Ankylosing spondylitis (AS) is a type of arthritis that primarily affects the spine. It inflames the joints and ligaments that normally allow the back to move. As the disease progresses it can affect other joints, such as those between the ribs and spine and those of the shoulders, hips, knees and feet. It can also cause inflammation in the eyes, lungs and heart valves.

Patients with AS experience chronic pain and stiffness in the lower back. Inactivity tends to make symptoms worse. Many patients experience back pain in the middle of the night and may feel stiff in the morning.

Several genes have been linked to AS, but its exact cause remains unknown. It is believed to be an autoimmune disease, meaning the patient's immune system is hyperactive and attacks the body's own tissues.

Treatments including medications and exercise can help reduce the pain and stiffness. The earlier treatment for AS begins, the more effective it is likely to be.