Dorsal exposures of the craniovertebral junction are common in neurosurgical practice. This exposure is used for many procedures, including the following:
Decompression of the high cervical spine for congenital or degenerative spinal stenosis
Occiput to cervical or atlantoaxial fusions
Decompression for Chiari I malformations
C2 ganglionectomy for occipital neuralgia
Intradural tumor removal or spinal cord exposure in the high cervical spine or dorsal brainstem
The approach is straightforward, but because of the complex anatomy and biomechanical function of this area in the spine, special considerations are necessary when exposing this region. It is appropriate for treating lesions from the posterior fossa to the upper cervical spine, and lesions dorsal to or intrinsic to the spinal cord can be approached in this fashion. Ventral pathology that has displaced the cord laterally, such as ventral meningiomas, can often be treated with the dorsal approach as well. Lesions directly anterior to the spinal cord are better treated with ventral or lateral skull base approaches.
4.2 Anatomy
Several complex muscular attachments in the upper cervical spine and occiput allow for the complex motions in this area. More than 50% of the axial rotation and flexion/extension of the neck occurs between the atlanto-occipital and atlantoaxial articulations. The posterior musculature responsible for these motions will thus be stripped from its bony attachments during a standard dorsal approach to the spine ( ▶ Fig. 4.1). Unnecessary damage to these soft tissues should be avoided, and careful suture closure at the end of the procedure will limit the amount of postoperative disability the patient experiences.
Fig. 4.1 (a–c) Muscular layers of the dorsal high cervical spine.
4.3 Preoperative Considerations and Patient Selection
In planning the surgical approach, the surgeon must ensure that the pathology can be treated from a direct dorsal approach. This is important for attempts at resection of lesions ventral to the spinal cord. For lateral pathology, the proximity and function of the vertebral arteries may need to be examined. Although this does not preclude the use of a dorsal approach, any anticipated need for a vascular dissection may warrant preoperative vascular imaging, and in most cases a computed tomographic angiogram or magnetic resonance angiogram will suffice. For highly vascular pathology, however, a formal angiogram may be necessary for precise visualization of the small vessels and will also be an opportunity for potential embolization of feeding vessels. For patients with cervical stenosis, the patient’s neck mobility in flexion and extension should be checked. Surgical positioning should not exceed the confines of this motion. In addition, the patient should be counseled on the potential for postoperative restrictions in neck range of motion and the potential for new cervical pain or disability.
4.4 Operative Procedure
4.4.1 Special Equipment
Mayfield skull clamp (Integra LifeSciences Corporation, Cincinnati, Ohio)
Self-retaining retractors (curved Wheatlander or cerebellar)
High-speed drill
Thin footplate Kerrison rongeurs (1, 2, and 3 mm)
Woodson elevator
Surgical hemostatic (Gelfoam; Pfizer, Inc., New York, New York, and powdered Gelfoam, bone wax)
4.4.2 Anesthetic Considerations
Coordination with the anesthesiologist is a critical element of the surgery. For cases with high cervical stenosis, a fiber-optic intubation may be necessary if neck movements result in narrowing of the spinal canal. For example, in patients with atlantoaxial subluxation, the sagittal canal diameter may be reduced in flexion. In contrast, patients with cervical spondylosis will typically have greater cord compromise in extension. The maintenance of adequate blood pressure for spinal cord perfusion is important for older patients and for those with cord compromise; invasive blood pressure monitoring is indicated in those cases. In addition, for surgery involving electrophysiological monitoring, coordination between the anesthesiologist and the electrophysiologist is necessary, especially when motor evoked potential monitoring is used.
4.4.3 Positioning
Patients can be placed into either the prone or lateral position. The lateral position has several advantages. First, the approach to lateral lesions can be tailored so that gravity will assist in the retraction of sensitive structures such as the cerebellum. Second, blood naturally drains away from the surgical field, aiding with visualization. The prone position allows for a more symmetrical view of the spine and should be used for cases involving spinal instrumentation. The prone position is also more efficient for obtaining intraoperative radiographs and allows an assistant to stand on the side opposite the operating surgeon. For either position, the neck should be placed in slight flexion for patients with stenosis and in slight extension for those with subluxations. The bed should be placed into a slight reverse Trendelenburg to elevate the head above the level of the heart; this not only facilitates venous drainage but also minimizes the risk of iatrogenic blindness form ocular engorgement and posterior ischemic optic neuropathy.
4.4.4 Operative Technique
Muscular and Soft Tissue Dissection
Most incisions will be made in the midline, although paramedian approaches may also be used for laterally located lesions. After skin incision, the nuchal fascia is identified. The midline will contain an avascular plane that can be used to minimize muscular bleeding. Bovie electrocautery should be used to open this fascia in the sagittal plane until the spinous processes are encountered. Dissection should proceed in the subperiosteal plane over the laminae or occiput. Lateral retraction with a Cobb elevator is useful during bony exposure. At the occipitoatlantal and atlantoaxial junctions, there is typically a gap where there is no bony protection of the neural elements. The bifid prominence of the C2 spinous process will aid in localization of the level. Because of the lack of bony coverings, care should be exercised when using electrocautery in this region to avoid inadvertent durotomies. The posterior atlantoaxial ligament between the occiput and atlas and the ligamentum flavum between the atlas and axis can be easily dissected from the overlying soft tissues. The easiest way to accomplish this is to expose the base of the occiput at the foramen magnum, the posterior arch of C1, and the laminae of C2 with electrocautery. Blunt spreading of the midline will then open the muscles, revealing these ligaments. Self-retaining retractors can then be placed.
Laminectomy
After soft tissue dissection of the appropriate spinal levels, bony removal can begin if necessary. For patients without significant spinal stenosis, a Leksell rongeur can be used to remove the relevant spinous processes. A Woodson elevator is then used to open the midline raphe of the posterior atlantoaxial ligament and ligamentum flavum. The sublaminar space is usually capacious, and the Woodson elevator can be used to free any epidural adhesions. Thin footplate (1, 2, and 3 mm) Kerrison rongeurs can be used to remove laminar bone at C1 and C2. The bone at the rim of the foramen magnum tends to be thick and may need to be thinned before removal. Exposure to the lateral masses is typically performed, resulting in a laminectomy that is 15 to 18 mm wide ( ▶ Fig. 4.2 a).
Fig. 4.2 (a) Bony margins for standard midline laminectomy for central decompression or spinal cord exposure (red); location of the C2 ganglion of ganglionectomy (blue). (b) Entry and starting points for spinal instrumentation: midline occipital keel fixation (yellow), sublaminar wiring (purple), C1 lateral mass screws (blue), C2 crossing laminar screws (pink), C2 pedicle screws (red), and C1–2 transarticular screws (green).
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