Ankylosing spondylitis (AS) is type of chronic arthritis that primarily affects the spine. The joints between the vertebrae of the spine and ligaments that allow the back to move become inflamed, sometimes leading to the joints and bones fusing together. This inflammation often leads to stiffness of the back and hips.

The name of the disease describes its destructive nature. “Ankylosing” means stiff or rigid, "spondyl" refers to the spine, and "itis" means inflammation. AS belongs to a family of diseases known as spondylarthropathies that attack the spine. These diseases include psoriatic arthritis, Reiter's syndrome (a form of reactive arthritis, sparked by infection) and enteropathic arthritis (arthritis related to digestive disorders such as inflammatory bowel disease or celiac disease).

While AS progresses, it can also cause inflammation of other organs in the body, including the eyes, lungs and heart valves. Anemia (reduced number of red blood cells) is also associated with the chronic inflammation of ankylosing spondylitis.

The impact of AS varies widely from individual to individual. Some patients experience episodes of back pain that come and go, whereas others have chronic symptoms that worsen and cause severe joint and back stiffness, loss of motion and deformity.