Ankylosing spondylitis is a form of arthritis that affects the spine. The term is derived from two Greek words meaning "bent spine” and “inflammation." Ankylosing spondylitis is a chronic condition that causes painful inflammation of the joints between the vertebrae in the spine, and between the spine and pelvis. In advanced cases, the vertebrae may fuse together, further limiting movement and resulting in a hunched over posture. Occasionally, ankylosing spondylitis can involve other joints or more rarely, organs in the body, such as the heart and lungs.
Ankylosing spondylitis affects men more often than women and is usually diagnosed in late adolescence or early childhood. There does appear to be a genetic component to ankylosing spondylitis, which has to do with an antigen called HLA-B27. Although many people who have the HLA-B27 antigen never develop ankylosing spondylitis, it is considered a significant risk factor. While about 7% of the US population is HLA-B27 positive, 90-95% of those with ankylosing spondylitis are HLA-B27 positive. Only about 5-6% of individuals who are HLA-B27 positive will ever develop ankylosing spondylitis. Research into other risk factors and potential triggers that lead to ankylosing spondylitis is ongoing.
There are several theories regarding the exact reason for the inflammation associated with ankylosing spondylitis. One possibility is that foreign DNA, such as from a virus, may trigger the body’s immune system, but the immune system over-reacts and starts attacking its own cells, leading to chronic inflammation. At this time, the exact cause of the inflammation seen in ankylosing spondylitis is not known. One of the effects of chronic inflammation of the ligaments around the bone, particularly in the spine, is triggering of new bone growth, which can cause individual vertebrae to fuse together. Fusion of the vertebrae can lead to permanent reduced mobility of the spine, as well as stiffening of the rib cage. When the ribcage becomes less flexible, the chest cannot expand as fully as usual, and this lower capacity to take in air causes difficulty with breathing.