What criteria are used to diagnose AS

• Inflammatory pain and stiffness beginning in the sacroiliac joints with subsequent spread to the lumbar, thoracic, and cervical regions. Inflammatory back pain differs from mechanical back pain and is characterized by morning stiffness (>30 minutes), improvement with exercise, awakening in the second half of the night by pain, and alternating buttock pain

• Limitation of spinal motion in the coronal and sagittal planes

• Decreased chest expansion relative to normative values for age and sex

• Spinal deformity

• HLA-B27 antigen test positivity. This finding must be interpreted with caution. Although up to 90% of white patients with AS have HLA-B27, the gene is present in up to 8% of the white population, and less than 1% of persons in the United States develop AS

• Imaging studies. Arthritic changes in the sacroiliac joints have traditionally been considered the hallmark for diagnosis of AS. Additional radiographic findings include ossification of the spinal ligaments, squaring of the lumbar vertebrae and kyphotic spinal deformities. Recent studies have shown that evidence of sacroiliitis on plain radiographs is a late finding and occurs 5 to 10 years following disease onset. Evidence of sacroiliitis on magnetic resonance imaging (MRI) and thoracic MRI evidence of costovertebral joint inflammation are thought to represent the earliest detectable changes of AS on imaging studies. Figure 69-2

Figure 69-2. Coronal fat saturation FSE T2-weighted image shows increased periarticular signal about the left sacroiliac joint (arrowheads and asterisks) consistent with bone marrow edema and increased signal within the sacroiliac joint (black and white arrows). These findings correlated with active left sacroiliitis in this patient with ankylosing spondylitis. (From Bennett DL, Ohashi K, El-Khoury GY. Spondyloarthropathies: Ankylosing spondylitis and psoriatic arthritis. Radiol Clin North Am 2004;42(1).)

Figure 69-2. Coronal fat saturation FSE T2-weighted image shows increased periarticular signal about the left sacroiliac joint (arrowheads and asterisks) consistent with bone marrow edema and increased signal within the sacroiliac joint (black and white arrows). These findings correlated with active left sacroiliitis in this patient with ankylosing spondylitis. (From Bennett DL, Ohashi K, El-Khoury GY. Spondyloarthropathies: Ankylosing spondylitis and psoriatic arthritis. Radiol Clin North Am 2004;42(1).)

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