Positioning and Initial Dissection

The patient is placed in a lateral decubitus position with the head fixed by a three-point pin head holder in a neutral position. Intraoperatively, somatosensory and motor evoked potentials are monitored. Specific anesthetic techniques are needed to allow real-time neurophysiological monitoring throughout the operation. The soft tissue and bony extent of the tumor are carefully noted on the preoperative evaluation of the patient′s diagnostic imaging studies, which include thin-section magnetic resonance imaging (MRI) and computed tomography (CT). Any significant extension of the tumor into the foramen magnum and posterior fossa can be accessed through the lateral transcondylar approach, although the entire tumor may not need to be exposed.

Two alternative incisions can be used. The choice is based on whether the approach needs to be extended cephalad to the foramen magnum or the cervical spine. If the tumor extends above the foramen magnum, a wide C-shaped incision is centered on the ear and extended inferiorly, posterior and parallel to the sternomastoid muscle. To access a purely cervical tumor, an inverted L-shaped incision is used. The horizontal limb of the incision is made at the nuchal line, and the vertical limb starts from the mastoid tip and proceeds vertically along the lateral aspect of the neck ( Fig. 29.1 ). If a posterior cervical fusion is needed, a separate midline incision can be made posteriorly, independent of the lateral cervical incision.

The dissection proceeds down along the muscle layers to the lateral masses of the vertebrae. The subcutaneous dissection along the inferior aspect of the incision must be performed carefully, with the surgeon looking for the accessory nerve as it exits the sternocleidomastoid muscle to enter the trapezius. This nerve can sometimes be mistaken for the cervical plexus of nerves formed by the branches of the upper cervical roots. The sternocleidomastoid muscle is detached from the mastoid and suboccipital region and reflected anteriorly ( Fig. 29.2A ). The splenius capitis and the longissimus capitis are detached from the mastoid region. The transverse process of C1 acts as a constant landmark that can be palpated easily just caudal to the mastoid process. The deepest muscle layer is formed by the uppermost slip of the levator scapulae and the superior and inferior oblique muscles and the rectus capitis lateralis, all of which attach to the transverse process of C1 ( Fig. 29.2B ). These attachments at the C1 transverse process are dissected and reflected inferiorly. The small segmental muscles deep to this layer are incised in a linear manner to access the lateral aspect of the spine.

Identification of the Vertebral Artery

Typically, the vertebral artery is identified, isolated, and mobilized mostly when extradural tumors are being treated. The lateral dissection through the muscles of the posterior triangle can produce troublesome bleeding from the rich plexus of veins. The vertebral artery is surrounded by a periosteal sheath, which encloses a venous plexus that communicates with the epidural plexus. The artery is identified by bony and muscular landmarks instead of by its pulsation, which may be imperceptible ( Fig. 29.3 ). Between C2 and C1, the artery can be located caudal to the inferior border of the inferior oblique muscles. In the coronal plane, the artery exits at the superior portion of the foramen transversarium of C2, posterior to the plane at which it enters the foramen from below. Between C2 and C1, the ventral ramus of the C2 root crosses the lateral surface of the artery. After emerging on the superior surface of the C1 foramen, the artery turns posteriorly along the upper surface of the C1 posterior arch where it lies deep to the superior oblique muscle. The venous plexus about the vessel closely adheres to the joint capsule of the atlanto-occipital articulation as it winds around the joint to enter the dura. The artery can be variably redundant above C1 and between C2 and C1 to accommodate the motion at these spinal segments. Keeping the head and neck neutral minimizes distortion of the course of the vertebral artery and facilitates the dissection.

The arterial wall is exposed by opening the surrounding periosteal sheath and venous plexus. The venous plexus is coagulated and incised longitudinally and then opened circumferentially to expose the full circumference of the artery. The foramen transversarium is opened with rongeurs, and the artery is displaced from C1, if needed, to access the ventral C1-C2 region for extradural pathology ( Fig. 29.4 ). Similarly, the foramen transversaria of C2-C3 can be opened, isolating the entire length of the artery as needed. Mobilizing the artery allows access to the entire lateral aspect of the spinal column (articular facets, lamina, transverse process, and side of the vertebral body).