Axial spondylitis is a type of inflammatory arthritis involving the spine and/or sacroiliac joints.
If you have axial spondylitis, you may or may not also have sacroiliitis. If sacroiliitis is present on your x-ray, your doctor will likely diagnose you with ankylosing spondylitis, or AS. Ankylosing spondylitis is the most common type of inflammatory arthritis (spondylitis).
If you have axial spondylitis without sacroiliitis, you may be diagnosed with axial undifferentialed spondyloarthritis. Some people diagnosed with axial undifferentialed spondyloarthritis eventually develop sacroiliitis and may then be diagnosed with AS. Others do not, but their axial undifferentiated spondyloarthritis is not just a mild form of AS. It is a disease all its own, and the pain can be just as great.
Axial spondylitis is strongly related to the presence of the HLA-B27 gene. A genetic test for HLA-B27 is available. Along with ankylosing spondylitis, two other types of inflammatory arthritis associated with the presence of HLA-B27 are psoriatic arthritis and enteropathic arthritis. If you have either of these forms of inflammatory arthritis and you have the HLA-B27 gene, you'll likely experience pain and other symptoms in your spine.