Undifferentiated Spondyloarthritis
The last post in our spondyloarthritis series. When it looks like SpA but doesn't fit a specific box.
This is the last post in our spondyloarthritis series. If you missed the earlier ones, we've covered the primer, psoriatic arthritis, ankylosing spondylitis, IBD-associated arthritis, and reactive arthritis. Today is the group that doesn't quite fit into any of those.
That group is called undifferentiated spondyloarthritis.
The name is exactly what it sounds like. These are patients who clearly have spondyloarthritis (the joint pattern, the enthesitis, the dactylitis, sometimes inflammatory back pain) but don't have any of the defining features that would put them into one of the specific subtypes. No psoriasis. No inflammatory bowel disease. No recent infection that triggered it. No clear axial disease pattern that meets the criteria for ankylosing spondylitis.
Why the category exists
You might ask why we have this category in the first place. If they have spondyloarthritis, why not just call it that and move on?
A couple of reasons.
Some of these patients will eventually develop features that put them into one of the specific subtypes. Someone with undifferentiated SpA might develop psoriasis a few years down the road and reclassify as psoriatic arthritis. Someone else might develop inflammatory back pain and axial features and end up with a diagnosis of ankylosing spondylitis. So the category is a placeholder for patients whose disease hasn't fully declared itself yet.
Other patients will stay undifferentiated their whole lives. They never develop psoriasis, never develop IBD, and their disease just keeps behaving like an undifferentiated SpA.
Either way, treating them as spondyloarthritis is the right move.
How I approach it
In my practice, when a patient presents like this, I usually treat them the same way I would treat psoriatic arthritis. The reason is that psoriatic arthritis has the broadest set of treatment options that map well to what undifferentiated SpA patients tend to have going on. Peripheral joint involvement, enthesitis, sometimes dactylitis, sometimes axial symptoms.
The medication categories are all on the table. NSAIDs, sulfasalazine, methotrexate, TNF inhibitors, IL-17 inhibitors, IL-23 inhibitors, JAK inhibitors. The choice depends on what they've got going on and how severe it is.
The one exception is if I suspect there might be some hidden IBD lurking. In that case, I steer away from IL-17 inhibitors (which can worsen IBD) and lean toward the medications that cover both gut and joints. Sometimes I'll coordinate with GI to help sort things out.
If a patient also has uveitis, I avoid etanercept (Enbrel). Etanercept doesn't work as well for uveitis as the other TNF inhibitors, so I'll reach for something like adalimumab or infliximab instead.
If undifferentiated SpA later declares itself as PsA, AS, or IBD-associated arthritis, we adjust treatment accordingly. But for a lot of these patients, treating like PsA works well and we stay the course.
HLA-B27
Like the other spondyloarthritis subtypes, HLA-B27 can be helpful in undifferentiated SpA but doesn't make the diagnosis. Positive B27 in the right clinical context supports the diagnosis. Negative doesn't rule it out. Most of the general population who are B27 positive don't have any spondyloarthritis at all.
We covered this more in the primer post, so I won't rehash it here.
Wrapping up the series
That's the last of the six main groups. Spondyloarthritis is a bigger and more common category than most people realize.
The good news is that most spondyloarthritis is treatable. Some of it very treatable. The treatment landscape has changed a lot in the last twenty years and we now have a lot of good options.
If you've been dealing with joint symptoms that fit the pattern of any of these conditions (asymmetric joint pain, back pain that improves with movement, heel pain that won't quit, a swollen finger or toe, joint symptoms alongside psoriasis or IBD), a rheumatology evaluation is worth it.
That's what I do here at Restore Rheumatology. Direct care for arthritis and autoimmune disease, in Oakdale, Minnesota. If you think any of this might apply to you, get in touch.
