What is peripheral spondyloarthritis? Identifying proportion, phenotype and burden in post hoc analysis of the ASAS-PerSpA study

Background: Little is known about the prevalence, phenotype, and burden of peripheral spondyloarthritis (pSpA). The objective of the study is to compare the phenotype and burden of disease of pure pSpA to that of pure psoriatic arthritis (PsA), pure axial SpA (axSpA), and combined forms of SpA. Methods: This is a post hoc analysis of 4,185 patients from the cross-sectional ASAS-Peripheral involvement in SpA (PerSpA) study. Patients were approached in 2 ways: the first approach was based on the rheumatologist's diagnosis (diagnostic approach) and the second one was based on the fulfillment of ASAS or CASPAR classification criteria (classification criteria approach). Demographics, disease phenotype, and burden were compared among pure pSpA, PsA, axSpA, and the combined forms. Findings: The proportion of pSpA was 31.5% of SpA using the classification criteria approach and 10.3% using the diagnostic approach. pSpA was pure (i.e. without axSpA or PsA) in 16.8% of pSpA using the criteria, and in 62.3% using the diagnostic approach. Using classification 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, but lower use of biologics using both approaches. Interpretation: The proportion of pSpA varies when using the classification criteria or the diagnostic approach. pSpA occurred in a pure form less frequently than PsA and axSpA and had intermediate features but a higher disease burden.

Automated summary

The study compared the phenotype and burden of pure peripheral spondyloarthritis to pure psoriatic arthritis, pure axial spondyloarthritis, and combined forms of spondyloarthritis. Pure pSpA patients had a high prevalence of peripheral joint disease, synovitis, enthesitis, positive HLA-B27, high C-Reactive Protein level, and inflammatory back pain. They had a significantly higher disease burden compared to pure PsA and pure axSpA.