Clinical and imaging characteristics of patients with axial psoriatic arthritis: Baseline findings from the GESPIC-axial-PsA cohort

Background Axial disease is a domain of psoriatic arthritis (PsA), referred to as axial-PsA. Despite recent advances and increasing recognition, axial-PsA remains poorly defined; validated classification criteria and longitudinal cohorts are lacking. Objectives We aimed to characterise axial-PsA and delineate phenotypes in an imaging-confirmed cohort. Design GESPIC-axPsA, a dedicated arm of the German Spondyloarthritis Inception Cohort (GESPIC), is a prospective, monocentric, study including patients with psoriasis and clinically diagnosed axial-PsA confirmed by imaging. Baseline assessments included clinical phenotyping, laboratory testing, biosample-collection, radiographs of the axial skeleton, and MRI of the whole spine and sacroiliac joints (SIJs). Methods The analysis was primarily descriptive, characterizing baseline clinical and imaging features of patients with imaging-confirmed axial-PsA; categorical and continuous variables were summarized using appropriate measures based on their distribution, and subgroup comparisons were performed using chi-square or Fisher’s exact tests for categorical variables and Mann–Whitney U tests for continuous variables after assessing normality. Results We enrolled 107 axial-PsA patients (55% female; mean age 44.8 years); 78.5% had active psoriasis, 78.6% had inflammatory back pain (IBP), and 48.6% were HLA-B27 positive. Radiographic sacroiliitis was present in 45.7%. On MRI, active/structural SIJ lesions were seen in 53.3%/73.3%; spinal active/structural lesions in 60.0%/53.3%. Both Classification criteria for axSpA (ASAS) and PsA (CASPAR) were simultaneously fulfilled in 60.7%. We identified 20 patients (18.7%) with spine-only involvement—spinal MRI lesions without SIJ lesions on radiography or MRI. Compared with the remainder, the spine-only group was older, more often female, less frequently HLA-B27–positive, and had back pain less consistent with IBP (all p<0.05). Conclusions In our imaging-confirmed axial-PsA cohort using standardized whole-spine and SIJ MRI, we observed marked phenotypic heterogeneity. Approximately one fifth had isolated spinal involvement; this subgroup differed by age, sex, HLA-B27, and IBP features, with imaging distributions distinct from axSpA—supporting the need for disease-specific classification criteria.