Q1: What are the features seen in the lateral radiographs of the cervical spine (p 711)?

Lateral radiographs of the cervical spine show increased atlas-dens interval anteriorly and reduced posterior atlas-dens interval in flexion views but normal intervals in the extension views. The spinal canal shows corresponding reverse changes. In addition, degenerative changes are seen at the C5–C6 intervertebral disc space.

Q2: What abnormalities are seen on the MRI scan (p 711)?

Sagittal T2 weighted MRI scan of the cervical spine shows narrowing of the bony spinal canal at the arch of C1 vertebra. This in turn gives rise to generalised narrowing and distortion of the spinal cord at the craniomedullary junction. There is some signal change within the cord itself at this level.

Q3: What is the likely diagnosis?

Anterior atlantoaxial subluxation involving supra-axial spine.

Q4: How can this condition be treated?

Atlantoaxial subluxation, when symptomatic or is severe, can be treated by posterior cervical fusion with or without stabilisation at this level. Arthrodesis with autologous corticocancellous bone graft is augmented by various stabilisation techniques.

Discussion

Rheumatoid arthritis is a chronic, inflammatory disorder characterised by symmetric polyarthritis involving multiple joints. The cervical spine can be involved in 17%–86% of patients with rheumatoid arthritis. It can involve the supra-axial or subaxial spine with the craniovertebral junction being most often affected.¹ Cervical instability is the most serious and potentially lethal manifestation of rheumatoid arthritis. Also, a patient with rheumatoid arthritis and an unstable cervical spine will present a major anaesthetic problem if not stabilised. Atlantoaxial subluxation can be anterior, posterior, or lateral with the anterior type being most common.² It is usually the result of the destruction of joints, ligaments, and bone caused by erosive synovitis involving atlantoaxial, atlanto-odontoid, and atlanto-occipital joints.³ The inflammatory destruction via synovitis of the transverse ligament leads to anterior subluxation of the atlas on the axis. Erosion of the odontoid process frequently coincides with this process. Protrusion of the odontoid process posteriorly into the spinal canal can result in clinical symptoms or signs such as suboccipital pain or neuralgia and myelopathy. Further progression of the disease involves loss of alar and capsular ligament integrity and leads to further erosion of the dens.¹ If the inflammation persists in the atlantoaxial joints, their cartilage and bone structures will be eroded, the joint spaces will narrow down, and the atlas falls down around the axis (atlantoaxial impaction). When the atlantoaxial joint surfaces are eroded they become rough and the instability decreases (box 1).^2,4

Anterior atlantoaxial subluxation implies a widening of the joint space between the anterior arch of the atlas and the odontoid process. Radiographic diagnostic criteria have been developed as descriptors of existence and advancement of atlantoaxial subluxation. Two of the most commonly used criteria are anterior and posterior atlantoaxial intervals (AADI and PADI respectively). The determination of these intervals involves constructing a line that connects the centroids of the anterior and posterior rings of the atlas on a lateral plain radiograph at maximal flexion. The AADI is the distance along this line that measures the difference between the posterior surface of the anterior arch of the atlas and the anterior surface of the dens. The PADI, which is complementary to AADI, is the distance between posterior surface of dens and anterior surface of posterior arch of atlas. The normal AADI is 1 to 2 mm in adults (sometimes referred to as atlantodens interval, ADI).³ In children, the ADI may be as much as 4.5 mm and can show an increase of 0.5 mm in flexion. Atlantoaxial subluxation is defined as the AADI >3 mm or PADI ⩽14 mm.¹

Atlantoaxial subluxation of 9 mm reduces the area of the spinal canal (space available for the cord) to 60%. Theoretically, full rotation of 47° would further reduce the spinal canal to 21% which must cause compression of the cord (as cord normally occupies 27 to 30% of the spinal canal). The rheumatoid pannus also contributes to medullary compression in cases of atlantoaxial subluxation.³

The preoperative PADI value is a more reliable indicator of the development and severity of paralysis. Moreover, PADI is a better predictor than AADI of whether postoperative neurological recovery is expected. This is because PADI correlates closely with the space available for the spinal cord.^1,5

Patient controlled flexion and extension views are evaluated to determine the AADI and PADI. Instability is present with a 3 mm of AADI difference in flexion and extension views, although radiographic instability in rheumatoid arthritis is common and is not an indication for surgery. Cervical spine surgery is seldom indicated solely by radiographic findings. These patients should be examined by with MRI to get more information about possible spinal cord compression and also to visualise other soft tissues, such as pannus, before a final decision on surgical treatment.²

The assessment of cervical disease in patients with rheumatoid arthritis can be difficult due to coexisting systemic illnesses, neurological abnormalities, and rheumatoid polyarthropathy.⁵ The Ranawat scale is often used to grade rheumatoid myelopathy because its coarse grading structure accepts the major musculoskeletal disability in rheumatoid arthritis (see box 2).

Rheumatoid cervical spine changes are usually treated conservatively. The indications for surgery are well established in the symptomatic patient. Severe pain may be alleviated and neurological deterioration relieved. Controversy still exists, however, over the role of prophylactic procedures in asymptomatic patients.⁵ Surgery has been recommended in cases with extensive subluxation or gross instability even without neurological deficit to avoid development of myelopathy.

The commonest surgical procedure is posterior C1–C2 fusion and wiring⁵ with additional halovest stabilisation or C1–C2 transarticular screw fixation. Atlantoaxial subluxation that is not reducible may require removal of the posterior arch of atlas for cord decompression followed by occiput to axis fusion. An AADI of >7–10 mm or a posterior space (PADI) of less than 13 mm is a relative contraindication to surgery in other areas and the spine should be stabilised first.

Final diagnosis

Anterior atlantoaxial subluxation of the cervical spine in rheumatoid arthritis.