Positioning

Patient positioning depends primarily on the nature and location of the offending lesion. A prone position is used, as it avoids distortion of the upper cervical anatomy. Rotation of the table and/or the microscope is required to allow adequate visualization lateral and ventral to the cervicomedullary junction.15 Some authors prefer a lateral decubitus position without any flexion of the neck so as to not compromise the cervicomedullary junction, which is often already distorted by a preexisting compressive lesion.16 This position also avoids alteration of the course of the vertebral artery from C2 to its dural entrance. Some authors still prefer and recommend the sitting position for surgery.14

For lesions located anterolateral to the cervicomedullary junction, a modified “park bench” position is generally preferred17 ( Fig. 42.1 ). The contralateral arm and shoulder are kept in a dependent fashion below the level of the body to increase the amount of cranial flexion and rotation. The head of the patient is positioned so that the ipsilateral mastoid tip is the highest point of the surgical field. The ipsilateral shoulder is gently retracted or pulled to increase the distance between the patient’s head and the shoulder, thus increasing the degree of rotation of the microscope. Three key movements of the head are performed before it is fixed in a modified Mayfield head holder: (1) the head is flexed so that the chin is 1 cm from the sternum; (2) the head is then rotated contralateral to the lesion, thus increasing the angle between the atlas and the foramen magnum to the maximum; and (3) the head is laterally flexed to ∼30° toward the contralateral shoulder ( Fig. 42.2 ).

Incision

Although some authors use a straight vertical or “lazy S” incision, we prefer a “hockey stick” incision. The incision starts at the tip of the mastoid and is brought superiorly above the nuchal line if a wide exposure, including the ventrolateral pons, is required; otherwise, it can be extended just above the mastoid tip and curved medially to reach the midline. The incision is then extended inferiorly at the midline to the spinous processes of C2 and C3, depending on the inferior degree of exposure required ( Fig. 42.3 ).

The skin flap is elevated while preserving the fascia of the neck muscles. Leaving a fascial cuff attached to the bone facilitates reconstruction of the fascial-muscular plane at the conclusion of the procedure and allows watertight closure of this plane, minimizing chances of cerebrospinal fluid (CSF) leak.

Muscle takedown is conducted as a single flap, which is then rotated anteriorly and retracted with fishhooks attached to rubber bands. The rubber bands are then attached under tension to a Leyla bar. Retraction offered by fishhooks is advantageous for many reasons, including

1. 
   

   They provide “dynamic retraction,” which can be easily adjusted intraoperatively.

   
   
2. 
   

   By applying retraction and at the same time downward tension on the skin flap, it is possible to flatten the edges of the wound, thus decreasing the depth of the operative field.

Exposure

While taking down the muscle flap, it is important to recognize several bony landmarks. The transverse process of C1 is an important landmark in estimating the extent of lateral exposure. The posterior arch of the atlas is followed laterally and superiorly all the way to the sulcus arteriosus.

Exposure of the C2 lamina as far laterally as possible facilitates adequate retraction of the paraspinous muscles and allows adequate visualization and lateral extension of the approach even in patients with a short, thick neck.

The extradural portion of the vertebral artery is an important landmark for any lateral approach to the foramen magnum. The vertebral artery is identified in its extradural portion at the level of the sulcus arteriosus. The degree of exposure and mobilization of the vertebral artery vary according to the type of pathology, degree of involvement of the artery itself, and need for proximal control in its extradural portion.

In most cases, isolation of the vertebral artery may not be necessary. More often it is sufficient to recognize and identify the artery at its extradural/intradural transition, so it can be protected during condylar drilling. Isolation and skeletonization of the vertebral artery are indicated when the artery is encased by tumor and significant manipulation of the vessel is anticipated. In such a situation, the surgeon must be aware of the generous venous plexus surrounding the artery, which can be a source of tedious bleeding during exposure ( Fig. 42.4 ).

Depending on the inferior extent of exposure required, the ipsilateral portion of the posterior arch of the atlas can be removed with rongeurs or with a high-speed drill. Either a craniotomy or a craniectomy can then be performed. The superior extent of bone removal is dependent upon the degree of superior exposure required ( Fig. 42.5 ).

Partial drilling of the occipital condyle increases the degree of lateral exposure ( Fig. 42.6 ). In addition, partial removal of the posterior third of the condyle offers a flat exposure of the cervicomedullary junction, thus minimizing the need for retraction. The amount of condylar resection varies with the modification of the lateral approach being used. A summary of the different modifications of the far lateral approach variants and their key characteristics is given in Table 42.1.

In our experience, the majority of lesions affecting the intradural vertebral artery can be easily dealt with through limited drilling of the occipital condyle. Similarly, the majority of intradural mass lesions, such as meningiomas of the foramen magnum, can be easily resected without extensive condylar drilling by taking advantage of the additional space created during intratumoral debulking. Therefore, any unnecessary procedural steps, such as extensive condylar drilling (more than the posterior third) and exposure of the vertebral artery with coagulation of the venous plexus, should be avoided.18