Overview of Axial Spondyloarthritis

Doctor pointing to Sacroiliac Joint on skeleton
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Axial Spondyloarthritis is a type of chronic inflammatory arthritis involving the spine and/or sacroiliac joints. Together, the head, rib cage, and spine are called the "axial skeleton." Axial spondyloarthritis is one form of spondyloarthritis or spondyloarthropathy, which is a family of rheumatoid conditions where inflammation of the spine and sacroiliac joints is a key distinguishing feature.

These conditions are related because they share many of the same symptoms, including (as mentioned) inflammation in the spine, pelvis, and other joints, as well as intestines, eyes, ligaments and tendons. While the diseases in the “family” share the key feature of inflammation, each—with one exception discussed below—has its own unique distinction.

Your doctor will likely diagnose you into one of the “family members” based on a number of things, not the least of which is inflammation in a specific area of the body, such as your eyes, heels, intestines or back. Diagnosing involves a physical exam, a conversation about your symptoms, and/or imaging and lab tests. 

If you have axial spondyloarthropathy, 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. 

Radiographic and Non-Radiographic

Along those lines, researchers have proposed two types of axial spondyloarthropathy—the type where arthritic changes show up on x-ray/CT scan and the type where it doesn’t. When changes on x-ray/CT scan are apparent, this is called radiographic axial spondyloarthropathy (acronym AxSpA.) And when no changes are apparent, it’s called non-radiographic axial spondyloarthropathy (acronym nr-axSpA.)

Nearly all people diagnosed with ankylosing spondylitis fit into the category of AxSpA, as discussed above. People with reactive, enteropathic, psoriatic, and others may fall into either category.

A 2015 study published in RMD Open comparing ankylosing spondylitis and non-radiographic axial spondyloarthritis found many similarities between these two related conditions. But the researchers also found three main differences—in mobility and function vs limitation, degree of inflammation, and whether males or females were more predisposed.

Generally, males are at the higher risk for ankylosing spondylitis (aka the radiographic variety we’re speaking of here, i.e., AxSpA.) But in this study and other done in recent years, researchers have been discovering that more females show signs of the non-radiographic axial spondyloarthritis (nr-AxSpA) variety.

This may be because MRI is starting to be used on non-radiographic cases. There's a big difference between what radiography—which, as mentioned above, is an x-ray or CT scan—can reveal about axial spondyloarthritis and what non-radiographic measurements, which are typically done using MRIs, can reveal. While an MRI may not yield all the information about axial spondyloarthritis, it does shed new light—including the early development of the inflammation, something that radiography cannot provide.

One of the big “a-ha's” that has resulted from the use of MRI to detect signs of inflammation is that, counter to what was previously thought, sacroiliitis (and possibly the related condition of ankylosing spondylitis) may well affect men and women equally. That said, the jury is still out. Most researchers will tell you more studies are necessary before they can confirm this new fact about ankylosing spondylitis high-risk groups.

Undifferentiated Spondyloarthritis

If you have axial spondylitis without sacroiliitis, you may be diagnosed with axial undifferentiated spondyloarthritis. Some people with this condition 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.

By the way, undifferentiated spondyloarthritis is the category people with some type of chronic inflammatory arthritis are placed into if they do not also have a distinguishing feature, as discussed earlier.

Genetic Origins

Axial spondyloarthritis is related to the presence of a gene known as HLA-B27. 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.

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