Ankylosing spondylitis (AS)

Ankylosing spondylitis is a form of inflammatory arthritis that mainly affects the spine and the sacroiliac joints where the spine meets the pelvis, part of a larger group of conditions called axial spondyloarthritis. Current understanding points to immune cells and inflammatory cytokines, particularly along the interleukin-23/17 pathway, driving inflammation at the spine and at the entheses, the points where ligaments and tendons attach to bone, which over time can lead to stiffening and, in some people, fusion of parts of the spine. The HLA-B27 gene is strongly associated with the condition, present in a large majority of people diagnosed, though having the gene alone does not mean someone develops AS.

Reported prevalence varies considerably across populations, and research describes a male predominance in classic AS, with onset almost always before age 50.

Common signs

Deep, ongoing back and buttock pain and stiffness that is typically worse with rest and in the morning and eases with movement. Reduced spinal flexibility, and sometimes hip or other joint involvement, eye inflammation (uveitis), and fatigue.

This is a plain-language summary for orientation, not a diagnosis. Mechanisms are described as current understanding, not settled fact. Work with your own clinician for your care.