Axial SpA vs Axial PsA: Different Entities?

Are there clinically significant differences between axial spondyloarthritis with psoriasis and psoriatic arthritis with axial symptoms? Does it matter?

It all depends on whom you ask, but right now the evidence seems to be tipping in favor of the "splitters" who cite evidence supporting their contention that axial spondyloarthritis (axSpA)/ankylosing spondylitis (AS) with psoriasis and psoriatic arthritis (PsA) with axial symptoms are distinct clinical entities that require more precise diagnosis and treatment.

"Lumpers," in contrast, argue that they are different points on the same clinical spectrum.

The debate is not just of academic interest, but has real consequences for patients, say specialists on both sides of the aisle.

Overlapping features, different presentations

"Axial SpA and axPsA have overlapping features but also meaningful differences in genetics, clinical presentation, imaging, and immunophenotype. Efforts are underway to develop classification criteria for axPsA to aid research efforts as well as clinical diagnosis and management," Philip J. Mease, MD, director of rheumatology research at Swedish Medical Center/Providence–St. Joseph Health in Seattle, and colleagues contend.

In an editorial published in the International Journal of Rheumatic Diseases, Dr. Mease and colleagues noted that, although HLA-B*27 is a genetic risk factor for both axPsA and axSpA, some HLA-B alleles are significantly associated with axPsA, whereas other alleles are associated with axSpA.

In addition, while genes in the interleukin-23 and IL-17 pathway are associated with increased risk for axSpA, genes in the IL-13 pathway have been identified as risk markers for axPsA, they noted.

Two cohorts better than one?

Dafna Gladman, MD, professor of medicine at the University of Toronto and senior scientist at the Schroeder Arthritis Institute at Toronto Western Hospital, and colleagues have a unique perspective on the similarities and differences between the disease entities.

Dr Dafna Gladman

Her group's research uses data on cohorts of patients treated in two separate clinics at Toronto Western Hospital: one for patients with PsA, and one for patients with axial spondyloarthritis, including those with ankylosing spondylitis, nonradiographic axSpA, and spondylitis associated with inflammatory bowel disease.

"Our work has shown that there are differences, and one of the reasons that it's now important is that the anti–IL-23 medications, both the IL-12/23 inhibitor ustekinumab [Stelara] and the IL-23 inhibitor guselkumab [Tremfya] work for psoriatic arthritis, whereas IL-23 did not work in ankylosing spondylitis, so that provided further impetus to look into the distinction between the two groups," Dr. Gladman said in an interview.

Dr. Gladman and colleagues published a study in Rheumatology in which they compared clinical presentations and features of patients with AS with or without psoriasis with patients with axPsA.

They found that patients with AS with or without psoriasis tended to be younger, had a higher proportion of males to females, and were more likely to be positive for HLA-B*27. Patients with AS also had more back pain at presentation, worse axial disease activity scores, worse global assessments by physicians, and higher grades of sacroiliitis, and they were more likely to be taking biologic agents.

"What that showed, right off the top, that whether we're looking at the total group or we're looking specifically at those patients who have psoriasis or don't have psoriasis, they are different from those with psoriatic arthritis with axial disease," she said.

They concluded that "axPsA seems to be a distinct entity."

Two clinics, same presentation

Because the aforementioned study included all patients with PsA with or without peripheral disease, the investigators decided to filter out some of the background noise and conduct a second study in which they compared patients who presented to the two clinics with the same presentation, either with spinal disease and psoriasis to the spondylitis clinic, or with psoriasis and isolated axial disease to the PsA clinic.

The results, published in Annals of the Rheumatic Diseases, showed that just 2.03% of patients with PsA had isolated axial disease, and an additional 29.38% had axial and peripheral disease.

In this study, "you can see that even in that group there are distinct differences. The patients that are labeled psoriatic spondylitis are different from those that are labeled ankylosing spondylitis with psoriasis," Dr. Gladman said.

Isolated axial disease in patients with PsA was associated with HLA-B*27 positivity and lower Health Assessment Questionnaire scores. In addition, patients who were HLA-B*27 positive also had a nearly eightfold higher risk for developing peripheral disease over time.

Patients with isolated axial PsA were significantly more likely to be diagnosed at an older age (mean, 37.44 vs. 29.65 years), had higher Psoriasis Area Severity Index scores and a higher likelihood of having psoriatic nail lesions than patients with AS with isolated axial disease and psoriasis.

In contrast, patients with isolated axSpA with psoriasis were more likely to have inflammatory back pain, spinal pain, joint pain/swelling, and areas of localized tenderness, and they had greater severity of morning stiffness.

