Axial Spondyloarthritis vs. Ankylosing Spondylitis: What's the Difference?

Spondyloarthritis (also called spondyloarthropathy) refers to a family of inflammatory rheumatological conditions that can affect the joints, including the joints of the spine, as well as other bodily systems.

Axial spondyloarthritis can be considered a subset of spondyloarthritis. This type primarily affects the axial joints, which are those found in the spine, chest, and pelvis.

Ankylosing spondylitis is generally considered a specific and severe subset of axial spondyloarthritis.

Spondyloarthropathies

Spondyloarthropathies are the broadest definition of the disorder. They are a group of related inflammatory illnesses with some overlapping characteristics in terms of symptoms and genetic links. Overall, these related conditions are believed to affect up to 2% of the population.

Some of these symptoms and characteristics include:

• Inflammation in the spine and pelvis bones
• Inflammation of peripheral joints
• Inflammation of part of the eye (uveitis)
• Inflammation where ligaments and tendons connect to bones (enthesitis)
• Inflammation of the bowel
• Skin lesions
• Association with the genetic factor HLA-B27

The subsets of spondyloarthritis were once classified according to their associated condition. These conditions and groupings included:

• Psoriatic spondyloarthritis, in people with psoriatic arthritis
• Enteropathic spondyloarthritis, in people with inflammatory bowel disease (IBD)
• Reactive arthritis, associated with a urinary tract infection, digestive tract infection, or elsewhere in the body
• Undifferentiated spondyloarthritis, with initially unclear patterns of symptoms
• Ankylosing spondylitis, a common form that affects the joints in your spine

Healthcare providers now place these variations into two main groupings:

• One is called axial, or axial-predominant, spondyloarthritis.
• The other is called peripheral, or peripheral-predominant, spondyloarthritis.

These names more accurately describe the way that symptoms sometimes overlap between the types.

Axial vs. Peripheral Spondyloarthritis

These two main divisions of spondyloarthropathies are based on the parts of your body that are most affected.

In peripheral spondyloarthritis, people have pain and other symptoms that affect joints such as the knees, fingers, or toes.

In axial spondyloarthritis, the arthritis symptoms affect the axial joints. These joints are found in the spine, chest, and the sacroiliac joint (hip bone).

Regardless of type, diagnosis of all spondyloarthropathies requires a physical exam, a conversation about your symptoms, and usually imaging and lab tests. There is also quite a bit of overlap in terms of the ways these different forms of spondyloarthropathy are treated.

Ankylosing Spondylitis

Ankylosing spondylitis (AS) is a term that many people with the condition, and healthcare providers who care for them, still use. The term "spondylitis" simply means "inflammation of the vertebrae," the small bones that make up your spine.

People with ankylosing spondylitis experience inflammatory back pain. This pain may arise with other symptoms that include:

• Limited movement of the spine
• Inflammation where ligaments and tendons connect to bones
• Pain or redness in one eye (uveitis)

Some people with ankylosing spondylitis experience severe impacts and mobility challenges that may interfere with being able to work.

Historically, X-rays were an important part of diagnosing the condition. When viewed with X-ray images, there are visible signs of substantial inflammation of the sacroiliac joints (termed sacroiliitis). These are the joints where the lower part of the spine (the sacrum) connects to the pelvis.

Because the diagnosis has relied on imaging, it gave rise to another set of terms used to describe the disorder and its progression.

These terms are called:

• Radiographic axial spondyloarthritis (rAxSpA), which is strongly linked with an AS diagnosis; sometimes the terms have been used interchangeably
• Non- or pre-radiographic axial spondyloarthritis (nrAxSpA)

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Non-Radiographic Axial Spondyloarthritis

Magnetic resonance imaging (MRI) began to be used more commonly in the 1980s. Because of the way it works, MRI can pick up more detail than conventional X-rays, revealing more information about what is going on.

Practitioners began to notice that although some people had many of the symptoms of ankylosing spondylitis, changes in chronic inflammation couldn’t be seen on X-ray. However, those people did have inflammation of the sacroiliac joint that could be seen with an MRI scan.

In other words, these people had inflammation and inflammatory changes that weren't as severe or advanced as those of people already diagnosed with ankylosing spondylitis. These people were eventually categorized as nrAxSpA.

Ankylosing Spondylitis and nrAxSpA

Researchers and clinicians still aren’t sure about the relationship between the non-radiographic and radiographic forms of axial spondyloarthritis, or how it defines ankylosing spondylitis.

In fact, a recent study of 2,080 people with axial spondyloarthritis, who were identified as either nrAxSpA or having a variant of rAxSpA, found that the current differentiation offers limited benefit in terms of understanding how well treatment will work and what kind of outcomes can be predicted.

Some people diagnosed with nrAxSpA go on to later develop the symptoms of classic ankylosing spondylitis that's considered rAxSpA. The inflammation of the sacroiliac joint becomes severe enough to be viewed on an X-ray, with their sacroiliitis defined as radiographic later on.

Yet this may not always happen. One study reviewed the progression in 94 people who were diagnosed six years earlier with axial spondyloarthritis and found 83% of them showed no changes in their sacroiliac joints during that time.

Seven people progressed from non-radiographic to rAxSpA during that time, but nine people actually changed from radiographic to nrAxSpA.

Your gender assigned at birth may be a contributing factor to any progression to ankylosing spondylitis, with more females diagnosed with nrAxSpA forms than with ankylosing spondylitis.

People with nrAxSpA tend to have a shorter length of disease and lower levels of inflammatory markers. They also tend to have less severe disease. There may be genetic differences, too, but researchers are still working to understand that.

Currently, the treatment of the two conditions is very similar. It may include:

• Physical therapy
• Exercise
• Over-the-counter pain medications
• Prescription treatments

Multiple Diagnoses

You're not likely to have both non-radiographic axial spondyloarthritis and another form of spondyloarthropathy. Currently, researchers are generally categorizing other classic forms of spondyloarthropathies as kinds of peripheral spondyloarthropathy.

For example, you might be told you have peripheral spondyloarthritis with psoriasis, peripheral spondyloarthritis with inflammatory bowel disease, or peripheral spondyloarthritis from a preceding infection (reactive arthritis).

People with these conditions are generally not considered in the axial spondyloarthritis group. However, they still do sometimes have symptoms involving the axial joints.

It’s often helpful to understand the basic terminology that professionals use to discuss your disease. Don’t hesitate to ask your healthcare provider to explain if you don't understand it. Keep in mind that the treatment can overlap among the different types of the condition, and the words are less important than knowing what's going on in your body and how to manage your symptoms.