What are the diagnostic steps and treatment options for sacroiliitis?

Diagnostic Approach to Sacroiliitis

Begin with conventional radiography of the sacroiliac joints as the first-line imaging modality, followed by MRI of the SI joints if radiographs are negative or equivocal but clinical suspicion remains high. 1, 2

Clinical Presentation to Recognize

• Age and symptom pattern: Axial spondyloarthropathy typically presents before age 45 with chronic pain (≥3 months duration) that is insidious in onset 1
• Inflammatory characteristics: Morning stiffness, pain that improves with exercise but not rest, pain awakening in the second half of the night, and alternating buttock pain 1
• Physical examination maneuvers: When 3 of 6 provocative tests (Patrick's Test, Thigh Thrust, Gaenslen's Test, Distraction, Compression, Sacral Thrust) are positive, sensitivity is 94% and specificity is 78% for sacroiliac joint pain 1
• Laboratory markers: Consider HLA-B27 and C-reactive protein as supportive data, though no single test is pathognomonic 1

Imaging Algorithm

Step 1: Conventional Radiography (First-Line)

• Order radiographs of the sacroiliac joints plus symptomatic spine regions (at minimum cervical and lumbar spine) 1, 2
• Limitations to recognize: Radiography has low sensitivity (19%-72%) for early disease, with radiographic findings often lagging 3-7 years behind symptom onset 1, 2
• What to look for: Chronic erosions, sclerotic changes, and ankylosis as sequelae of inflammatory sacroiliitis 2
• Interobserver reliability: Only fair to moderate agreement, so negative radiographs do not exclude disease 2

Step 2: MRI of Sacroiliac Joints (When Radiographs Negative/Equivocal)

• MRI without contrast is generally sufficient, with sensitivity of 79% and specificity of 89% for axial spondyloarthropathy 2
• Required sequences: T1-weighted sequences PLUS fat-suppressed fluid-sensitive sequences (T2-weighted fat-saturated or STIR) 1, 2
• Evaluate both: Active inflammatory lesions (primarily bone marrow edema) AND structural lesions (erosions, new bone formation, sclerosis, fat infiltration) 1
• Contrast-enhanced imaging: May be considered for initial evaluation to improve conspicuity of subtle inflammatory lesions, but has not been shown to significantly increase diagnostic accuracy 1, 2

Critical pitfall: Bone marrow edema can appear in up to 30% of healthy controls, but deep lesions extending ≥1 cm from the articular surface are more specific for axial spondyloarthropathy 2

Step 3: CT of Sacroiliac Joints (Alternative When MRI Contraindicated)

• Use CT when: Radiographs show equivocal abnormalities OR MRI cannot be performed 1
• Advantages: Superior to radiography for detecting subtle bone erosions and reparative changes 1
• Limitation: Lacks sensitivity for direct inflammatory changes before structural damage occurs 1

Step 4: MRI of Spine (When SI Joints Normal but Suspicion High)

• Consider spine MRI when: Radiographs and MRI of SI joints are negative but clinical suspicion persists 1
• Rationale: Isolated spine inflammatory involvement occurs in 6-23% of cases 1
• Protocol requirement: Must include fluid-sensitive sequences (STIR or T2-weighted fat-saturated), as standard disc disease protocols may lack necessary fat suppression 1
• Note: Spine MRI is NOT generally recommended for initial diagnosis 1, 2

Modalities NOT Recommended

• Bone scintigraphy/SPECT: Low to moderate sensitivity with variable specificity; not routinely recommended 2
• Ultrasound: Not recommended for diagnosis of sacroiliitis 1, 2
• PET/CT: Not routinely obtained as initial imaging; uncertain utility 2

Differential Diagnosis Considerations

For acute presentations with fever and unilateral involvement, consider pyogenic sacroiliitis—the most common cause of acute sacroiliitis 3:

• Diagnostic approach: Technetium bone scan to localize, then CT or MRI for detailed characterization 3
• Confirmation: Fluoroscopic-guided fine-needle aspiration for culture 4
• Risk factors: Intravenous drug use, trauma, identifiable infection focus elsewhere (though 44% have no identifiable risk factors) 4

Common Diagnostic Pitfalls

• Radiographs miss >50% of patients with structural changes when compared to CT 2
• Standard spine MRI protocols for disc disease may not include fat suppression necessary to detect inflammatory changes 1
• Equivocal MRI findings: Bone marrow edema alone is nonspecific; look for deep lesions and structural changes 2
• Delayed diagnosis: Average 7-year lag between symptom onset and radiographic changes emphasizes need for MRI in young patients with short symptom duration 1