Spondyloarthritis (SpA) is a type of arthritis that involves joints along the spine as well as hips, shoulders, knees and ankles.

When there is arthritis in a joint, it can be warm and swollen and painful to move or tender to touch. These problems happen because of the inflammation (swelling) in the joint. When the arthritis is in a large, deep-seated joint, such as the hip or along the spine, the swelling can be hard to see, or sometimes not able to be seen, on physical exam. This means radiology imaging such as CTs or MRIs is needed. In SpA, swelling often also happens where tendons attach our muscles to bones or where ligaments attach to bones. These areas are called enthesis (plural entheses).

SpA can happen in children and adults. There are many forms of SpA.

The most advanced form of SpA is ankylosing spondylitis (AS). To have AS there must be proof of that the spine is involved on the X-ray. When this happens in children it is called juvenile ankylosing spondylitis (JAS).

A far more common type of SpA in children is called enthesitis-related arthritis (ERA). See details below in "Signs and Symptoms." 

SpA most often happens in people who are 20-30 years old, but one out of each six or seven cases starts in the teen years. If you have not gotten AS by the time you are 45, chances are good that you won't. The exact number of people with SpA is not known. It happens in about five to nine out of each 1,000 people. Since about half of these people have AS, there are close to 500,000 cases in the US alone.