Ankylosing Spondylitis

The Facts

Ankylosing spondylitis (AS) is a type of arthritis that affects the ligaments of the spine where they attach to the bone. Other joints, such as the hips and shoulders, may also be similarly affected. AS causes pain, stiffness, and inflammation at the affected joints.

About 0.5% of Americans have AS. Having a family member with AS increases your risk of developing the condition, since the disease is at least partly hereditary. People with a certain molecule, called HLA B27, on the surface of their cells are also more likely to get AS. Having both HLA B27 and a family history further increases your risk: about one in five of these people will develop AS. If you carry this molecule, however, without a family history, the chance of getting this condition is less than 1 in 10.

AS affects about three times as many men as women. People with AS tend to be of medium height with a slim build and a long, narrow face. Most people are first diagnosed between the ages of 15 and 40 years. However, younger and older people can also be affected.

Causes

The cause of AS is not completely understood, but it's believed to be at least partly related to genetics. AS is more common in people with a family history of the condition. Scientists believe that AS is "triggered" by something in the environment, such as an infection, for people whose genes put them at risk of AS. The immune system responds to this trigger, producing chemicals that cause inflammation in the spine and other joints of the body. There is no evidence, however, that an infection causes the disease.

It is also known that people with a molecule called HLA B27 on the surface of their cells are at higher risk of developing AS. HLA B27 can be passed down from parent to child. Although it increases the risk of AS, not everyone with HLA B27 will get AS.