Surgical Approaches to Craniovertebral Junction Congenital Malformations, Chiari Malformations, and Cranial Settling (Invagination)




Surgical Anatomy of the Craniocervical Junction


The craniocervical junction extends from the occipital bone to the second cervical vertebra. Understanding the surgical anatomy of the bony structures of the lower clivus to C2, associated articular processes and ligaments, muscular attachments, and relationships to the vertebral artery and lower cranial nerves is crucial for managing diseases in this region. The musculature of the neck is an important part of the surgical anatomy in this area. Many important muscles attach to this bony structure, including the superior and inferior obliques, levator scapulae, and rectus capitis muscles ( Fig. 8-1, A ). The longus capitis muscle attaches to the lower clivus, and the rectus capitis muscle attaches anterior to the ventral surface of the occipital condyle.




Figure 8-1


Bony and soft tissue anatomy of the craniocervical junction.


The occipital bone is divided into a squamosal segment, a basal segment, and paired occipital condyles. The basal portion of the occipital bone lies anterior to the foramen magnum and is joined to the sphenoid bone by the clivus, which arises at a 45-degree angle. The occipital condyles, located lateral to the anterior half of the foramen magnum, articulate with the atlas in a cup-and-ball mechanism. The C1 transverse process projects anteriorly and laterally (see Fig. 8-1, B ).


Several ligaments are encountered at the craniocervical junction traversing from ventral to dorsal. The anterior longitudinal ligament covers the dens anteriorly. Posterior to the dens is the cruciate ligament, which is made of the transverse ligament and associated superior and inferior longitudinal bands. The cruciate ligament is followed by the tectorial membrane (the rostral extension of the posterior longitudinal ligament) and then the dura (see Fig. 8-1, C ).


Cranial nerves IX and X arise from the lateral aspect of the medulla, and cranial nerve XI arises from the medulla and upper cervical spinal cord and travels upward dorsal to the dentate ligament. These nerves exit the skull base through the jugular foramen, medial to the jugular bulb. Anterior and superior to the occipital condyle is the hypoglossal canal, which transmits the hypoglossal nerve (cranial nerve XII).


The vertebral artery enters the transverse foramen of C2 and ascends through the C1 transverse process. Surrounded by a rich venous plexus, much like the C2 nerve root, it wraps around the superior aspect of the C1 lamina in the sulcus arteriosus by crossing medially behind the articular capsule of the atlantooccipital joint to penetrate the dura. This segment of the vertebral artery above C1 runs in the anatomic suboccipital triangle, which is bordered by three muscles: the obliquus capitis superior extends from the occipital bone to the C1 transverse process; the obliquus capitis inferior extends from the C1 transverse process to the C2 spinous process; and the rectus capitis posterior major extends from the C2 transverse process to the occipital bone.


Basilar invagination is the term for a developmental anomaly of the craniovertebral junction (CVJ) in which the odontoid prolapses into the foramen magnum. It is the prototypic disease process that may be associated with other bony and central nervous system (CNS) abnormalities at the CVJ, such as Chiari malformation, and it necessitates surgical intervention; therefore this pathology and its management will be the focus of this chapter. Reported etiologies of basilar invagination include basiocciput (clivus), occipital condyle, or atlas hypoplasia; incomplete ring of C1 with spread of the lateral masses; achondroplasia; and atlantooccipital assimilation. Between 25% and 35% of patients with basilar invagination also have associated CNS abnormalities, including Chiari malformation, syringomyelia, syringobulbia, and hydrocephalus.




Basilar Invagination


The literature inaccurately uses the terms basilar invagination, basilar impression, cranial settling, and platybasia synonymously ; however, basilar impression refers to the acquired form of basilar invagination, the result of softening of the bone at the base of the skull in cases of Paget disease, osteomalacia, tumor, infection, hyperparathyroidism, osteogenesis imperfecta, Hurler syndrome, and rickets. Cranial settling, a form of basilar impression, is a term reserved for cases of rheumatoid arthritis, whereas platybasia is a flattening of the skull base that presents in combination with basilar invagination or in isolation.


Indications for Surgical Treatment


Basilar invagination can result in progressive and profound neurologic deficits and death if left untreated. The majority of patients come to medical attention with some degree of neurologic disturbance ; this is an absolute indication for surgical treatment. Surgery may also be performedprophylactically to prevent development of potentially irreversible neurologic deficits. Finally, a subset of patients may be neurologically intact with mild basilar invagination, which may be followed clinically and radiographically for progression.


Before surgical treatment, patients with basilar invagination may undergo a trial of cervical traction in an attempt to reduce the odontoid. As a general rule, basilar invagination that is not reducible is treated with anterior decompression followed by posterior occipitocervical fusion.


Anterior Surgical Approaches


Transoral-Transpalatopharyngeal Approach


The transoral-transpalatopharyngeal approach can be used to access the anterior CVJ. The lower clivus, atlas, and axis readily come into view via this approach ( Fig. 8-2 ). When combined with palatal or mandibular splitting procedures, additional rostral and caudal exposure may be attained.




Figure 8-2


Exposure from a standard transoral approach from inferior clivus to middle to lower C2 body.


The operative procedure consists of positioning the patient supine with fiberoptic orotracheal intubation. A throat pack is placed, and the oropharynx is prepped with 10% povidone-iodine and hydrogen peroxide. Prophylactic antibiotics are given to the patient, usually a third- or fourth-generation cephalosporin. The oral cavity and pharynx are exposed with the help of a Dingman or Crockard oral retractor ( Fig. 8-3 ). Retracting the soft palate superiorly with a suture and transnasal rubber catheter can enhance exposure. Additional exposure may be obtained by incising the soft palate in the midline, starting on one side of the base of the uvula and retracting laterally.




