The most hazardous complication of ankylosing spondylitis is compression of the cord from seemingly minor trauma that has resulted in fracture–dislocation of the cervical (or lumbar) vertebrae. Fox and colleagues treated 31 such patients in a 5-year period; the majority of unstable fractures that required surgical fixation were in the cervical region, and several patients had fracture–dislocations at two levels. The instability at the upper spinal levels may be difficult to detect radiologically, and caution should be observed in allowing patients to resume full activity after a neck injury if the cervical spine is involved by ankylosing spondylitis. Careful flexion and extension radiograph views usually, but not always, demonstrate the instability.
As mentioned briefly earlier, multiple arachnoid cysts in the thoracic or lumbar region are associated with ankylosing spondylitis (and with Marfan syndrome).
Rheumatoid Arthritis of the Spine
The spinal changes of rheumatoid arthritis differ somewhat from those of ankylosing spondylitis although the latter too may be a cause of atlantoaxial dislocation (see further on under “Anomalies at the Craniocervical Junction”). The ligaments that attach the odontoid to the atlas and to the skull and the joint tissue are weakened by the destructive inflammatory process. The subsequent dislocation of the atlas on the axis may remain mobile or become fixed and give rise to an intermittent or persistent paraparesis or quadriparesis. Similar effects may result from a forward subluxation of C4 on C5 (see Nakano et al). Atlantoaxial dislocation is known to be a cause of collapse and sudden death. If the upper cervical cord is compressed, the odontoid process must be removed and C1-C2 decompressed and stabilized. Other levels of the spine are less frequently affected.
Ossification of the Posterior Longitudinal
Compressive cervical myelopathy caused by this process occurs almost exclusively in patients of Japanese extraction and has been demonstrated to us as an almost mundane finding by colleagues in Hawaii. The clinical signs are much the same as those of cervical spondylosis, but the radiologic appearance of cancellous bone along a segment of the posterior longitudinal ligament is unique. The ligamentous calcification can be seen on plain films, CT and MRI as a vertical longitudinal row of calcification and may be mistaken for spondylotic change. The ossified areas may enlarge enough to form islands of bone marrow. Laminoplasty with enlargement of the spinal canal has been successful.
This unusual myelopathy has usually been considered in discussions of the motor neuron disorders because of its characteristic features of chronic wasting of one, or less often, or both hands and forearms without sensory changes or long tract signs. It appears, however, that the damage in this disease is from intermittent compression of the lower cervical cord and gradual deterioration of the motor neurons in the anterior grey matter. Hirayama’s group pointed out that in the young men who were affected, the mechanism of cord damage is a buckling of the dorsal dural sac and an intermittent anterior displacement and ligamentous compression of the cord during flexion of the neck. Although the disorder is highlighted as occurring mainly in Asia, we continue to see cases from the
United States sporadically.