Craniovertebral Junction Deformities

The craniovertebral junction (CVJ) is subject to deformities caused by trauma, congenital disorders, degenerative disease, infection, and tumors. The goals of management of pathology of the CVJ are to identify instability, decompress neural elements, and provide structural support for the head. Instability can be identified using a number of craniometric and morphometric indices. Many of these criteria were developed in the pre–computed tomography (CT) and magnetic resonance imaging (MRI) era, and therefore a description of new indices using “newer” technologies will be presented along with historical ones. The criteria of instability requiring stabilization differ depending on the underlying pathology. For instance, instability caused by acute trauma has tightly defined criteria for instability as opposed to the chronic instability caused by degenerative disease such as rheumatoid arthritis. The plethora of grading systems causes some confusion regarding management decisions. Attempts have been made to create treatment algorithms for pathology of this complex region; however, high-quality medical evidence relating to many important questions is not available. Therefore, treatment decisions are made with a reliance on a thorough knowledge of the biomechanics, anatomy, and physiology of the CVJ. New technology continues to drive improvement of diagnosis, management, and outcomes of CVJ disease. This chapter reviews the diagnosis and management of this complex region.

Clinical and Radiographic Anatomy of the Craniovertebral Junction

The CVJ consists of the occiput, atlas, axis, and associated ligaments. The CVJ is a compromise between strength and flexibility. The bones and ligaments provide structural support for the head and protect the brain stem and upper cervical spinal cord. Concurrently, these structures allow for significant movement in flexion-extension, lateral bending, and rotational planes. The following discussion is limited to an overview of the anatomic landmarks and indices used to define abnormal relationships, because discussions of the embryology and anatomy of the CVJ are presented elsewhere in this text.

The normal relationships among the occiput, atlas, and axis have been studied and are well described. The use of CT with sagittal reconstruction and multiaxial MRI has greatly enhanced the understanding and definition of abnormal anatomic relationships at the CVJ. Various measurements have been described to delineate normal from abnormal pathologies. In other words, there are different indices for trauma, degenerative disease, cranial settling, and basilar impression.

Trauma

Injury to the CVJ can manifest as ligamentous injury or fracture of the occiput, atlas, or axis. These injuries are as follows: occipital condyle fractures, atlanto-occipital dislocation (AOD), atlas fractures or C1 burst fractures, and C2 fractures of the odontoid or pars interarticularis. Radiographic criteria have been established to help assess clinical stability. Although many of these criteria are used traditionally, they are by no means standardized criteria for each type of injury. Determining the instability of these fractures is of primary importance in determining management.

Degenerative Disease

Abnormalities of bone metabolism, degeneration of synovial joints, or abnormal stresses placed on the CVJ can result in basilar impression. The principles of diagnosis and treatment remain the same, regardless of the cause.

Rheumatoid arthritis (RA) is the most common degenerative disorder of the CVJ. RA is characterized by destruction of synovial joints. The disease is estimated to affect 0.8% of the Caucasian adult population in the United States, about 2.2 million people. The cervical spine is the second most commonly involved region of the body. The degenerative changes seen in the cervical spine are progressive in nature. Translational subluxation of C1-2 occurs first, followed by vertical subluxation of C1 on C2. Compression of the spinal cord and brain stem occurs as the lateral mass joints are eroded by inflammatory synovitis and the odontoid ascends through the atlas and the foramen magnum. Oda and colleagues found a predictable progression of transverse subluxation to reducible vertical subluxation to irreducible vertical atlantoaxial subluxation. Fujiwara and colleagues redemonstrated this progression and also noted an association between the severity of RA and the progression of subluxation. Patients with less severe RA develop transverse subluxation, those with RA of moderate severity develop a combination of transverse and vertical subluxation, and those with more severe RA develop vertical subluxation. Basilar impression is the ascension of the odontoid process into the posterior cranial fossa and is defined by an abnormal position of the dens with respect to the foramen magnum. As the dens ascends into the posterior fossa, variable symptoms, which include but are not limited to myelopathy and lower cranial nerve deficits, develop. Although motor weakness and sensory changes due to myelopathy are the most common signs, the earliest sign of spinal cord dysfunction is posterior column function. The incidence of cervical fusion in patients with rheumatoid arthritis has been decreasing possibly due to the use of new medical therapies.

Determination of transverse C1-2 instability is performed using the anterior dental interval (ADI) and posterior anterior dental interval (PADI). An ADI greater than 3 mm and a PADI less than 14 mm is considered to be abnormal. Vertical subluxation is measured using the Ranawat method. The vertical distance between the center of the pedicles on the axis to a line connecting the ventral and dorsal arches of the atlas is measured. If this distance is less than 13 mm in men and 15 mm in women, vertical subluxation is diagnosed.

Many indices are used to screen for basilar impression from plain radiographs. These indices use bony anatomic landmarks and are the Clark station, McRae line, Chamberlain line, McGregor line, Redlund-Johnell criterion, Ranawat criterion, Fischgold-Metzger line, and Wackenheim line. Riew and associates evaluated the sensitivity and specificity of these standard screening measurements. The most sensitive measurements (the tests with the fewest false-negative results) are the Wackenheim line, at 88%, and the Clark station, at 83%. The Redlund-Johnell criterion is the most specific measurement (it has the fewest false-positive results) at 76%. The Redlund-Johnell measurement has the highest positive predictive value (PPV) of 68%. The Wackenheim line has a positive predictive value of 48%. The Fischgold-Metzger line has a negative predictive value of 100%. The McRae line has the lowest negative predictive value of 75%. The study also found that identification of bony landmarks is difficult and precludes accurate application of these measurement techniques in many cases. Riew and associates have recommended a combination of tests to screen for basilar invagination: the Clark station, the Redlund-Johnell criterion, and the Ranawat criterion.

