Rheumatoid arthritis with vertical subluxation and an associated anterior atlantoaxial subluxation at C1-C2

Findings

Figure 1: Axial CT image of the brain displayed in bone windows at the level of the posterior fossa. The dens is subluxed superiorly into the foramen magnum.
Figure 2: Lateral radiograph of the cervical spine in the neutral position. The superior half of the odontoid process is difficult to visualize secondary to overlapping osseous structures. The position of the anterior arch of the atlas in relation to the dens is also not clearly demonstrated. Chronic degenerative changes are incidentally noted at C3-C4.
Figure 3 and Figure 4: T1-weighted and T2-weighted sagittal MR images of the cervical spine. The dens is seen to extend superiorly, occupying a position within the foramen magnum. The anterior arch of the atlas has an abnormal position in relation to the odontoid process, maintaining a position equivalent to the plain film finding of Clark station II. The posterior aspect of the anterior atlas also has a position more than 2.5 mm from the anterior border of the dens.


Diagnosis: Rheumatoid arthritis with vertical subluxation and an associated anterior atlantoaxial subluxation at C1-C2


Vertical subluxation is a malalignment disorder of the cervical spine affecting the C1-C2 junction in patients with rheumatoid arthritis. The definition of vertical subluxation is synonymous with other frequently used terms such as atlantoaxial impaction, vertical atlantoaxial subluxation, odontoid upward migration, cranial settling, and basilar invagination of the odontoid process. Vertical subluxation has been reported in 5 to 22% of patients with rheumatoid arthritis. The inflammatory arthropathy of rheumatoid arthritis alters the synovial structures of the cervical spine and predisposes patients to anterior, vertical, lateral and/or posterior atlantoaxial subluxation. Although anterior subluxation is the most frequent to occur, vertical subluxation is considered to have the highest potential for associated neurological complications.

Rheumatoid arthritis causes progressive erosive changes of the osseous and articular structures at C1-C2, which may subsequently lead to collapse of the facet joints. This process of vertical subluxation stems from extension of the superior aspect of the odontoid process into the foramen magnum. Potentially devastating neurological complications or sudden death can occur as the result of compression of the adjacent brainstem, spinal cord, cranial nerve roots, spinal and/or vertebral arteries. Vertical subluxation may also appear in association with anterior subluxation, which results from the disruption of the transverse atlantoaxial ligament by pannus formation.

Initial evaluation can be performed with plain radiographs. Not all patients with inflammatory arthropathy of the cervical spine are symptomatic, demonstrating the need for a high index of clinical suspicion in patients with chronic rheumatoid arthritis. A lateral radiograph of the cervical spine in a neutral position can be evaluated for vertical subluxation. A multitude of techniques, with varying degrees of sensitivity and specificity, exist for the radiographic analysis of vertical subluxation on plain film. A common methodology includes the use of a MacGregor’s line, which extends from the posterior hard palate to the occiput. An 8 mm extension of the dens above the MacGregor’s line in men and 9.7 mm in women has been described as proof of vertical subluxation. Clark station is another simple method, with the odontoid process divided into three equal parts in length within the sagittal plane. The most superior section is station I, the middle section is station II, and the most inferior section is station III. If the anterior arch of the atlas lies anterior to station II or III of the dens, then vertical subluxation is present. A potential pitfall of radiographic analysis is nonvisualization of the dens, which may occur as a result of overlapping osseous structures, osteopenia or extensive erosion of the dens by rheumatoid arthritis. In addition to the evaluation for vertical subluxation on the lateral film, the distance between the anterior arch of the atlas and the dens must be measured. A distance of greater than 2.5 mm in adults indicates an associated anterior subluxation.

Cross-sectional imaging can be utilized in difficult cases or symptomatic patients with equivocal plain films to establish a diagnosis. The multiplanar capabilities of computed tomography (CT) and magnetic resonance (MR) imaging can better define the anatomic relationships among the occiput, axis and atlas than plain film radiographs. Any destructive osseous changes related to rheumatoid arthritis are also best demonstrated by CT. When neurological compromise is suspected, the modality of choice for brain or spinal cord injury is MR.