Overview of Ankylosing Spondylitis
Ankylosing spondylitis (AS) is one of a group of chronic conditions called spondyloarthropathy or spondyloarthritis, which also includes reactive arthritis, psoriatic arthritis, and Reiter's syndrome. Ankylosing spondylitis is an inflammatory disease that primarily affects the axial skeleton—the spine and the sacroiliac joint. The sacroiliac joint is located between the sacrum (bone at the base of the spine) and the pelvis.
The term "ankylosing" refers to stiffness or immobility, and "spondylitis" is inflammation of one or more vertebrae in the spine. Initial symptoms of AS include pain, tenderness, and stiffness in the lower back.
AS causes inflammation, swelling, and lesions in the areas where ligaments attach to bone, particularly in the spine and pelvis. As the condition progresses, the lesions undergo a process called ossification, which is the formation of new bone.
The spine is made up of individual bones called vertebrae, which are interconnected and are cushioned and separated by intervertebral disks (fibrous cartilage that encloses a central mass). The excess build-up of bone bridges across the intervertebral disks and starts to connect the vertebrae. Eventually, fusion of the vertebrae prevents the back from rotating and bending normally.
Ankylosing spondylitis also can affect the breastbone (sternum), hips, shoulders, and peripheral joints (e.g., knees). When this occurs, pain and stiffness gradually progresses to a complete loss of function in the affected joint.
Incidence & Prevalence of Ankylosing Spondylitis
Ankylosing spondylitis usually develops during adolescence (late teens) or in early adulthood. The average age of onset is 23 years old. In approximately 5 percent of cases, symptoms develop after the age of 40. Juvenile onset AS develops in patients younger than 16 years of age. Ankylosing spondylitis affects men approximately 3 times more often than women, and the disease is often more severe in men.