summary Sacroiliitis is an inflammatory condition of the sacroiliac joint that can cause persistent low back and posterior pelvic pain Diagnosis is made with contrast-enhanced MRI demonstrating fluid/inflammation in the SI joint and possible abscess formation Treatment is usually nonoperative, consisting of NSAIDs and rest, with IV antibiotics reserved for infection. Surgical debridement is indicated in the presence of an abscess Epidemiology Demographics most commonly presents in teenagers to middle-age adults males > females Associated conditions commonly part of ankylosing spondylitis or Reiter's syndrome Etiology Pathophysiology can stem from a traumatic event or infection pregnancy may predispose often stems from chronic inflammation of the SI joints can lead to fibrosis and ossification within the SI joints Associated conditions ankylosing spondylitis associated with HLA-B27 approximately 1-2% of all individuals are HLA-B27 positive Reiter's syndrome oligoarticular arthritis, conjunctivitis, and urethritis Presentation Symptoms pain with prolonged standing difficulty climbing stairs generalized low back pain weakness of the hip musculature on the affected side morning stiffness Physical exam FABER test pain with flexion, abduction, and external rotation of the hip ankylosing spondylitis may be associated with the following: spinal flexion deformities typically beginning in the thoracic and lumbar spine Imaging Radiographs may show some erosive changes in the bone, although these findings are nonspecific may show calcifications or sclerosis within the SI joint MRI is the study of choice with gadolinium T2-weighted images may show fluid/inflammation in the SI joint and/or a possible abscess Studies Labs WBC usually normal can be elevated in cases of infection ESR/CRP usually elevated blood cultures positive in ~50% of cases HLA-B27 check rheumatoid factor (typically negative in ankylosing spondylitis) Treatment Depends on underlying cause infection IV antibiotics until symptoms improve and CRP levels normalize followed by oral antibiotics surgery may be necessary if antibiotics alone fail or if a large abscess is present trauma or overuse rest, activity modification, NSAIDs, and corticosteroid injections indications most cases resolve with rest and activity modification pregnancy observation symptoms typically resolve after childbirth when occurring as part of a broader spondyloarthropathy aggressive physical therapy, NSAIDs, and/or TNF inhibitors severe symptoms may require TNF inhibitors or other biologic agents Prognosis Depends on the underlying cause, but most cases eventually resolve