What are the signs of sacroiliitis?

Signs of Sacroiliitis

The key signs of sacroiliitis include inflammatory back pain (present in 70-80% of patients), tenderness with direct palpation of the sacroiliac joints, and radiological evidence of sacroiliac joint inflammation on imaging studies. 1, 2

Clinical Signs

Inflammatory Back Pain Characteristics

The hallmark clinical presentation includes specific features that distinguish sacroiliitis from mechanical back pain:

• Insidious onset before age 45 years with chronic duration (≥3 months) 1, 2
• Prolonged morning stiffness lasting more than 30 minutes 3, 1
• Improvement with exercise but not with rest - a critical distinguishing feature 1, 2
• Night pain, particularly awakening in the second half of the night 1
• Alternating buttock pain indicating sacroiliac joint involvement 1, 2
• Lower back and buttock pain as the typical initial site of involvement 1

Physical Examination Findings

• Direct palpation tenderness over the sacroiliac joints is a key examination finding 3
• Pain with provocative maneuvers of the sacroiliac joints during physical examination 4

Radiological Signs

Plain Radiography

• Sacroiliitis grade ≥2 bilaterally or grade ≥3 unilaterally on x-ray examination (sensitivity 66%, specificity 68%) 3, 2
• Radiological evidence occurs in 20-50% of patients with inflammatory bowel disease-associated spondyloarthritis 3

Important caveat: Conventional radiographs may be normal in early and acute stages of sacroiliitis, as it may take several years of inflammation before radiological damage becomes visible 3, 5

MRI Findings (Gold Standard for Early Disease)

• Active inflammation on fluid-sensitive sequences (STIR or fat-saturated T2-weighted) showing bone marrow edema (sensitivity 78%, specificity 88%) 3, 6, 2
• MRI can identify early sacroiliitis in symptomatic patients with normal plain radiography (non-radiographic spondyloarthritis) 3
• T1-weighted sequences demonstrate structural changes and fatty replacement 6

For patients aged <40 years with inflammatory back pain lasting >3 months, early MRI assessment using T1-weighted spin-echo, STIR, and fat-saturated T2-weighted sequences is recommended to identify non-radiographic sacroiliitis. 3

Laboratory Signs

Inflammatory Markers

• Elevated C-reactive protein (sensitivity 35%, specificity 91%) - though notably absent in many patients 2
• Elevated ESR - but sensitivity is only 50% in ankylosing spondylitis patients 3

Genetic Marker

• HLA-B27 positivity (sensitivity 50%, specificity 90%) is found in 74-89% of patients with axial spondyloarthritis 1, 2
• In IBD-associated sacroiliitis, HLA-B27 is found in 25-75% of patients with ankylosing spondylitis but only 7-15% with isolated sacroiliitis 3

Critical pitfall: HLA-B27 has lower prevalence in IBD-associated sacroiliitis than in idiopathic ankylosing spondylitis, making it unreliable as a standalone diagnostic test 3

Associated Manifestations

Musculoskeletal

• Peripheral arthritis affecting large joints (most commonly knees) in 30-50% of patients, presenting in an oligoarticular, asymmetric pattern 1, 2
• Enthesitis (inflammation at tendon insertion sites) 2

Extra-Articular

• Uveitis (inflammatory eye disease) 1, 2
• Psoriasis 1, 2
• Inflammatory bowel disease (Crohn's disease or ulcerative colitis) 3, 1, 2

Diagnostic Approach Algorithm

For patients with chronic back pain >3 months with first symptoms <45 years:

1. Assess for inflammatory back pain features (sensitivity 74-81%, specificity 25-44%) 2
2. If inflammatory back pain present OR HLA-B27 positive, proceed to imaging 3
3. Order plain radiographs of sacroiliac joints first 3
4. If radiographs show sacroiliitis (grade ≥2 bilaterally or ≥3 unilaterally), diagnosis confirmed 3
5. If radiographs normal but clinical suspicion high, obtain MRI of sacroiliac joints 3, 2
6. Refer to rheumatologist for further evaluation when sacroiliitis confirmed by imaging 3

Common pitfall: The diagnosis is frequently delayed 4.9 to 8 years from symptom onset because early radiographs may be normal and inflammatory back pain symptoms occur in 5-6% of the general population, requiring additional confirmatory features 1