningiomas can arise from the clival region and extend toward the anterolateral surface of the brainstem.1–4 They may expand superiorly to reach the sellar and suprasellar areas, inferiorly to the foramen magnum and craniovertebral junction, and laterally to the middle cranial fossa, tentorium, and cerebellopontine angle. One single surgical corridor may not allow full exposure of the entire lesion. The majority of these lesions can be approached posteriorly by suboccipital, retrosigmoid, or far lateral craniotomies or anteriorly using two approaches: the transoral or the endonasal route.

Concerning the far lateral approach, the posterolateral exposure of the craniovertebral junction has proved to be very helpful in the management of lesions in this area.5 It is an extension of the standard suboccipital approach, designed to maximize exposure of the lateroventral craniocervical junction.6,7 It allows removal of the occipital bone including the posterior aspect of the occipital condyle and posterior arch of C1.

The main anatomic landmarks are the vertebral artery and the hypoglossal nerve. Following the basic principle of cranial base surgery, the angle of view is increased by bone removal. The initial steps of this approach are as follows: dissection of occipital–cervical muscles with the exposition of suboccipital triangle, lateral suboccipital craniotomy, posterolateral occipital partial condylectomy, and, finally, exposure of vertebral artery until its entrance into the dura mater. When the intention is reaching the anterior and lateral medulla regions, as in tumors of the lower clivus, the inferior third basilar artery, or vertebral basilar junction aneurysms, the far lateral approach allows a tangential, unobstructed view of the posterolateral cervicomedullary area.

Advances in endoscopic endonasal skull base surgery have led to the development of new routes to areas beyond the midline skull base.8 In the past decade, cranial base surgery has been enriched by the introduction of endonasal techniques to access lesions located anterior or anterolateral to the brainstem. Modifications of the standard transsphenoidal route, especially with the use of the endoscope, have allowed additional exposure of the suprasellar, retrosellar, and retroclival areas, providing a direct view.9,10 Since the beginning of the 1990s, the historical target of the endoscopic endonasal approaches has been the midline skull base, initially limited to the sellar region and over time expanded to areas from the cribriform plate down to C2. Anatomic knowledge and intraoperative image guidance have led to the expansion of such routes to areas beyond the midline skull base. Previous studies have shown that the lateral limits of the endoscopic endonasal transclival approach can be overcome by additional bone removal, and feasible surgical corridors to Meckel′s cave, the cerebellopontine angle, and the ventrolateral brainstem have been described.11–13 Actually, anatomic14–17 and clinical reports concerning extended endoscopic approaches have demonstrated the possibility of reaching areas that extend from the cribriform plate down to the craniovertebral junction and C218 to remove both extradural and intradural tumors via a pure transclival endoscopic endonasal approach.19–24

29.1.2 Surgical Steps

Positioning

An accurate patient positioning is important. The patient′s head is flexed until the chin is 1 cm from the sternum, rotated contralaterally to the lesion, and flexed 30 degrees laterally toward the contralateral shoulder, so that the ipsilateral external auditory meatus and the mastoid bone are at the highest point, allowing the angle between the atlas and the foramen magnum to be increased. The neurosurgeon should pay attention to the jugular veins to avoid compression, which could cause a venous return impairment and brain swelling. For this reason, an axillary roll is placed and the contralateral arm rests on a Krauss armrest. The elevated arm is distracted inferiorly toward the foot of the table to provide more room for the surgeon above the shoulder. All pressure points are carefully padded with foam or gel pads and the patient is secured to the operating table with adhesive tape to allow safe rotation of the table during the operation to improve the surgeon′s line of sight.

After the patient has been induced general anesthesia and properly catheterized, the hair should be combed with a brush used for washing the hands, soaked in detergent solution so as to facilitate the shaving that should be performed up to 2 cm from the region of the surgical incision.

Skin Incision

After the positioning and trichotomy, the marking of the skin incision is done, in such a way that the two endings form an imaginary straight line that adequately simulates the separation of the skin flap and the consequent bone exposure. The most important external anatomic landmarks are represented by inion, asterion, C2 spinous process, and mastoid apex, and these should be marked. The transverse process of C1, which may be touched in the middle way between the mastoid tip and the posterior angle of the mandible, can be localized as well. The incision begins in the midline, approximately 5 cm below the inion and goes straight upward until 3 cm above the external occipital protuberance. Then it turns laterally to the asterion and finally it turns downward and laterally over the sternocleidomastoid muscle posterior edge, approximately 5 cm below the mastoid apex ( Fig. 29.1 ). Alternatively, the skin and galea are elevated first to expose the underlying pericranium above the superficial neck fascia, which may be harvested as a fascial graft for later watertight dural closure. The pericranium and the superficial fascia are then elevated to expose the underlying musculature and the occipital artery ( Fig. 29.2a ). It is important to maintain midline orientation by palpating the spinous processes of the upper cervical vertebrae. Preservation of a muscle “cuff” at the level of the superior nuchal line is helpful to correct approximation of the musculature at the end of the procedure and in prevention of a cerebrospinal fluid (CSF) leak ( Fig. 29.2b ).

