VIENNA — While there is no doubt that some people with psoriatic arthritis (PsA) have axial symptoms, data presented at the annual European Congress of Rheumatology do not appear to add much to what is already known about axial PsA or to further the cause of differentiating it from axial spondyloarthritis (axSpA).
In both the AXIS study and Reuma.pt, around one in three patients with PsA were found to have axial involvement. Notably, the percentage of people with axial PsA was found to vary according to how imaging information was interpreted in the AXIS study. Both studies were discussed during the Axial Involvement in PsA and SpA session at EULAR 2024.
The One-Million-Dollar Question
“So, the one-million-dollar question: What is it, really?” Philippe Carron, MD, PhD, Ghent University Hospital, Ghent, Belgium, said in the presentation that started the session. Despite PsA being described more than 60 years ago, “we still have no internationally accepted definition or a consensus on how we should define these patients and how we should screen them,” he said.
“There are some believers that it is just a form of axial SpA with concomitant psoriasis, but also some people that think that the axial PsA is a typical disease, with typical characteristics which are different from axial disease,” Dr. Carron said.
The lack of consensus makes it difficult to estimate just how many people have axial PsA. Reported prevalences range from 5% to 70%, “all caused by which criteria that you’re using to define axial involvement,” Dr. Carron added.
There are, however, two things that can be agreed upon, according to Dr. Carron. First, the prevalence of axial involvement in people with early PsA is “much, much lower” than that of more established disease. Second, exclusive axial involvement is seen in “just a minority of PsA patients.” Most people with axial disease also have peripheral disease, he added.
Imaging findings in axial PsA “are quite similar to those seen in axial SpA,” although Dr. Carron also said that there were some distinct differences. Radiographic sacroiliitis occurs in around 25%-50% of people with axial PsA, and atypical syndesmophytes are more often found in people with axial PsA than in those with axSpA.
Shared Characteristics
But are axial PsA and axSpA separate diseases or part of the same disease continuum? That’s a question that is still very much open for debate, said Sofia Ramiro, MD, PhD, a senior researcher at Leiden University Medical Center, Leiden, the Netherlands, and rheumatology consultant at Zuyderland Medical Center in Heerlen, the Netherlands.
While many studies have looked to answer this question, there is a big methodological problem — the studies largely cannot be compared as they have used different definitions of axSpA.
Take a patient with inflammatory back pain, psoriasis, and oligoarthritis, Dr. Ramiro said. If the patient goes to one rheumatologist, they may get a diagnosis of axSpA, but if they go to a different rheumatologist, they may get a diagnosis of axial PsA.
“This is influenced by training, expertise, by beliefs, and by belonging to ASAS [Assessment of Spondyloarthritis International Society] or to GRAPPA [Group for Research and Assessment of Psoriasis and Psoriatic Arthritis],” Dr. Ramiro suggested. It’s “a diagnostic bias” that is very difficult to overcome and makes direct comparisons between patient populations recruited into clinical studies “extremely challenging.”
To confuse matters more, axial PsA and axSpA share common characteristics: Inflammatory back pain, HLA-B27 positivity, elevated levels of C-reactive protein (CRP) or a higher erythrocyte sedimentation rate, and structural lesions in the sacroiliac joints and spine.