Ankylosing spondylitis (AS) – Bechterew disease - is a rheumatic inflammatory disease involving the sacroiliac and spinal joints with manifest radiographic damage of the SIJs. The disease symptoms in the form of back pain and gradual 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 etiology of AS is yet unknown, but there is, among other things, a hereditary factor, since many patients have a hereditary tissue type HLA-B27.

The diagnosis of AS may still be delayed several years (doi:10.1093/RHEUMATOLOGY/KEAA807). This is partly because the disease usually begins gradually and the symptoms are often fluctuating with periods of minor or no symptoms. There are no biochemical tests that can confirm the diagnosis except that the presence of the tissue type HLA-B27 and/or elevated CRP (c-reactive protein) support it.

According to the internationally accepted criteria for AS, a definitive diagnosis requires typical SIJ changes by radiography, which, however, only show structural joints and/or bone damage usually first visible several years after the onset of symptoms.

As several therapeutic possibilities are available today, it has become important to diagnose AS early and thereby potentially diminishing the development of structural damage. MRI has gained importance in this context, as MRI can visualize active inflammatory changes before the occurrence of structural damage. CT, which is better than radiography for visualizing the SIJ can show joint changes earlier than by radiography, but can only reveal structural joints and/or bone changes unless using modern Dual Energy technique.

In later stages of the disease the diagnosis is often based on radiography and cross-sectional imaging (MRI or CT) is usually used only to estimate the disease activity or detect complications such as spinal fractures.