A new study provides evidence that two conditions that fall under the umbrella of spondyloarthritis — isolated axial disease in patients with psoriatic arthritis (PsA) and isolated axial disease in patients with ankylosing spondylitis (AS) accompanied by psoriasis — are different clinical entities and may need different treatments. These relatively rare rheumatologic conditions, defined by their back involvement, have considerable clinical overlap and are often lumped together under the label axial spondyloarthritis.

This is a hot topic and current matter of debate within the scientific community: Are axial PsA and axial AS two separate diseases or just two phenotypes under the spondyloarthritis umbrella? said Fabian Proft, MD, a rheumatologist and researcher at Charité Universitätsmedizin Berlin, commenting on the new study, which was published online in Annals of the Rheumatic Diseases.

Dr Fabian Proft

Both conditions belong to the spectrum of spondyloarthritis, but with varying viewpoints on nomenclature. They have intersections and overlaps, but not all treatments are equally effective for both. “We need to better understand their differences and similarities,” Proft said, adding that the new study is noteworthy for the size of the population included, its long-term follow-up data, and the researchers’ depth of experience treating these patients.

The researchers are based at the University of Toronto, which has separate clinics dedicated to PsA and to AS, said Dafna D. Gladman, MD, professor of medicine at the university, codirector of the PsA clinic, and corresponding author for the new study. The two clinics follow the same standardized protocols, including clinical, radiographic, genetic, and laboratory assessments. Even though the patients present quite similarly, she credits referring physicians for recognizing the distinctions by their referrals to the PsA or AS clinic.

According to previous research, pure axial PsA, without peripheral involvement, is rare, affecting about 2%-5% of patients with PsA. For this study, an observational cohort of 1,576 patients from the PsA clinic included 31% (n = 495) with axial disease, 2% (n = 32) with isolated axial PsA, and 29% (n = 463) with both axial and peripheral involvement. A total of 25 of the patients with isolated axial PsA ultimately developed peripheral disease by their most recent clinic follow-up visit. In a second cohort of 1,688 patients with AS, nearly 5% (n = 68) had isolated axial disease with psoriasis.

Dr Dafna D. Gladman

“In our logistic regression analysis, isolated axial PsA was found to be a different clinical entity than isolated AS with psoriasis. They are not the same patients,” Gladman said. The patients with isolated axial PsA were older at diagnosis, more likely to have psoriatic nail lesions, and less likely to have inflammatory back pain than were patients with isolated axial AS and accompanying psoriasis.

When interviewed in early September, Gladman was preparing to fly to Ghent, Belgium, to participate in a debate at the International Congress on Spondyloarthritides, taking the pro position on the thesis: Is axial inflammation in PsA distinct from axial spondyloarthritis? Taking the con position was to be Robert Landewé, MD, PhD, of Amsterdam University Medical Center in the Netherlands.

“This is an old debate, splitters versus lumpers,” Gladman told this news organization. “My message is that when you place patients in more homogeneous groups, you can learn more and perhaps find better opportunities for treating their disease.” For example, even with the similarities, do these patients need to be treated with different medications? Medications for psoriasis, including those targeting the interleukin-23 cytokine, may not be effective for AS, but patients with axial PsA may not get them because of the association with axial AS.

“Now is the opportunity to really understand what — if any — are the differences between various components of this disease group. If you lump people together, you may miss the forest for the trees,” Gladman said. “If, at the end of the day, we find out these patients essentially are the same, I will lump. But until we have proved that there are no important differences, I will split.” She added that it is important for practicing rheumatologists to make the correct diagnosis so that they know to access certain drugs.

Proft credited Gladman and colleagues’ study for adding another piece of the puzzle to better understand differences and similarities for these two axial diseases. He noted, however, that the study did not include MRI scans for every participating patient, which could have given a deeper picture.

“International efforts are being made to recruit patients for a multinational, multicenter study of axial involvement in PsA,” which will include MRI data, Gladman said. She and Proft are both part of AXIS, the Axial Involvement in Psoriatic Arthritis cohort, now recruiting patients for such a study. AXIS is a joint project of the Assessment of SpondyloArthritis international Society and the Group for Research and Assessment of Psoriasis and Psoriatic Arthritis.

“We don’t have final answers yet, although we have given evidence to support the differences.” The proof is in the pudding, she said, and that pudding will be the clinical trials.

The University of Toronto Psoriatic Arthritis Program is supported by a grant from the Krembil Foundation. The study authors declared no competing interests. Proft reported receiving research support from Novartis, Eli Lilly, and UCB, and fees for consulting and serving on speakers bureaus from AbbVie, Amgen, Bristol-Myers Squibb, Celgene, Eli Lilly, Hexal, Janssen, Merck Sharp & Dohme, Novartis, Pfizer, Roche, and UCB.

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

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