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.