Dr. Gladman noted that, although AS and PsA are associated with the same gene that encodes for the IL-23 receptor, each condition is associated with a different single-nucleotide polymorphism.

Same disease, different flavors?

But as Mark Twain said, it is difference of opinion that makes horse races, and some specialists in rheumatology say that axSpA amd axPsA are just two sides of the same coin.

Dr Shailendra Singh

"There are always different schools of thought. I believe that they are not different diseases, but a spectrum of diseases," said Shailendra Singh, MD, a rheumatologist at Unity Health Medical Center in Searcy, Ark., and past president of the Arkansas Rheumatology Association.

In an interview, Dr. Singh said that the spectrum ranges from diseases with primarily axial involvement, such as AS, to those with primarily peripheral involvement, such as reactive arthritis.

He pointed out that these conditions have overlapping symptoms, including enthesitis, dactylitis, and uveitis, and inflammatory arthritis.

Daniel Wendling, MD, PhD, from the Centre Hospitalier Régional Universitaire de Besançon (France), Université de Franche-Comté, and colleagues agreed.

"The criteria currently available for both SpA [ASAS (Assessment of Spondyloarthritis International Society) criteria] and PsA [CASPAR (Classification for Psoriatic Arthritis) criteria] are classification criteria, not diagnostic criteria. They are not very stringent and are not exclusive. Thus, the same patient can easily be classified simultaneously in both entities, making the distinction between axSpA with psoriasis and axPsA theoretical," they wrote in an editorial published in Joint Bone Spine.

They cited as an example of the allegedly fuzzy criteria a prospective study conducted by the investigators in Bath, England, in which modified New York criteria for AS were met by 24% of patients with AS, and CASPAR criteria for PsA were met by an equal number of patients with AS.

Therapeutic implications

Dr. Wendling and colleagues acknowledge the differences cited in studies by Dr. Gladman, Dr. Mease, and others between patients with axPsA and those with axSpA, but argue that the differences are not that great and not so clear.

"It should also be emphasized that, although some differences between axPsA and axSpA reach statistical significance, they are mostly at the margin, with low odd ratios," they wrote.

"It is also important to consider the variability in the definition of axPsA, sometimes simply 'physician reported' and elsewhere based on the modified New York radiographic criteria; the latter are only present late in the course of the disease, and this may induce bias," they continued.

Dr. Singh agreed that, as noted by Dr. Gladman, some patients will respond to anti–IL-17, anti–IL-23, and anti–IL-12/23 agents, whereas others will have better responses with tumor necrosis factor (TNF) inhibitors, and still others, such as those with peripheral involvement in the hands and feet may fare better with nonbiologic disease-modifying antirheumatic drugs such as methotrexate.

Answers to come?

Dr. Gladman noted that the information available to date about the efficacy of IL-23 inhibition in axPsA is based on a post hoc analysis of the PSUMMIT 1 and 2 controlled trials in PsA, and is not definitive.

The randomized, controlled STAR trial, currently recruiting patients, is designed to see whether guselkumab can reduce axial symptoms and inflammation in patients with active axPsA.

"What I say is, there is a rationale for [anti–IL-23] to work in psoriatic arthritis, and not work in ankylosing spondylitis," she said.

In contrast, IL-17 inhibitors, anti-TNF agents, and Janus kinase inhibitors show efficacy against both axPsA and AS. Rituximab is ineffective against PsA, but has shown efficacy against AS, especially in patients with neurologic complications from anti-TNF agents.

"There may be other medications that would work more specifically in axial psoriatic arthritis that don't work in ankylosing spondylitis, but at least recognizing that there may be some differences, and that therefore a correct diagnosis should be obtained, might be important," she said.

Ideally, the picture will become clearer with results from the ongoing Axial Involvement in Psoriatic Arthritis cohort, a joint project of ASAS and the Group for Research and Assessment of Psoriasis and Psoriatic Arthritis. The multinational, cross-sectional study is designed "to systematically evaluate clinical and imaging manifestations indicative of axial involvement in patients with PsA and to develop classification criteria and a unified nomenclature for axial involvement in PsA that would allow defining a homogeneous subgroup of patients for research."

Stay tuned.

Dr. Gladman's research is supported by a grant from the Krembil Foundation. Dr. Singh disclosed research support from various companies. Funding sources and conflict of interest disclosures from other works cited are contained in their respective references.

This article originally appeared on MDedge.com, part of the Medscape Professional Network.

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