Diagnostic Approach to Unilateral Sacroiliac Joint Pain

Primary Differential Diagnoses

The American College of Radiology suggests that unilateral sacroiliitis occurs particularly in psoriatic spondyloarthritis and reactive arthritis, whereas ankylosing spondylitis typically presents bilaterally 1, 2
The age of 55 is slightly older than typical for axial spondyloarthropathy, which characteristically presents before age 40-45 years, though late presentations do occur 2, 3
Inflammatory pain characteristics include insidious onset, improvement with exercise but not rest, pain occurring at night, morning stiffness >30 minutes, and alternating buttock pain 2, 3
Associated features to assess include uveitis, psoriasis, inflammatory bowel disease, peripheral joint involvement, and HLA-B27 positivity 2, 3

The American College of Radiology recommends determining if pain has inflammatory characteristics versus mechanical features 2, 3
The initial imaging should include plain radiographs of the sacroiliac joints, which have an appropriateness rating of 9/9 by the American College of Radiology 5, 6
Complementary spine radiographs should also be included, at minimum cervical and lumbar 7, 6
If radiographs are negative or equivocal with high clinical suspicion, proceed to MRI of sacroiliac joints without contrast, which has a sensitivity of 79% and specificity of 89% 6
The MRI protocol must include T1-weighted sequences and fat-suppressed fluid-sensitive sequences, and look for bone marrow edema extending ≥1 cm deep to the articular surface 6, 4

HLA-B27 testing should be ordered alongside initial radiographs if inflammatory features are present, according to the American College of Radiology 3
Inflammatory markers such as C-reactive protein should also be checked, though these may be normal in axSpA 3
Diagnostic SI joint block with local anesthetic can confirm the joint as pain source, though false-positives and false-negatives occur 6

The American College of Radiology advises against relying on imaging alone, as radiographs have fair to moderate interobserver agreement and miss early disease 7, 6
Inflammatory sacroiliitis should not be dismissed based on age >45 years alone, as diagnosis is frequently delayed 4.9-8 years from symptom onset 3
CT, bone scan, ultrasound, or PET/CT should not be ordered as initial imaging, as these are not appropriate for initial evaluation 5, 7, 6