The insidious onset of low back pain and/or stiffness is often the initial symptom of ankylosing spondylitis. The hallmark of ankylosing spondylitis is symmetrical sacroiliitis that is often bilateral (Fig. 287-2) . Sacroiliitis develops early but may take 7 to 10 years to become evident by conventional radiography. Pain is anatomically localized over the sacroiliac joints and less commonly radiates down the posterior of the thigh. Patients usually complain of inflammatory back-pain--prolonged spinal stiffness each morning that is relieved only by increased activity or anti-inflammatory

therapy. Other constitutional features (e.g., fever, anorexia, weight loss) are not uncommon at the onset. With progressive axial involvement, pain and stiffness result in difficulty with ambulation and activities of daily living. The cervical spine is involved late in the disease.

A peripheral asymmetrical oligoarthropathy is seen in up to 30% of patients with ankylosing spondylitis. Synovitis of the hip can be destructive and may lead to concentric loss of joint space, especially in men. Other involved joints include the ankles, wrists, shoulders, elbows, and small joints of the hands or feet.

Extra-articular disease in ankylosing spondylitis primarily affects the eye. Ocular involvement is seen in up to 40% of patients and is more frequently observed in HLA-B27+ individuals. Uveitis is manifested as acute, unilateral orbital pain accompanied by photophobia and progressive loss of vision if untreated. Aortitis, aortic insufficiency, and conduction defects are uncommon. Other uncommon manifestations include mitral valve disease, myocardial dysfunction, pericarditis, pulmonary fibrosis, and amyloidosis.

Restricted spinal movement results from the axial stiffness and paraspinal muscular spasm that accompany inflammatory spondylitis, with or without intervertebral or zygapophyseal ankylosis. A loss of normal lumbar lordosis is a frequent observation in early disease. Fixed forward flexion, especially at the hip and neck, is seen after years of progressive disease. Chest expansion, as measured by the inspiratory minus expiratory chest circumference, is normally greater than 5 cm. Patients with ankylosing spondylitis demonstrate diminished expansion ( < 4 cm). Schober's test is performed to examine lumbar spine mobility. While the patient stands upright with heels together, a 10-cm span is marked from the 5th lumbar vertebra cephalad. Upon maximal forward flexion, the distance between marks is remeasured. Normal spinal flexion expands the skin surface area over the flexed spine to greater than 15 cm. Flexion in patients with spondylitis and limitation of spinal motion measures 14 cm or less.

Laboratory tests support the inflammatory nature of the disease: an elevated erythrocyte sedimentation rate or C-reactive protein, anemia of chronic disease, or mild elevations in alkaline phosphatase. Elevated IgA may be present, but other autoantibodies are noticeably absent. HLA-B27 determination is seldom necessary to establish the diagnosis. However, in questionable cases without distinctive radiographic changes, the presence of HLA-B27 may be of diagnostic value.

Radiographs demonstrate normal mineralization before the onset of ankylosis. Once present, ankylosis results in marked immobility and subsequent generalized osteoporosis. Sacroiliitis is indicated by erosions (leading to "pseudowidening"), ileal sclerosis, or fusion of the inferior synovial-lined portion of the sacroiliac joint (see Fig. 237-1) . These findings are easily observed on plain radiographs of the pelvis and seldom require computed tomography or magnetic

resonance imaging (MRI) for diagnosis. In selected instances, MRI may accurately diagnose periarticular disease such as plantar fasciitis. Axial radiographic findings also include marginal bridging syndesmophytes, interapophyseal joint fusion, and "squaring" of lumbar and thoracic vertebrae. Collectively, these findings may produce the classic appearance of a "bamboo spine" (Figs. 287-3 and 287-4) .

The clinical course and disease severity are highly variable. Inflammatory back pain and stiffness are prominent early in the disease, whereas chronic, aggressive disease may produce pain and marked axial immobility or deformity. In patients with new, refractory spinal pain, intervertebral fracture should be considered. An earlier age of onset and diagnosis portends a more severe outcome. Moreover, patients with ankylosing spondylitis are at risk for complications, some of which may be life threatening. These complications include restrictive lung disease, cauda equina syndrome, post-traumatic intervertebral fractures, osteoporotic compression fractures, or spondylodiscitis.

The diagnosis of ankylosing spondylitis is suggested by (1) young age at onset, (2) strong family history of low back pain, (3) low back pain lasting more than 3 months, (4) prolonged morning stiffness, and (5) symptomatic improvement with activity or exercise. Ankylosing spondylitis must be distinguished from other causes of mechanical or degenerative low back pain. The differential diagnosis also includes other spondyloarthropathies, osteitis condensans ilii, diffuse idiopathic skeletal hyperostosis, and other causes of hyperostosis (Table 287-4)