Ankylosing spondylitis (AS) is a type of arthritis that causes inflammation in the joints of the spine. The most common areas affected are the sacroiliac joints, which are the joints at the base of the spine that connect the spine and the pelvis, as well as the joints between the vertebrae. Other joints, such as the hips and shoulders, may also be similarly affected. AS causes pain, stiffness, and inflammation at the affected joints.
AS is the most common of the arthritis conditions known as spondylopathies. The second most common spondylopathy occurs in people with psoriasis.
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 to about 15% to 20% if a first-degree relative (e.g., a parent) has it. However, if you carry this molecule without a family history, the chance of getting this condition is 1% to 2%. 92% of Caucasian patients with AS are HLA B27-positive, compared to only 50% of African Americans.
AS affects about 3 times as many men as women, but it may be that the disease is less recognized among females. Most people are first diagnosed between the ages of 15 and 40 years. However, younger and older people can also be affected.
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. One theory is 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 by 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.