What is the significance of unilateral sacroiliitis?
Sacroiliitis, or inflammation of the sacroiliac joint, can be caused by both inflammatory and infectious processes. When the process is bilateral, it is generally the cause of a rheumatologic disease such as ankylosing spondylitis or inflammatory bowel disease–related spondyloarthropathy (particularly if symmetric) or reactive arthritis or psoriatic arthritis (particularly if asymmetric). Although unilateral sacroiliitis can be caused by one of the asymmetric seronegative spondyloarthropathies (reactive or psoriatic arthritis), it should also raise concern for infection. Therefore, patients with unilateral sacroiliitis often need sacroiliac joint aspiration, typically under fluoroscopic or CT guidance, for definitive diagnosis.