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 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 feet. There may be inflammatory changes located to the sacroiliac joints in the acute stage. The disease usually disappears 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.

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

Patients with clinical 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 spondyloarthropathy. 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.

There are less frequent forms of spondyloarthropathy, e.g. arthropathy associated with the dermal disease pustulosis palmoplantaris, often referred to as pustulotic arthro-osteitis. Involvement of the sacroiliac and/or the spinal joints as part of juvenile arthritis - juvenile spondyloarthropathy - may also occur. These rheumatic disorders can be separated from the more common types by clinical and/or imaging findings.