Introduction: Spondyloarthritis (SpA) is a group of heterogeneous diseases that includes axial SpA (axSpA), such as ankylosing spondylitis and axial non-radiographic SpA, and Psoriatic Arthritis (PsA) with peripheral and/or axial involvement (axPsA). Currently, it is not well known if the characteristics and burden of the disease in patients with axPsA are similar to that of patients with axSpA.

Objectives: To compare the demographic, clinical and structural features between patients with axSpA and axPsA.

Methods: Data from an observational prospective cohort including all patients with SpA initiating biological therapy because of predominant axial manifestations from 2002-2019 in a university hospital were analyzed. AxSpA and axPsA were defined in clinical practice according to the prescribing rheumatologist, based on clinical features and complementary examinations. Demographic information, laboratory tests, disease presentation, sacroiliitis according to modified New York criteria in the pelvis X-ray, disease activity indexes (ASDAS and BASDAI) and concomitant treatment before starting biological drug were collected from the electronic medical record and biologic database. In the statistical analysis, chi square or the exact Fisher’s test was used for categorical and t-Student or U-Mann Whitney for continuous variables, according to the distribution of the data. Then, the association between demographic and clinical features and each disease was analysed using univariable and multivariable logistic regression models.

Results: Out of 352 included patients, 287 (81.5%) had axSpA, and 65 had axPsA (18.5%). Baseline characteristics are shown in Table 1. Mean baseline ASDAS was 3.3 ± 0.9 and 3.1 ± 1.0 for axSpA and axPsA, respectively. No significant differences at baseline were observed between axSpA and axPsA for most of the characteristics including: gender, age at diagnosis, age at starting biologic, disease duration before biologic, smoking habit, CRP, disease activity, enthesitis, dactylitis, inflammatory bowel disease (IBD), patient global assessment and sulfasalazine use. However, there were differences between diseases in some relevant characteristics. AxSpA patients had less peripheral involvement (41.5 vs 78.5%, p = 0.004), more uveitis (15.3 vs 3.1%, p = 0.03) and were more frequently HLA-B*27 positive (72.3 vs 34.1%, p < 0.001), in comparison to axPsA patients. They also had better baseline physician global assessments (PhGA) (37.4 vs 44.4, p = 0.02), and a higher grade of radiographic sacroiilitis. AxSpA patients used less global baseline concomitant therapy with conventional DMARDs (p = 0.001), methotrexate (p < 0.001) and prednisone (p < 0.01), whereas they used more sulfasalazine (p = 0.003) than axPsA patients. After running multivariate analyses, the absence of peripheral manifestations (OR = 4.7; p < 0.001) and the positivity of HLA-B27 (OR = 5.4; p < 0.001) were independently associated with axSpA.

Conclusions: Despite being spondyloartrithis with many common traits, axSpA and axPsA present some differences in clinical practice. Whereas axSpA patients are more frequently HLA-B27 positive, axPsA have more peripheral involvement. These differences in clinical presentation between both diseases may contribute to variances in therapeutic management, such as increased use of baseline concomitant therapy in axPsA patients who initiate biological therapy.