Causes of Sacroiliitis
Sacroiliitis is primarily caused by inflammatory spondyloarthropathies (particularly ankylosing spondylitis), with less common causes including infectious/septic processes, post-traumatic inflammation, and inflammatory bowel disease-associated arthropathy. 1
Primary Inflammatory Spondyloarthropathies
Ankylosing spondylitis represents the classic and most common cause of bilateral sacroiliitis, occurring predominantly in males with onset before age 45 years. 1 This condition demonstrates:
- Strong HLA-B27 association (74-89% of cases), though this marker alone is insufficient for diagnosis 1, 2
- Insidious onset of inflammatory back pain lasting ≥3 months with characteristic features including morning stiffness >30 minutes, improvement with exercise but not rest, and alternating buttock pain 2, 3
- Progressive ankylosis that can lead to complete joint fusion if untreated 1
Other seronegative spondyloarthropathies causing sacroiliitis include psoriatic arthritis, reactive arthritis (including Reiter's syndrome), and enteropathic arthritis. 4, 5, 6
Inflammatory Bowel Disease-Associated Sacroiliitis
Radiological sacroiliitis occurs in 20-50% of patients with both ulcerative colitis and Crohn's disease, making IBD a major secondary cause. 1, 2 Key distinguishing features include:
- Only 1-10% of IBD patients with sacroiliitis progress to full ankylosing spondylitis, despite high prevalence of radiographic changes 1
- Lower HLA-B27 prevalence (25-75% in IBD-AS, only 7-15% in isolated sacroiliitis) compared to idiopathic ankylosing spondylitis 1
- Axial arthropathy runs independent of intestinal disease activity, unlike peripheral arthritis in IBD 1
- This represents a distinct clinical entity requiring different diagnostic and therapeutic considerations 7
Infectious Causes
Pyogenic (septic) sacroiliitis represents an important infectious cause that can occur post-traumatically or hematogenously. 1, 6 Additional infectious etiologies include:
- Tuberculous sacroiliitis 5
- Brucellosis-associated sacroiliitis 5
- These infectious causes can lead to unilateral sacroiliitis and complete joint ankylosis even in HLA-B27 positive individuals without spondyloarthropathy 6
Mechanical and Degenerative Causes
Post-traumatic inflammation can cause sacroiliitis following direct injury to the sacroiliac joint. 1, 6 Additionally:
- Osteoarthritis can affect the sacroiliac joints and cause inflammatory changes 5
- Age-related degenerative changes can mimic inflammatory sacroiliitis on imaging, particularly showing bone marrow edema on MRI in up to 30% of healthy controls 8
Metabolic Causes
Gout can cause sacroiliitis through crystal deposition in the sacroiliac joints. 5
Critical Diagnostic Pitfalls
The diagnosis is frequently delayed by 5-8 years from symptom onset because early inflammatory sacroiliitis may show normal conventional radiographs. 1, 3 Important caveats include:
- HLA-B27 positivity alone is unreliable as a diagnostic test, particularly in IBD patients where prevalence is lower than idiopathic ankylosing spondylitis 1
- Bone marrow edema on MRI is not specific for inflammatory sacroiliitis and can be seen in postpartum patients, athletes, chronic back pain patients, and healthy controls 8
- Unilateral sacroiliitis should prompt consideration of infectious or post-traumatic causes rather than assuming spondyloarthropathy 6
- Inflammatory back pain symptoms occur in 5-6% of the general adult population, so additional clinical features and imaging confirmation are essential 3