Ankylosing spondylitis (AS) is one of the group of seronegative forms of arthritis, which also includes Reiter's syndrome and psoriatic arthritis. There is a close association with the HLA-B27 type, and the disease is more than three times more common in men than in women. In many patients, the disease seems to progress slowly for years, primarily with back pain, and then stabilizes without much disability. In some cases the disease progresses to total spinal fusion, producing a typical bamboo spine on x-ray criteria. In all patients, the presence of sacroiliac joint disease on radiographs is required to make the diagnosis. There are two major neurological complications of AS: a cauda equina syndrome of unclear etiology, and several types of cervical spinal cord compression due to dislocation or deformity.
Ankylosing spondylitis is less common in black populations and in people of Japanese ancestry, in parallel with the lower expression of HLA-B27 in these groups. There is a considerable familial incidence of AS. Approximately 20 percent of patients with AS have joint disease in the peripheral joints, not in the vertebral column or pelvis, and it is in this group that the neurological complications seem to be most frequent.
Patients with the cauda equina syndrome of AS have a gradual and relatively symmetrical loss of function in the L5, S1, and S2 roots. Both motor and sensory fibers are affected. Foot drop, weakness of plantar flexion, and perianal sensory loss are typical. Nearly all patients develop bladder and bowel incontinence. The cervical spine disorder most often associated with AS is instability at the craniocervical junction because of ligamentous laxity at the level of the odontoid process. y Some patients have stepwise subluxation of the cervical spine, resembling that seen in seropositive rheumatoid arthritis. In either case, myelopathy may develop, often with significant sensory loss in the hands, as well as quadriparesis. As in rheumatoid arthritic disease, there may be marked instability of the cervical spine or craniocervical junction with much local pain, sudden changes in neurological symptoms with postural change, and eventual requirement for surgical fusion. Patients with AS may have uveitis or cardiac disease. Fractures of the fully or partially fused spine, especially if osteopenia exists, are common.
The cauda equina syndrome can be confused with lumbar disc disease, but the distinguishing features of the former are symmetry and early loss of bladder function. The craniocervical instability of AS produces a myelopathy that may be insidious and difficult to distinguish from general weakness, myopathy, nerve entrapment, or generalized neuropathy.
The diagnosis should be confirmed radiologically; there are specific criteria for the diagnosis of AS.  Cauda equina syndrome is accompanied by a distinctive radiological change, consisting of a wide patulous distention of the subarachnoid space in the sacral region, often with erosion of the overlying sacral bone. The cause of this distended space is unknown, nor is it clear how this distention contributes to nerve root dysfunction. y CSF examination does not suggest an arachnoiditis. The cervical spine should be fully visualized by MRI, and the position of the odontoid should be carefully noted. In rheumatoid arthritis patients, asymptomatic widening of the space between the odontoid and the arch of C1 (normally 3 mm or less) is observed. In AS, unrecognized fracture, odontoid dislocation, or atlantoaxial dislocation can occur.
Unfortunately, no specific therapy is available for patients with the cauda equina syndrome. A scattering of case reports indicate that some authors have attempted decompression of the thecal sac. Decompressive laminectomy
is of no value. Fractures or cervical dislocations may require fusion.
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