Non-radiographic axial spondyloarthritis (nr-AxSpA) is a form of inflammatory arthritis affecting the spine. The condition primarily causes pain and inflammation in the lower back.
Nr-AxSpA exists on the same disease spectrum as ankylosing spondylitis (AS). The primary difference between nr-AxSpA and AS is that the former does not show visible joint damage on X-rays, while the latter does. As such, doctors consider nr-AxSpA to be an early stage of AS.
This article describes what nr-AxSpA is, including its symptoms, diagnosis, and treatment. It also outlines the differences between nr-AxSpA and AS.
Nr-AxSpA is a type of spondyloarthritis, meaning it is a type of inflammatory arthritis that primarily affects the spine. Doctors refer to the condition as “non-radiographic” because X-ray imaging does not reveal any visible damage to the joints.
If nr-AxSpA worsens to the extent that joint damage is visible on spinal X-rays, doctors will then change the diagnosis to AS, a more severe form of spondyloarthritis.
Doctors consider both nr-AxSpA and AS to be forms of spondyloarthritis that exist at opposite ends of the same disease spectrum.
X-rays will not show evidence of nr-AxSpA because the inflammation has not yet caused visible damage to the joints. However, an MRI may indicate active inflammation in the soft tissues surrounding the sacroiliac (SI) joints. These joints connect the hip bones to the sacrum — the triangular bone that sits between the lower spine and the tailbone.
An MRI may also show swelling or edema in the bone marrow of the SI joints.
Nr-AxSpA does not occur due to mechanical issues in the spine. Instead, it results from an overactive immune system and associated chronic inflammation.
In nr-AxSpA, the immune system mistakenly attacks healthy body tissues, triggering a chronic inflammatory response. This response causes damage to healthy cells and tissues.
Experts do not understand why the immune system behaves in this way in people with nr-AxSpA, although genes appear to play a role. For example, the gene HLA-B27 is common among individuals with spondyloarthritis.
The symptoms of nr-AxSpA may resemble other forms of back pain, such as sprains, strains, and slipped discs. However, there are ways to differentiate the symptoms. Some examples are below.
Pain
Nr-AxSpA may cause back pain that wakes a person from their sleep or is present when they wake up.
Someone with nr-AxSpA may also feel significant morning stiffness that makes it challenging for them to start the day.
Because nr-AxSpA pain occurs in response to inflammation, other areas of the body may also become painful, particularly the knees and hands.
Exercise
While some forms of back pain improve when a person is resting, nr-AxSpA back pain may improve with movement and stretching.
Enthesitis
Enthesitis is the medical term for inflammation in areas where tendons or ligaments attach to bone. As nr-AxSpA progresses to AS, enthesitis can cause the bones of the spine to fuse. This can affect a person’s mobility.
Age
The symptoms of nr-AxSpA often begin before the age of 40 years, typically during a person’s late teens or early adulthood.
Other conditions
The chronic inflammation that has links to nr-AxSpA may trigger other inflammatory conditions, such as:
- Psoriatic arthritis: A skin condition that doctors characterize by the formation of psoriatic plaques, which appear as raised, inflamed, scaly areas of skin.
- Inflammatory bowel disease: A group of inflammatory disorders that includes ulcerative colitis and Crohn’s disease. The condition may cause symptoms, such as:
- abdominal pain, cramps, or bloating
- recurrent or bloody diarrhea
- weight loss
- extreme fatigue
- Uveitis: A type of inflammatory eye condition that may cause eye pain and blurred vision. Around 15% of people with nr-AxSpA develop uveitis.
There is no single definitive diagnostic test for nr-AxSpA. Instead, doctors diagnose the condition according to the following:
- a person’s symptoms
- imaging tests
- blood tests
- physical examination
A person’s family medical history could also be helpful.
Imaging tests
Doctors typically use MRI scans to confirm a diagnosis of nr-AxSpA, as this imaging technique can indicate an active inflammation of the SI joint.
However, for some people with nr-AxSpA, an MRI does not reveal any evidence of the condition.
Blood tests
Doctors may order blood tests that can help identify inflammation in the body. Examples include blood tests for C-reactive protein and erythrocyte sedimentation rate. While these tests cannot pinpoint the location of the inflammation, they can reveal that the body is undergoing an inflammatory response.
Other blood tests may check for the HLA-B27 gene present in around 83% of individuals with AS. Doctors cannot use this test alone to confirm an nr-AxSpA or AS diagnosis because around 98% of people with the gene never go on to develop spondyloarthritis.
The treatment for nr-AxSpA and AS are the same and focus on easing pain rather than preventing the disease from progressing.
The two mainstays of treatment involve nonsteroidal anti-inflammatory drugs (NSAIDs) and physical therapy.
If the above methods are ineffective, a doctor may recommend biologic drugs called tumor necrosis factor (TNF) inhibitors. These drugs target specific inflammatory molecules in the body. However, the Food and Drug Administration (FDA) has not yet approved TNF inhibitors to treat nr-AxSpA. That said, evidence suggests these drugs may slow the progression of nr-AxSpA if a person takes them early in the course of the disease.
Both nr-AxSpA and AS are forms of spondyloarthritis, so doctors may consider nr-AxSpA an early stage of AS. Around 10 to 40% of people with nr-AxSpA will develop AS over the course of 2–10 years.
The primary difference between the two conditions is that nr-AxSpA has not yet progressed enough to cause bone changes visible on an X-ray, while AS has.
Because X-rays cannot capture the early damage that may have links to nr-AxSpA, doctors may instead use MRIs to look for swelling in the soft tissues surrounding the joints.
Traditionally, a diagnosis of AS requires X-ray evidence of bone changes. These changes may take a decade to develop. In addition, X-rays can only show the damage the inflammation causes, while MRIs show active inflammation.
The symptoms of late-stage nr-AxSpA may not differ significantly from those of early-stage AS, since these stages of spondyloarthritis exist relatively close together on the disease spectrum.
Nr-AxSpA is a type of inflammatory arthritis affecting the spine. It belongs on the same disease spectrum as AS. Unlike AS, it has not progressed sufficiently to cause bone damage that shows on radiographic imaging.
One of the main symptoms of nr-AxSpA is back pain due to inflammation. The pain differs from those of mechanical back pain because it often begins in late adolescence or early adulthood and may improve rather than worsen with physical activity.
Treatments for nr-AxSpA may comprise physical therapy and NSAIDs. Additionally, doctors may recommend biologic drugs called TNF inhibitors that help to reduce inflammation.