What is sacroiliitis?

Sacroiliitis: Definition, Diagnosis, and Management

Sacroiliitis is inflammation of one or both sacroiliac (SI) joints, which connect the base of the spine (sacrum) to the pelvis (ilium), and is a key feature of axial spondyloarthritis that can cause significant pain, functional limitations, and reduced quality of life. 1

Clinical Features and Epidemiology

• Sacroiliitis accounts for up to 25% of cases of lower back pain and can have a debilitating effect on patient functionality 1
• Symptoms typically include inflammatory back pain that may radiate to the buttocks or legs, morning stiffness lasting >30 minutes, improvement with exercise but not with rest, and pain awakening during the second half of the night 2, 3
• Inflammatory back pain typically begins before age 45 and lasts more than 3 months, with these characteristics having approximately 75% sensitivity for axial spondyloarthritis 3
• Sacroiliitis is more common in males and can occur in various conditions including ankylosing spondylitis, psoriatic arthritis, reactive arthritis, and inflammatory bowel disease-related spondyloarthropathies 2, 4

Pathophysiology

• The pathophysiology is complex, involving inflammatory processes at the sacroiliac joint that may be triggered by genetic, immunological, and environmental factors 4
• HLA-B27 is found in 25-75% of patients with inflammatory bowel disease and ankylosing spondylitis, but only in 7-15% of patients with isolated sacroiliitis 2
• Radiological evidence of sacroiliitis is common in both ulcerative colitis and Crohn's disease, occurring in 20-50% of patients, but progressive ankylosing spondylitis with syndesmophytes occurs in only 1-10% of patients 2

Diagnostic Approach

Initial Imaging

• Radiographs of the sacroiliac joints (anteroposterior view of the pelvis) are recommended as the first-line imaging modality for initial evaluation of suspected sacroiliitis 2, 3
• Complementary spine radiographs (cervical and lumbar) should be performed, especially if symptoms are referable to these areas 2, 3
• Radiographs demonstrate chronic erosions, sclerotic changes, and ankylosis as sequelae of inflammatory sacroiliitis 2, 3
• Limitations of radiography include:
  • Low sensitivity (19%-72%) for early disease detection 2, 3
  • Radiographic changes may take several years of inflammation before becoming visible 3
  • Interobserver agreement for radiographic findings is only fair to moderate 3

Advanced Imaging

• MRI of the sacroiliac joints should be considered when radiographs are negative but clinical suspicion remains high 2, 3
• MRI can detect inflammatory changes 3-7 years before radiographic structural findings appear 3
• Fluid-sensitive MR sequences (T2-weighted fat-saturated or short-tau inversion recovery [STIR]) allow identification of characteristic bone marrow lesions and areas of soft-tissue inflammatory change 2, 5
• In patients unable to undergo MRI, CT may be helpful, demonstrating improved sensitivity over conventional radiography for detection of subtle bone erosions and reparative changes 2, 5
• Early assessment using T1-weighted spin-echo, STIR, and fat-saturated T2-weighted sequences is recommended for patients aged less than 40 years with inflammatory back pain lasting more than 3 months 2

Management Considerations

• Early diagnosis is crucial as it allows for timely intervention that may prevent disease progression and improve outcomes 3, 6
• Treatment typically begins with conservative approaches including physical therapy and analgesics for symptom relief 1
• For refractory cases, interventional methods such as corticosteroid injections, prolotherapy, radiofrequency ablation, and even SI joint fusion surgery may be considered 1
• Anti-TNF-α and other biologic therapies may help reduce inflammation and potentially slow disease progression 5
• Regular imaging follow-up (typically no more frequently than every 2 years) is recommended to monitor disease progression 5

Important Clinical Considerations

• Differential diagnosis should include other causes of lower back pain, such as degenerative disc disease, facet joint arthropathy, and mechanical SI joint dysfunction 1, 6
• Sacroiliitis can be unilateral or bilateral, with bilateral involvement more common in inflammatory conditions 7
• Paraspinal muscle atrophy may be present even in early or non-radiographic axial spondyloarthritis, though it's more common in advanced disease 5
• When requesting MRI to evaluate for sacroiliitis, specify the need for appropriate sequences (including fat-suppressed fluid-sensitive sequences) 5