In ankylosing spondylitis (AS) several parts of the lower spine become inflamed, including the bones in the spine (vertebrae) and spinal joints. Over time this can damage the spine and lead to the growth of new bone. In some cases this can cause parts of the spine to join up (fuse) and lose flexibility (ankylosis).
The exact cause of AS is unknown, but it is thought to be related to a combination of genetic and environmental factors.
Genetic factors
Research has shown that 9 out of 10 people with AS carry a particular gene known as human leukocyte antigen B27 (HLA-B27). Having this gene does not necessarily mean you’ll develop AS. It’s estimated 8 in every 100 people in the general population have the HLA-B27 gene, but most do not have AS. It’s thought having this gene may make you more vulnerable to developing AS. The condition may be triggered by 1 or more environmental factors, although it’s not known what these are.
Testing for this gene may be carried out if AS is suspected. However, this test is not a very reliable method of diagnosing the condition because some people can have the HLA-B27 gene but not have ankylosing spondylitis.
Environmental factors
Environmental factors such as infections, may also play a role in the development of AS. If your genes already make you prone to AS, something else like an injury or a bacterial infection could trigger it to start. Scientists don’t know exactly what these triggers may be.
One theory is that AS could start with a breakdown of the natural defenses in your intestines. This allows bacteria to pass into your bloodstream. That, in turn, may trigger changes in your immune response that lead to AS.
Risk Factors
Anyone can get it. But your chances of having ankylosing spondylitis are higher if:
- Others in your family have the disease.
- You’re 17-45 years old. AS usually, but not always, starts when you’re a teen or young adult. This is earlier than other types of arthritis and rheumatic diseases.
- You’re a man. Men are at least twice as likely to get the disease as women.
- You have gastrointestinal infections regularly.
- You have another autoimmune disease, like psoriasis, Crohn’s disease, or ulcerative colitis.
More research is however needed to fully understand the the interplay of genetic and environmental factors in the development of AS.