Characterising axial psoriatic arthritis in clinical practice: a retrospective analysis from the Rheumatic Diseases Portuguese Registry.

Objective: To determine the proportion of axial psoriatic arthritis (axPsA), describe how it is diagnosed in clinical practice, and identify clinical and demographic characteristics independently associated with axPsA. Methods: A multicentre retrospective observational study was conducted using data from patients registered in the Rheumatic Diseases Portuguese Registry (Reuma.pt) with a diagnosis of PsA or spondyloarthritis with psoriasis. Peripheral involvement was defined as the presence of peripheral joint disease. Axial involvement was defined by the presence of physician-reported spondylitis and/or imaging findings suggestive of axial disease. Patients were divided into four non-exclusive groups axPsA, axial involvement with or without peripheral joint disease; ax-PsA_only, axial involvement exclusively; peripheral PsA (pPsA)_only, peripheral joint disease, without axial involvement; pPsA, peripheral joint disease with or without axial disease. Results: This study included 2,304 patients. axPsA was present in 37.1% of patients and axPsA_only in 8.1%. The diagnosis was made based on suggestive imaging findings in 30.1%, and on physician judgement in 69.9%. axPsA was independently associated with HLA-B27 positivity (odds ratio [OR]=2.90; p<0.001), enthesitis (OR=1.64; p<0.001), and younger age at symptom onset (OR=0.97; p<0.001); pPsA_only with dactylitis (OR=1.89; p<0.001) and nail dystrophy (OR=1.42; p=0.010). axPsA_only was independently associated HLA-B27 positivity (OR=3.35; p<0.001) and uveitis (OR=2.56; p=0.004); pPsA with nail dystrophy (OR=2.11; p=0.002), dactylitis (OR=18.18; p<0.001), and enthesitis (OR=1.74; p=0.031). Conclusion: Axial involvement is present in over one-third of PsA patients, often in association with peripheral joint disease. axPsA patients demonstrate distinct clinical and demographic characteristics compared to those without axial disease.