Ankylosing spondylitis is a type of rare arthritis in which there is a long-term inflammation of the joints of the spine. Typically the joints where the spine joins the pelvis are also affected. Occasionally other joints such as the shoulders or hips are involved. Eye and bowel problems may also occur.

Stiffness of the affected joints generally worsens over time. Although the cause of ankylosing spondylitis is unknown, it is believed to involve a combination of genetic and environmental factors. More than 85% of those affected in the UK have a specific human leukocyte antigen known as the HLA-B27 antigen. "The underlying mechanism is believed to be an autoimmune or autoinflammatory".

Diagnosis is typically based on the symptoms with support from medical imaging and blood tests.

"AS is a type of serongative spondyloarthropathy", meaning that tests show no presence of rheumatoid factor antibodies.

Onset is typically in young adults. Males and females are equally affected. Men are more likely than women to experience changes to the bones and fusion, and thus they were being picked up using X-ray. Women are more likely than men to experience inflammation rather than fusion.

There is no cure for ankylosing spondylitis. Treatments may improve symptoms and prevent worsening. This may include medication, exercise, physical therapy, and surgery in rare cases.

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