Ankylosing spondylitis is a chronic, usually progressive, condition in which inflammatory changes and new bone formation occurs at the attachment of tendons and ligaments to bone (enthesopathy). Sacroiliac joint involvement (which limits lumbar flexion) is the hallmark of ankylosing spondylitis, although 20-30% of patients also have larger peripheral joint involvement.
The cause of ankylosing spondylitis is unknown. Risk factors include HLA-B27 tissue antigen, and a positive family history.
Ankylosing spondylitis may be associated with psoriasis, ReiterÕs syndrome (venereal and dysenteric), inflammatory bowel disease (ulcerative colitis and CrohnÕs disease), uveitis and iritis.
Elevated serum titers of antibodies to certain enteric bacteria, particularly Klebsiella pneumoniae, are common in ankylosing spondylitis patients. There is an antigenic interrelatedness between B27 and certain enteric bacteria.
Recent research has established a link between sex hormones (elevated luteinizing hormone, inversion of estradiol/testosterone ratio, and diminished testicular reserve for testosterone, and slightly increased for estradiol) in ankylosing spondylitis.
The erythrocyte sedimentation rate (ESR) is elevated in 80% of cases, but it correlates poorly with disease activity and prognosis. Rheumatoid factor is negative. The synovial fluid shows mild leukocytosis and decreased viscosity. Cerebrospinal fluid has increased protein. An EKG will show conduction defects. The Wright-Schober test for lumbar spine flexion is abnormal. Measurement of respiratory excursion of chest wall is decreased. Less than 2.5 cm is virtually diagnostic.
Imaging studies (radiographs) of the lumbar spine and sacroiliac joint will show degenerative changes. An early sign is sclerosis on both sides of the sacroiliac joint, followed by ankylosis. The "squaring" of vertebral bodies and ossification of annulus fibrosis gives a characteristic appearance of "bamboo spine". Symmetric erosive changes will appear in larger joints, including pericapsular ossification, sclerosis, and a loss of joint space.
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