Background/Purpose: Peripheral spondyloarthritis (pSpA) shows features that overlap with psoriatic arthritis (PsA), axial spondyloarthritis (axSpA) and other forms of SpA and is unsatisfactorily defined despite the introduction of the ASAS classification criteria in 2011. The objective of this study was to estimate the prevalence of pSpA among patients with SpA, to identify the phenotype and burden of patients with pure pSpA and to compare these findings with that of pure PsA, pure axSpA and combined forms of SpA.

Methods: This is a post hoc analysis of the ASAS-Peripheral involvement in SpA (PerSpA) study, based on 4,185 patients diagnosed by their rheumatologists with pSpA, PsA and axSpA. Two parallel approaches were used to define the disease subtypes (Fig.1). First, the criteria approach categorized the patients in 6 groups: pure disease (fulfilling either pSpA, or axSpA ASAS classification criteria, or CASPAR criteria) and combined disease (patients who fulfill more than one of the above-mentioned criteria). Second, the diagnostic approach categorized the patients in 6 groups as well: pure disease (diagnosed with pSpA, axSpA or PsA as the main SpA disease, exclusively) and combined disease (patients with overlapping features according to rheumatologists’ judgement). Socio-demographic characteristics, clinical, biological, imaging phenotype, and disease burden were compared among patients with pure pSpA, PsA, axSpA and the combined forms of SpA (grouped together), using Chi-square, ANOVA and Kruskal-Wallis as appropriate.

Results: The prevalence of all pSpA was 31.5% using the criteria approach and 10% using the diagnostic approach (Fig.1). pSpA was pure in only 17% of pSpA using the criteria approach (compared to 34% pure PsA and 68% pure axSpA), and in 62% of pSpA using the diagnostic approach (compared to 58% pure PsA and 75% pure axSpA). Patients with pure pSpA had a socio-demographic profile and disease phenotype with “intermediate” features between PsA and axSpA (Fig.2). Using criteria and diagnostic approach, respectively, pure pSpA patients had a high prevalence of peripheral joint disease (86 and 96%), synovitis (76 and 91%), and enthesitis (57 and 55%), a positive HLA-B27 in 65 and 59%, a high C-Reactive Protein level in 51% and inflammatory back pain in 52 and 42%. However, compared to pure PsA and pure axSpA, they had a significantly higher disease burden (BASDAI, ASDAS, ASAS-HI, BASFI, FiRST, WPAI and EQ-5D), with scores being in a similar range to that of the combined forms of SpA (Table 1), when using the criteria approach. This higher disease burden was confirmed in the diagnostic approach in comparison with axSpA only. Still, patients with pure pSpA had lower use of biological Disease-Modifying Drugs (b-DMARDs) using both approaches.

Conclusion: The prevalence of pSpA varies when using the classification criteria or the rheumatologist’s diagnosis. It occurs in a pure form (i.e., associated with neither PsA nor axSpA) less frequently than PsA and axSpA. Pure pSpA has a distinct clinical phenotype with intermediate features between pure PsA and pure axSpA but with a higher disease burden compared to both diseases, and a lower use of b-DMRADs.

Figure 1. Prevalence of the pure and combined forms of spondyloarthritis (peripheral SpA (pSpA), psoriatic arthritis (PsA) and axial SpA (axSpA), using the criteria (ASAS and CASPAR) and to the diagnostic approach.

Figure 2. Phenotypic profile of patients with peripheral spondyloarthritis.

Table 1. Patient profile, disease phenotype and burden in patients with pure SpA compared to pure PsA, pure axSpA, and combined forms using the criteria approach.

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ACR Meeting Abstracts - https://acrabstracts.org/abstract/what-is-peripheral-spondyloarthritis-identifying-disease-phenotype-and-burden-a-post-hoc-analysis-of-the-asas-perspa-international-study/