Sacroiliitis is the inflammation in the sacroiliac joints. Using conventional x-rays to detect this can be problematic and it can take seven to ten years of disease progression for the changes in the joints to be serious enough to show up. This is the pathologic hallmark, and usually one of the earliest manifestations , of ankylosing spondylitis.
Sacroiliitis is the distinctive characteristic of ankylosing spondylitis, and its presence is required for the diagnosis under several sets of criteria, where it is the key symptom of all spondyloarthropathies, often associated with inflammatory back pain. The inflammation of either just one or both of the joints can extend the lower spine to the pelvis. It is often characterized by extreme pain and a severely restricted range of motion, and may be accompanied by other symptoms such as fever, skin conditions, and diarrhea.
Early sacroiliitis starts to manifest with small joint erosions and irregularities in the articular surfaces of the sacroiliac joint. Using conventional x-rays to detect this involvement can be problematic because it can take 7-10 years of disease progression for the changes in the SI joints to be serious enough to be viewable. Because it may often be asymptomatic, sacroiliac films are used to identify early radiographic changes such as blurring of the joints. This is by far the most useful clinical indicator of SA, where it can manifest itself by lower back pain, sometimes extending as far as the buttocks.
Infection with an organism of unknown origin or exposure to an unknown antigen in a genetically susceptible patient (HLA-B27-positive) is hypothesized to result in the clinical expression of a spondyloarthropathy. Infective arthritis may represent a direct invasion of joint space by a variety of microorganisms, including bacteria, viruses, mycobacterium, and fungi. Reactive arthritis, a sterile inflammatory process, is the consequence of an infectious process located elsewhere in the body. Infectious, immunologic, and genetic etiologies have been advanced where approximately 90% of patients with ulcerative colitis or Crohn’s disease who develop spondylitis or sacroiliitis are HLA-B27 positive.