**Fig. 29.2** Anatomic pictures showing the dissection of the soft tissue, the galea (a) and superficial layer of the muscles (**a, b**) . A (small), asterion; EOP, external occipital protuberance; OA, occipital artery; SCM, sternocleidomastoid; SNL, superior nuchal line; SpC, splenius capitis; SsC, semispinalis capitis; T, trapezius.

Muscle Dissection

From an anatomic point of view, three layers of muscles are identified during the dissection: the superficial layer, which includes the trapezius and sternocleidomastoid muscles; the middle layer, which consists of the splenius capitis, longissimus capitis, and semispinalis capitis ( Fig. 29.2a, b ) muscles; and the deep layer of muscles composed of the rectus capitis posterior major medially, the inferior oblique inferiorly, and the superior oblique muscle superolaterally. These three muscles represent the suboccipital triangle ( Fig. 29.3a ). The muscular stage can be divided into two steps. First, the sternocleidomastoid muscle is detached laterally. Then, all other occipital–cervical muscles are detached medially and inferiorly, until the suboccipital triangle and the vertebral artery inside are identified ( Fig. 29.3b ).

**Fig. 29.3** Anatomic pictures showing the dissection of the suboccipital triangle (purple). C1, atlas; IOM, inferior oblique muscle; RCPm, rectus capitis posterior minor; RCPM, rectus capitis posterior major; SOM, superior oblique muscle; ST, suboccipital triangle.

In taking down the muscle and skin flap, several landmarks are identified and followed. The transverse process of C1 is a valid landmark for lateral exposure. After the spinous processes of C1 and C2 are identified, the lamina of C2 is exposed, as is the ipsilateral portion of the posterior arch of C1 ( Fig. 29.4a ). The posterior arch of C1 is followed laterally to the sulcus arteriosus, which marks the medial limit of the vertebral artery ( Fig. 29.4b ). It is important to localize and identify the vertebral artery itself along with its surrounding venous plexus to better protect the artery during drilling of the posterior portion of the occipital condyle.

**Fig. 29.4** Anatomic pictures showing the dissection of the suboccipital triangle and the vertebral artery. A, asterion; C1, atlas; dm, dura mater; fm, foramen magnum; IOM, inferior oblique muscle; RCPM, rectus capitis posterior major; SOM, superior oblique muscle; TP, transverse process; VA, vertebral artery.

Vertebral Artery Exposition

The suboccipital triangle is composed of the rectus capitis posterior major muscle above and medially, the superior oblique muscle above and laterally, and the inferior oblique muscle below and laterally. It is covered by the semispinalis capitis muscle medially and the splenius capitis muscle laterally. Its floor is formed by the posterior atlantooccipital membrane. The suboccipital triangle involves the dorsal ramus of the Cl nerve root and the V3 horizontal segment of the vertebral artery. After identifying the suboccipital triangle under the semispinalis muscle, the surgical strategy is to perform a thorough dissection of vertebral artery ( Fig. 29.4 ). This triangle can be opened by detaching the insertions of the superior and inferior oblique muscles from the transverse process of C1 and reflecting them posteriorly. Exposure and control of the extradural vertebral artery can be achieved by identifying its extradural course from the foramen transversarium of C2 to the foramen magnum ( Fig. 29.5 ). Several small muscular branches and the posterior meningeal artery arise from the horizontal segment of the vertebral artery, which can be safely coagulated during surgery. In some cases, the posterior spinal artery and the posterior inferior cerebellar artery (PICA) arise extradurally and they can be injured. Subperiosteal dissection of the vertebral artery from the vertebral groove reduces bleeding from the venous plexus by leaving the periosteal sheath around the artery intact. The atlantooccipital membrane is sharply divided to expose the underlying dura ( Fig. 29.5 ).

**Fig. 29.5** Anatomic picture showing complete dissection of the craniovertebral junction. C1, atlas; C2, second cervical vertebra; C2 g, second cervical root ganglion; dm, dura mater; fm, foramen magnum; LM, lateral mass; mb, muscle branch of the vertebral artery; TP, transverse process; VA, vertebral artery.

Craniotomy

A retromastoid craniotomy is performed with complete exposure of transverse and sigmoid sinus as the superior and lateral limits. Inferiorly, the craniotomy should be extended until the edges of foramen magnum, and the posterior arch of C1 should be resected ( Figs. 29.6, 29.7 ). At this point, the posterior portion of the occipital condyle can drilled out to expose the hypoglossal canal. Once the drilling has been performed, the hypoglossal nerve comes into view ( Fig. 29.8a, b, Fig. 29.9 ).