The goals of treatments of basilar impression are to decompress the brain stem and spinal cord and to reestablish support for the head. Decompression of the neural elements can be achieved either directly or indirectly. Indirect decompression is performed via closed reduction through the use of traction and manual manipulation. Long-standing lesions are unlikely to be reducible. However, a trial of craniocervical traction is warranted. Use of a halo ring provides multiple points of skull fixation and allows for fixation to the thoracic vest once the deformity is reduced. Traction is started with a weight of approximately 7 pounds and is increased to a 15-pound maximum. Attempted reduction is generally limited for 5 to 7 days because the likelihood of further benefit is limited after this period and complications related to immobilization increase.

Special beds have been designed to help prevent complications of immobilization.

The ventral rheumatoid pannus often resolves once the C1-2 junction is fused, which can indirectly decompress the brain stem and spinal cord. However, occasionally a ventral transoral approach needs to be used to obtain adequate decompression. Intraoperative image guidance may be used to help with the decompression in a region where anatomic landmarks have been distorted. Endoscopic techniques have emerged to avoid the complications associated with the transoral approach.

Congenital Disorders

CVJ abnormalities are a common component of many congenital disorders. These disorders can be broadly grouped as connective tissue disorders (Down syndrome), Klippel-Feil syndrome, osteochondrodysplasia (e.g., achondroplasia), mucopolysaccharidoses (e.g., Morquio and Lesch-Nyan syndromes), and skeletal dysplasia (i.e., osteogenesis imperfecta), as well as other disorders of development including Goldenhar syndrome, Conradi syndrome, and the Klippel-Feil triad. CVJ abnormalities are most commonly seen in Down syndrome (20%) and Morquio syndrome (50%). The anatomic abnormalities of the craniovertebral junction observed in these disorders and their natural history can help guide treatment decisions.

The following discussion reviews the more common disorders of Down syndrome, Morquio syndrome, and achondroplasia to help delineate pertinent concepts. Description and management of other mentioned syndromes can be found in listed references.

CVJ abnormalities in these patients have variable causes. Atlantoaxial instability is a complex entity that has many causes. No matter the cause, the problem requiring fixation is the lack of fixation between the dens and the anterior arch of C1. Laxity of the normal ligamentous structures in the CVJ is one such cause and is likely the major cause in patients with connective tissue disorders, such as Morquio syndrome and Lesch-Nyhan syndrome. It is also a component of Down syndrome. Aberrant ossification of the dens occurs; this could be due to ligamentous laxity and disturbances in blood supply during development because of inordinate mobility Patients with skeletal dysplasia, such as osteogenesis imperfecta, have abnormal collagen deposition, resulting in brittle bones that easily develop multiple microfractures. Accumulation of these microfractures leads to ascension of the dens and medial skull base, causing basilar invagination.

Treatment of these lesions is guided by the symptoms and syndrome involved. For example, many children with Down syndrome have asymptomatic atlantoaxial instability. Large cohort studies have not demonstrated increased rates of neurologic injury in children with Down syndrome and atlantoaxial instability as compared with their peers without abnormal ADIs. These studies also do not demonstrate a protective effect of restricted activity. Reduction and stabilization of the CVJ in these cases is not indicated unless there are clear signs of brain stem or upper cervical spinal cord compression. Generally, more than 7 mm of atlanto-occipital subluxation has been used as a benchmark for recommending fusion. This is, in part, based on the work of Browd and associates, which demonstrated that atlanto-occipital instability in these patients with > 7 mm of subluxation is often progressive.

Morquio syndrome and other forms of skeletal dysplasia are often found to have an os odontoideum and ligamentous laxity. The cartilaginous os odontoideum deforms with flexion and extension. Radiographically, C1-2 instability manifests as changes in the ADI and is a late finding in affected children. By the time this condition is seen, myelopathy is nearly always present. These patients benefit from prophylactic fusion prior to the onset of myelopathy. The high prevalence of cervical myelopathy demands careful discussions between the surgeon and anesthesiologist as intubation is anything but benign in these cases. The ideal age for this operation has not been determined. Ransford and coworkers have suggested that the surgery be performed at 4 years of age unless myelopathic signs develop earlier. Dorsal occipitocervical fusion can result in complete ossification of the dens, which supports the role of ligamentous laxity in the formation of os odontoideum. It is important to note that other spine abnormalities and instability are not prevented by occipitocervical fusion and must be screened for in this population.

The treatment of patients with other congenital disorders of the CVJ varies depending on the syndrome, symptomatology, and relevant anatomy. The natural history of the disorder as well as the individual anatomic characteristics present in each patient take precedence in treatment decisions. Traction can be used if there is a suspicion that the lesion is reducible. Use of a halo ring allows for the application of corrective forces and subsequent fixation when reduction is completed. If a lesion proves to be irreducible, either after a trial of reduction or radiography, surgery is indicated to decompress the brain stem and spinal cord and to stabilize the CVJ. Traditionally, open surgical fixation, from an anterior or posterior perspective, has been the mainstay of treatment.

Open posterior cervical fixation carries with it a significant risk of injury to the vertebral artery, requiring increased attention to its course on preoperative studies. In the future, endoscopic surgery may provide options for less invasive techniques for accessing the occipitocervical junction, thereby allowing simpler but equally effective means of anterior surgical reduction or fixation to become more common.