|ECR 2019 / C-3035|
|Unilateral Acute Sacroiliitis - Think Infection, No Injection !|
Imaging of the sacroiliac joints is a unifying diagnostic tool for seronegative spondyloarthritis which includes plain radiographs as well as MR imaging.
The ASAS (Assessment of Spondyloarthritis International Society) diagnostic criteria for MR features of sacroiliitis is defined as focal high bone marrow signal intensity on the STIR / fat-suppressed T2-weighted images or contrast enhancement on the fat-suppressed T1-weighted images.
The bone marrow edema is typically symmetrical in ankylosing spondylitis within the subchondral/peri-articular bone and most commonly affects the lower and posterior thirds of the joints, usually commencing on the iliac side of the joint; with or without erosions. However, psoriatic and reactive spondyloarthritides can present with asymmetrical involvement or ocassionally with only unilateral sacroiliitis. However when the marrow oedema is unilateral, an infectious sacroiliitis should be excluded.
The inflammatory sacroiliitis usually remains confined to the anatomic borders and joint spaces. However involvement of the ligamentous portion as well as presence of adjacent soft tissue inflammatory oedema or communicating juxta-articular fluid collections are important imaging features that should raise suspicion for an uderlying infective cause.
Like septic arthritis in the appendicular skeleton, supporting clinical features of pyrexia, systemic upsets, acute or insiduous onset and increased inflammatory markers (ESR, CRP) with leukocytosis will further support an infectious aetiology. Further management will be dictated by image-guided aspiration of the joint fluid or juxta-articular collection to start targetted anti-microbial therapy.
The commonest organism causing pyogenic septic arthritis of the sacroiliac joints is Staphlococcus aureus however atypical organisms like Streptococcus, Salmonella, E.coli and Mycobacterium tuberculosis have been described in literature.
The other important infectious cause of spondylodisciitis as well as sacroiliitis in endemic areas is brucellosis with a pertinet history of consumption of unpasteurised cattle milk. In addition to suspicious imaging findings serological testing is mandatory to help in early diagnosis of this zoonosis and avoidance of parenteral or intra-articular cortico-steroids.