A universally accepted definition of axial psoriatic arthritis (axPsA) is lacking. We aimed to 1) assess the presence of axial involvement as defined by "at least unilateral grade 2 sacroiliitis (Uni2SI)" and 2) assess the radiographic progression of Uni2SI and identify risk factors for progression.
PsA patients participating in a prospective observational cohort were classified according to their highest sacroiliitis grade. The baseline features of patients with Uni2SI were compared to patients meeting the radiographic criteria of the modified New York Ankylosing Spondylitis (mNY AS) criteria. Risk factors were examined for progression from Uni2SI in a sub-group of patients with >1 follow-up radiographs. Logistic regression and a survival analysis were carried out and identified risk factors associated with radiographic mNY AS compared to Uni2SI.
Axial disease defined as ≥Uni2SI was detected in 612/1354 patients (45%). mNY AS sacroiliitis was observed in 477 patients (35%). Radiographic progression of Uni2SI was assessed in 154 patients, 80 (52%) progressed to mNY AS criteria within 5.5 years. At baseline, progressors were diagnosed at a younger age (35.6 vs. 38.9, p = 0.05), had less degenerative disc disease (OR = 0.47, p = 0.02), worse peripheral radiographic damage (OR=1.02, p = 0.03) and worse psoriasis (OR = 1.09, p = 0.01) compared to non-progressors. Patients with an elevated erythrocyte sedimentation rate were more likely to progress (HR = 1.83, p = 0.02), while patients with longer disease duration were less likely to progress (HR = 0.95, p = 0.02).
The radiographic mNY AS criteria appear to be suitable for defining axial PsA according to radiographs. MRI definitions are needed as well for the most appropriate definition of axial PsA.