Axial Involvement in PsA Still Defies Definition

VIENNA — While there is no doubt that some people with psoriatic arthritis (PsA) have axial symptoms, data presented at the European Alliance of Associations for Rheumatology (EULAR) 2024 Annual Meeting 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," 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," Carron added.

There are, however, two things that can be agreed upon, according to 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 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, 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]," 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.

AXIS Study 'Gives Answers'

More research into factors associated with axial PsA need to be performed to try to help define the condition and enable classification and ultimately treatment guidelines. This is where the AXIS study comes in.

The AXIS study is a joint project of ASAS and GRAPPA that was started in January 2019 with the aim of defining a homogenous subgroup of patients who could be studied.

"The objectives of the AXIS study are to determine the frequency of axial involvement in patients with PsA; to identify the frequency of active inflammatory and structural changes on imaging; and to identify factors associated with the presence of axial involvement in PsA," Murat Torgutalp, MD, of Charité – Universitätsmedizin Berlin, Berlin, Germany, said at EULAR 2024.

The study population consisted of 409 consecutively recruited patients diagnosed with PsA according to CASPAR criteria; all have had PsA for up to 10 years and were untreated with biologic or targeted synthetic disease modifying drugs at the time of inclusion.

Torgutalp, who is the study's primary research coordinator, reported that a diagnosis of PsA was made in 37% of the population when local investigators considered available clinical, laboratory, and imaging data. However, patients' imaging data were also centrally assessed, and when the local investigators were party to the expert imaging interpretations, the percentage of people diagnosed with PsA dropped to 27%.

"When we looked at the clinical characteristics, the presence of the back pain, particularly inflammatory back pain, HLA-B27 positivity, elevated CRP, and presence of active, inflammatory and structural changes in the sacroiliac joints and spine were associated with the final conclusion on the presence of axial involvement," Torgutalp said.

Despite the title of his presentation being "The Axis Study Gives Answers," Torgutalp presented lots of data without giving much insight into how they might be used. He concluded that "overall, there was a trend towards overestimation of the presence of imaging changes indicative of axial involvement across all imaging modalities" by the local investigators, he said.

Dennis McGonagle, MB, MCH, BAO, PhD, of the University of Leeds, Leeds, England, told Medscape Medical News that the AXIS study "is a noble, international effort across multiple countries to try and better understand axial PsA."

McGonagle, who was not involved in the study, added: "A lot of data are being generated, and a lot of analysis needs to be done to drill down to get a clear message that could influence practice."

Axial PsA in the Portuguese Population

Separately, Catarina Abreu, a rheumatology intern at Hospital Garcia de Orta, Almada, Portugal, presented some real-world data on axial PsA from Reuma.pt.

Of 2304 patients, 854 (37.1%) reportedly had axial PsA, which had been defined as physician-reported spondylitis or the presence of imaging findings suggestive of axial involvement. This included radiographic- or MRI-detected sacroiliitis or syndesmophytes seen on axial x-rays.

The majority (78.2%) of those with an axial PsA diagnosis had concomitant peripheral involvement, with 8.1% having exclusive axial disease.

About 70% of the axial PsA diagnoses had been made using clinical or laboratory findings alone, and 30% of diagnoses was based on imaging results. Of the latter, Abreu noted that patients who had imaging data available were more likely to be HLA-B27 positive and less likely to have dactylitis, with respective odds ratios (ORs) of 3.10 and 2.42.

Individuals with axial PsA were more likely to have enthesitis (OR, 1.92), although no data were available on whether this was axial or peripheral enthesitis. Tobacco exposure was also linked to an increased chance of having axial PsA (OR, 1.66).

Abreu noted that the "scarce number of available imaging exams" and other missing data in Reuma.pt may have led to an underdiagnosis of axial PsA.

"The difference that we found between axial and peripheral [PsA] are similar to the differences found in other studies that compared axial psoriatic arthritis with axial spondyloarthritis," Abreu said.

"So, we leave with the question that was already left before here: If these are different diseases or just different phenotypes of the same disease, and what implications will this have in the future?" Abreu concluded.

Carron received educational grants, speaker fees, or honoraria for other consultancy work from AbbVie, UCB, Pfizer, Eli Lilly, Novartis, Janssen, and Galapagos/Alfasigma.

Ramiro is an ASAS executive committee member and received research grants or consulting/speaker fees from AbbVie, Eli Lilly, Galapagos, Janssen, Merck Sharp and Dohme, Novartis, Pfizer, Sanofi, and UCB.

AXIS is supported by unrestricted research grants from AbbVie, Galapagos, Janssen, Eli Lilly, Novartis, Pfizer, and UCB. Torgutalp is the primary research coordinator for the study; he reported no financial conflicts of interest.

The Reuma.pt registry was developed with the financial support of the pharmaceutical industry and is currently supported by AbbVie, Amgen, AstraZeneca, Boehringer Ingelheim, Eli Lilly, Merck Sharp and Dohme, Novartis, Pfizer, and Sobi. Abreu reported no financial conflicts of interest.

Sara Freeman is a medical journalist and writer based in London, England. She is a regular contributor to Medscape Medical News, Medscape News UK, and other specialist healthcare media outlets.