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Inflammatory changes at the sacroiliac joint (SIJ) are a hallmark of axial spondyloarthritis (axSpA) which today usually is divided into radiographic axSpA and non-radiographic axSpA depending on the presence or absence of definite radiographic changes in accordance with the modified New York criteria for ankylosing spondylitis (DOI: 10.1002/art.37803). Radiographic axSpA is also named Morbus Bechterew.

In addition to this division, axSpA is also divided into the following disease entities according to previous European classification criteria for seronegative spondyloarthropathies (DOI: 10.1002/art.1780341003).

  • Ankylosing spondylitis (Morbus Bechterew)
  • Psoriatic arthritis
  • Reactive arthritis
  • Arthritis associated with inflammatory bowel disorders (enteropathic arthritis)
  • Undifferentiated spondyloarthritis

Additionally, there are special forms such as

  • Pustulotic arthro-osteitis or SAPHO (Synovitis, acne, pustulosis, hyperostosis, osteitis)

  • Juvenile spondyloarthritis

Ankylosing spondylitis

Ankylosing spondylitis (AS) is a rheumatic disease involving the SIJ with a potential concomitant involvement of spinal joints. The disease symptoms in the form of back pain and stiffness typically start between 20 and 30 years of age, but may occur earlier or later.
The disease is due to a rheumatic inflammation primarily involving the sacroiliac and spinal joints in addition to areas where tendons, ligaments and joint capsules are attached to bone, areas containing fibrocartilage and named entheses.
Untreated, the disease often results in increasing spinal stiffness due to fusion of the joints when the inflammatory changes heal.
The aetiology of AS is yet unknown, but there is, among other things, a hereditary factor, since many patients have a hereditary tissue type HLA B27.

Psoriatic arthritis

Psoriatic arthropathy is a chronic inflammatory rheumatic disease affecting patients with the dermal disease psoriasis. The joints of the extremities are most often involved, but approximately one third of the patients have inflammatory changes of the spine and/or the sacroiliac joints.

Reactive arthritis

Reactive arthritis is usually a transient acute arthritis appearing a few weeks after an infection somewhere in the body, mostly located in the urinary or the gastrointestinal tracts. It usually involves large joints, such as knees and ankles as well as the feet. There may be inflammatory changes located to the SIJ and /or spine in the acute stage. The disease usually vanishes after some months, but may recur and can develop into a chronic condition. This is especially the case in patients with the tissue type HLA B27. Patients with a chronic course often have involvement of the sacroiliac and/or spinal joints.

Enteropathic arthritis

Arthritis associated with the inflammatory bowel disorders ulcerative colitis and Crohn’s disease is usually named enteropathic arthritis. It is seen in 10-15% of patients with these bowel disorders and can involve peripheral joints as well as the sacroiliac joints and the spine.

Undifferentiated SpA

Patients with clinical or imaging signs of inflammation at the sacroiliac or spinal joints without other signs making it possible to establish one of the above-mentioned rheumatic diagnoses are usually grouped under the name undifferentiated spondyloarthritis. Psoriasis, typical imaging signs of AS or other SpA manifestations may occur later and contribute to a classification in one of the well-defined forms of SpA.

Special forms of SpA

There are less frequent forms of atypical spondyloarthritis, e.g., arthropathies associated with the dermal diseases, especially pustulosis palmoplantaris, often referred to as pustulotic arthro-osteitis or SAPHO (Synovitis, acne, pustulosis, hyperostosis, osteitis), which may involve the sacroiliac joints and the spine. Involvement of the sacroiliac and/or the spinal joints may also occur as part of juvenile arthritis - juvenile spondyloarthritis. These rheumatic disorders have similarities with the common forms of SpA, but also show differences clinically and/or by imaging.

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