What is the initial work-up for a patient suspected of having ankylosing spondylitis?

Initial Work-up for Suspected Ankylosing Spondylitis

Radiographs of the sacroiliac joints and spine are the first-line imaging modalities for the initial evaluation of patients with suspected ankylosing spondylitis (AS). 1

Patient Selection for Work-up

• Evaluate patients with chronic back pain (>3 months duration) whose symptoms began before age 45 1
• Focus on identifying inflammatory back pain characteristics, which have a sensitivity of 75% for axial spondyloarthritis 1
• Screen for features of inflammatory back pain:
  • Morning stiffness lasting >30 minutes
  • Improvement with exercise but not with rest
  • Pain awakening patient during second half of night
  • Alternating buttock pain 1

Initial Diagnostic Imaging

• Radiographs of the sacroiliac joints (anteroposterior view of the pelvis) should be obtained first 1
• Complementary radiographs of the spine (cervical and lumbar) should be performed, especially if symptoms are referable to these areas 1
• Radiographs demonstrate chronic erosions, sclerotic changes, and ankylosis as sequelae of inflammatory sacroiliitis 1

Limitations of Radiography

• Radiographs have limited sensitivity (19%-72%) for early disease 1
• Radiographic changes may take several years of inflammation before becoming visible 1
• Interobserver agreement for radiographic findings is only fair to moderate 1
• One study showed radiography missed more than half of patients with structural changes when compared to CT 1

Laboratory Testing

• HLA-B27 testing should be obtained if suspicion of AS remains high but no sacroiliitis is present on x-ray 2
• Consider inflammatory markers (C-reactive protein and erythrocyte sedimentation rate), though these are not invariably elevated in AS 3

Advanced Imaging

• MRI of the sacroiliac joints should be considered in cases with short duration of symptoms or when radiographs are negative but clinical suspicion remains high 1
• MRI can detect inflammatory changes 3-7 years before radiographic structural findings appear 1
• CT is not routinely recommended as an initial imaging modality for suspected AS 1

Diagnostic Criteria

• The Assessment of SpondyloArthritis International Society (ASAS) criteria are helpful in diagnosing AS 2
• Patients may have non-radiographic axial spondyloarthritis, which will evolve into typical AS in approximately 50% of cases 3

Common Pitfalls to Avoid

• Delaying diagnosis due to normal inflammatory markers (not invariably elevated in AS) 3
• Waiting for radiographic changes to appear before making the diagnosis (may take years) 1
• Failing to consider AS in young adults with chronic back pain (average delay in diagnosis is 8 years) 4
• Not evaluating for associated conditions such as anterior uveitis, inflammatory bowel disease, and peripheral arthritis 2

Next Steps After Initial Work-up

• If radiographs show sacroiliitis, diagnosis of AS can be established 1
• If radiographs are negative but clinical suspicion remains high, proceed to MRI of sacroiliac joints 1
• Consider referral to rheumatology for further evaluation and management, especially if initial work-up suggests AS 1

Remember that early diagnosis is increasingly important as effective treatments like NSAIDs and TNF inhibitors can significantly improve outcomes and quality of life 1, 2, 3.