How Are Axial Spondyloarthritis and Ankylosing Spondylitis Related?

When learning about spondyloarthritis, it's easy to get confused about terminology and categories. The terminology is particularly confusing because doctors have been changing the way these conditions are categorized over recent years, and not even medical professionals always use the terms consistently. But, there are some general things you can learn to help you keep the categories straight and to better understand them.

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 that affects primarily the axial joints (the joints of the spine, chest, and pelvis).

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

Types of Spondyloarthritis
Illustration by JR Bee, Verywell

Spondyloarthropathies

Spondyloarthropathies are the broadest category in the topic.

Spondyloarthropathies are a group of related inflammatory illnesses that have some overlapping characteristics in terms of their symptoms and genetics.

Some of these often overlapping characteristics include:

  • inflammation in the spine and pelvis bones
  • inflammation of peripheral joints
  • inflammation of a part of the eye (uveitis)
  • inflammation at insertion point of ligaments and tendons (enthesitis)
  • inflammation of the bowel
  • skin lesions
  • association with the genetic factor HLA-B27

Classically, people with spondyloarthropathy were often diagnosed into smaller subsets depending on other symptoms.

For example, people who had these symptoms along with psoriasis would be diagnosed with psoriatic spondyloarthritis. People who had inflammatory bowel disease along with these symptoms might be told they had enteropathic spondyloarthritis. People who didn’t fit neatly into a category might be told that they had undifferentiated spondyloarthritis.

Axial vs. Peripheral Spondyloarthritis

More recently, clinicians have started dividing spondyloarthropathies into axial spondyloarthritis and peripheral spondyloarthritis.

In axial spondyloarthritis, the arthritis symptoms affect the axial joints—the joints of the spine, chest, and the sacroiliac joint (hip bone).

In peripheral spondyloarthritis, people usually don’t have symptoms affecting these joints. Instead, they might have symptoms that affect the other joints, such as the knees or finger or toe joints.

However, it isn’t always that clear cut. Some people with axial spondyloarthritis have problems with one or more of their peripheral joints as well.

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.

Where Does Ankylosing Spondylitis Fit In?

How does this apply to you if you've been told you have a condition such as ankylosing spondylitis?

Ankylosing spondylitis (AS) is an older disease category, but one that many physicians and patients still use. The term "spondylitis" simply means "inflammation of the vertebra," the small bones that make up your spine.

AS is considered a variety of axial spondyloarthropathy, since it primarily affects the axial joints.

Patients with ankylosing spondylitis experience inflammatory back pain, limited movement of the spine, and sometimes other characteristic symptoms found in spondyloarthropathies, like enthesitis and uveitis. Often it can cause quite severe disability. Traditionally, it was thought to be more common in males compared to females, though it is now less clear if this is the case.

Historically, x-rays were an important part of diagnosing AS. When viewed under x-rays, you can also see signs of substantial inflammation of the sacroiliac joints (termed sacroiliitis) in a person with AS.

Because of this, ankylosing spondylitis is more or less synonymous with a newer term: radiographic axial spondyloarthritis. This is sometimes abbreviated as rAxSpA. You might also view ankylosing spondylitis as a more advanced or severe form of axial spondyloarthritis.

Ankylosing Spondylitis Doctor Discussion Guide

Get our printable guide for your next doctor's appointment to help you ask the right questions.

Doctor Discussion Guide Man

Non-Radiographic Axial Spondyloarthritis

Magnetic resonance imaging (MRI) began to be used more commonly in the 80s, 90s, and beyond. 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 some people had many of the symptoms of ankylosing spondylitis, although changes in chronic inflammation couldn’t be seen on x-ray. However, those patients 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 diagnosed with AS. These people were eventually categorized as “non-radiographic axial spondyloarthritis” or “pre-radiographic axial spondyloarthritis.” You might also see this abbreviated as nrAxSpA.

Non-Radiographic Axial Spondyloarthritis vs. Ankylosing Spondylitis?

Researchers and clinicians aren’t sure about the relationship between non-radiographic axial spondyloarthritis and ankylosing spondylitis yet.

Some people diagnosed with non-radiographic axial spondyloarthritis go on to later develop the symptoms of classic ankylosing spondylitis (i.e., radiographic axial spondyloarthritis), and the inflammation of the sacroiliac joint becomes severe enough that it can be viewed on an x-ray. In other words, their sacroiliitis goes from “non-radiographic” to “radiographic.” But, it also appears that many people don’t progress in this way.

Researchers still aren't sure how common progression is—it happens to some people, perhaps a majority, but not to everyone.

It is interesting to note that more women are diagnosed with non-radiographic axial spondyloarthritis than are diagnosed with ankylosing spondylitis. People with non-radiographic axial spondyloarthritis 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 other genetic differences too, but researchers are still working to understand that.

Currently, the treatment of the two conditions is very similar: Physical therapy, exercise, over-the-counter pain medications, and 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). So people with these conditions are generally not considered in this group. However, they still do sometimes have symptoms from problems with the axial joints.

There is a lot of treatment overlap for all the different forms of spondyloarthropathy, so the exact terminology for your diagnosis isn't always important.

A Word From Verywell

It’s often helpful to understand the basic terminology that clinicians and researchers use about your disease. If you are confused, don’t hesitate to ask your health care provider for clarification. That will give you a head start on dealing with and understanding and your condition. But also know that in this category, since treatment overlaps between the different types of the condition, the terminology isn't of utmost importance. The most important thing is for you to understand what's going on in your body and how you can best manage your symptoms.

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