Ankylosing spondylitis (AS) is a rheumatic disease, especially involving spinal joints and the joints between the sacrum and the ileum (the sacroiliac joints). The disease is associated with back pain and stiffness. The symptoms typically start between 20 and 30 years of age, but may occur earlier or later.
The disease is due to a rheumatic inflammation, a type of inflammation without bacteria. The inflammation usually attacks the sacroiliac and spinal joints in addition to areas where tendons, ligaments and joint capsules are attached to bone.
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 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.

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 unclassifiable 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.