What are the causes of sacroiliitis?

Causes of Sacroiliitis

Sacroiliitis is primarily caused by inflammatory spondyloarthropathies (including ankylosing spondylitis, psoriatic arthritis, reactive arthritis, and inflammatory bowel disease-associated arthritis), with less common causes including infectious/septic processes and post-traumatic inflammation. 1, 2

Primary Inflammatory Causes (Spondyloarthropathies)

The axial spondyloarthropathies represent the most common etiology of sacroiliitis, accounting for the majority of cases in clinical practice. 1

Ankylosing Spondylitis

• Represents the classic cause of bilateral sacroiliitis with progressive ankylosis occurring in 1-10% of patients with inflammatory bowel disease. 1
• Strongly associated with HLA-B27 positivity (74-89% of cases), though this is less reliable in IBD-associated disease. 1, 3, 4
• Typically presents before age 45 with insidious onset of inflammatory back pain lasting ≥3 months. 1, 3
• More common in males than females. 1

Inflammatory Bowel Disease-Associated Spondyloarthritis

• Radiological evidence of sacroiliitis occurs in 20-50% of patients with both ulcerative colitis and Crohn's disease. 1, 3
• Progressive ankylosing spondylitis develops in only 1-10% of IBD patients despite the high prevalence of radiographic sacroiliitis. 1
• HLA-B27 is found in 25-75% of IBD patients with ankylosing spondylitis but only 7-15% with isolated sacroiliitis. 1
• The axial arthropathy typically runs independent of intestinal disease activity, unlike peripheral arthritis. 1

Psoriatic Arthritis

• Part of the seronegative spondyloarthropathy spectrum causing sacroiliitis. 1, 4
• Can present with asymmetric sacroiliitis more frequently than ankylosing spondylitis. 5

Reactive Arthritis and Reiter's Syndrome

• Develops following genitourinary or gastrointestinal infections. 1, 5
• Asymmetric sacroiliitis is more common in reactive arthritis compared to ankylosing spondylitis. 5, 6

Infectious Causes

Septic/Pyogenic Sacroiliitis

• Bacterial infection of the sacroiliac joint represents an important differential diagnosis that requires urgent recognition. 7
• Can occur post-traumatically and may lead to complete unilateral sacroiliac joint ankylosis. 5
• MRI demonstrates spread of inflammation to surrounding muscles, distinguishing it from inflammatory spondyloarthropathies. 7
• More likely to present with unilateral involvement. 5

Mechanical/Traumatic Causes

Post-Traumatic Sacroiliitis

• Direct trauma to the sacroiliac joint can lead to inflammatory changes. 5
• May occur even in HLA-B27 positive individuals without developing features of spondyloarthropathy. 5

Key Diagnostic Considerations

The pattern of sacroiliac joint involvement provides important diagnostic clues:

• Bilateral symmetric sacroiliitis strongly suggests ankylosing spondylitis. 5, 7
• Asymmetric or unilateral sacroiliitis is more frequent in reactive arthritis, psoriatic arthritis, and IBD-associated disease. 5, 6
• Unilateral sacroiliitis should raise suspicion for septic sacroiliitis or post-traumatic causes. 5, 7

HLA-B27 testing has important limitations:

• While present in 74-89% of axial spondyloarthritis patients, it has lower prevalence in IBD-associated sacroiliitis. 3, 4
• HLA-B27 positivity alone is unreliable as a diagnostic test in IBD patients due to lower prevalence than idiopathic ankylosing spondylitis. 1

Clinical Pitfalls to Avoid

• Do not assume all sacroiliitis in HLA-B27 positive patients is due to spondyloarthropathy—infectious and traumatic causes must be excluded. 5
• Radiographic sacroiliitis is common (20-50%) in IBD patients, but most will not progress to ankylosing spondylitis. 1, 3
• Early inflammatory sacroiliitis may show normal conventional radiographs, requiring MRI for detection. 1, 7
• The diagnosis is frequently delayed by 5-8 years from symptom onset, emphasizing the need for high clinical suspicion. 4