Figure 8-3


Placement of the Dingman or Crockard oral retractor system.


A midline incision is made into the posterior pharynx extending from the rostral clivus to the C2–C3 disk space, exposing the prevertebral fascia and longus colli muscles. The posterior pharyngeal wall is dissected laterally, exposing the clivus, anterior arch of C1, and odontoid. Subperiosteal dissection of the anterior longitudinal ligament is then done with electrocautery up to a width of 3 cm to avoid injury to the eustachian tubes, vertebral arteries, and hypoglossal nerve laterally.


The C1 anterior arch is drilled out, and the inferior clivus is removed using a high-speed air-powered drill and Kerrison rongeurs. The dural attachment at the inferior edge of the clivus is dissected carefully at this point to avoid hemorrhage at the circular sinus (basilar plexus) and a cerebrospinal fluid (CSF) leak. The ligamentous attachments to the odontoid, the alar and apical ligaments, are similarly dissected carefully and sharply, and the odontoid is drilled and hollowed out. The cortical shell is then dissected from the surrounding ligaments, rostral to caudal. Caudal resection of the odontoid may create a mobile rostral fragment that compresses the cervicomedullary junction. When the odontoid is completely removed, the cruciate ligament and tectorial membrane are incised to expose the dura.


The wound is closed in layers, including longus colli and longus capitis muscles, posterior pharyngeal muscles, and posterior pharyngeal mucosa. Posterior procedures for both decompression and stabilization may need to be performed. These procedures may be performed during the same anesthetic setting, or they may be staged for another day. If the posterior approach is staged, the patient may need to be placed in a halo vest until definitive stabilization can be performed via an occipitocervical fusion.


A nasogastric tube is left in place for postoperative parenteral nutrition, and endotracheal intubation is maintained until oral and airway edema improves. Consideration should be given to performing the procedure with intraoperative neurophysiologic monitoring.


Advantages and Disadvantages of the Anterior Approaches


The entrance to the oral cavity between the upper and lower incisors must be at least 2.5 to 3.0 cm to have adequate loupe or microscope visualization and to introduce instruments in a transoral approach. This opening may not be possible in elderly patients with rheumatoid arthritis or in patients with mandibular disorders. These patients may require a median glossotomy, circumglossal approach, or mandibular splitting procedure to access the oral cavity.


Complications associated with the transoral approach include velopharyngeal incompetence, hypernasal speech and nasal reflux, dental injury, edema or tongue necrosis, upper airway obstruction from retropharyngeal edema, posterior pharyngeal wound dehiscence, dysphagia, odynophagia, pharyngeal cellulitis, meningitis, and temporomandibular joint syndrome.


Posterior Approaches


In cases of basilar invagination, posterior procedures may be performed for foramen magnum and posterior cervical decompression and for occipitocervical stabilization. Numerous methods of obtaining occipitocervical stabilization have been well described, including the use of methylmethacrylate, onlay bone graft with wires, contoured rods with wires, and metal plates with wires or screws. Internal fixation is advised to guarantee postoperative stability and to enhance the rate of arthrodesis. Occipitocervical constructs should extend at least down to C2.


Occipital Screw Technique


Prior to drilling, anatomic landmarks are identified. Four bony landmarks on the outer occipital cortex should be visible: 1) the posterior rim of the foramen magnum, 2) the superior nuchal line, 3) the inferior nuchal line, and 4) the external occipital protuberance ( Fig. 8-4 ). Safe placement of occipital instrumentation is between the inferior and superior nuchal line. Using the stop-drill or stepwise drill technique, occipital screws 4.0 to 4.5 mm in diameter may be placed in a bicortical fashion in 2-mm increments. Drill and screw trajectories should be angled medially toward the thick midline keel, and left and right occipital screws are staggered to avoid intersection of screw paths.




Figure 8-4


Bony landmarks of the occipital skull.


C1 Lateral Mass Screw Technique


The posterior arch of C1 is identified and followed laterally to visualize the lateral masses. Notably, there is a step-off between the medial aspect of the C1 lamina and the medial surface of the C1 lateral mass; this anatomic feature is different in adults whose medial C1 lamina is flush with the medial C1 lateral mass. Subperiosteal dissection of the C2 nerve roots and associated venous plexi from the junction between the posterior arch of C1 and lateral masses is performed to minimize bleeding. Alternatively, the C2 nerve roots and venous plexi can be coagulated with bipolar electrocautery and divided with little clinical significance. After palpating the medial and lateral surfaces of the lateral mass, a pilot hole may be drilled in the center of the lateral mass, usually no more than 2 to 3 mm from the medial surface. The rest of the placement of the C1 lateral mass screws proceeds using the technique described by Harms and Melcher, using either 3.5- or 4.0-mm diameter polyaxial screws ( Fig. 8-5 ). The drill and the screw trajectory are angled medially 0 to 5 degrees and aimed at the superior half of the anterior arch of C1 on fluoroscopy. Bicortical purchase is usually achieved about 4 mm from the anterior cortex of the anterior arch.


Jul 11, 2019 | Posted by in NEUROSURGERY | Comments Off on Surgical Approaches to Craniovertebral Junction Congenital Malformations, Chiari Malformations, and Cranial Settling (Invagination)

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