Sacroiliac Joint Septic Arthritis: A Case Report

Case Diagnosis: 29 y/o female with no past medical history presented with severe sacroiliac joint pain, later delineated to be septic arthritis.

Case Description or Program Description: 29-year-old female presented to the ED with severe pain in her left lower back with numbness/tingling into her left leg. She returned after being discharged the day before due to severe pain. Prior to her original arrival, she was immobilized for 12 hours during a car ride when her pain began. She was hypotensive with an elevated white count and lactic acid. She was started on broad spectrum antibiotics for suspected sepsis, blood cultures positive for MSSA bacteremia. An MRI showed non-specific findings. Repeat MRI 5 days later showed enhancement within the left iliac bone and sacrum adjacent to the sacroiliac joint, with no evidence of a drainable collection.

Setting: Community Hospital

Assessment/Results: Repeat MRI 9 days post discharge showed left sacroiliac joint enhancement, concerning for septic arthritis. She was unable to walk, utilizing a wheelchair. At an evaluation 1 month later, she had significant improvement of her symptoms within the last week when she was switched to Daptomycin.

Discussion (relevance): This case report describes the need to consider rare diagnoses in the differential diagnosis of back pain, especially when they present with sepsis. The differential for unilateral low back pain should include musculoskeletal causes such as muscle/ligament strains, disc herniations, fractures, infections, such as an epidural abscess and septic arthritis, as well as inflammatory conditions such as ankylosing spondylitis. Septic arthritis should be moved higher on the differential with known risk factors such as intravenous drug use, HIV infection, and pre-existing joint conditions.

Conclusions: This case describes an unusual presentation of septic arthritis in the sacroiliac joint. This is a rare condition, affecting 1-2% of individuals with septic arthritis. The etiology of this infection remains unclear, as she had no clear risk factors.