Axial psoriatic arthritis and ankylosing spondylitis: same or different? A real-world study with emphasis on comorbidities.

Axial involvement affects 25-70% of psoriatic arthritis (PsA) patients, depending on the criteria used for its definition. Efforts are underway to clarify the similarities and differences between axial-PsA and ankylosing spondylitis (AS). We aimed to compare, in a real-world setting, axial-PsA and AS, in terms of demographic, radiologic and clinical (musculoskeletal and extra-articular) characteristics, with a focus on comorbidities. All AS (New York criteria, n=128) and PsA patients (CASPAR criteria, n=78) with axial involvement who were regularly followed-up in the outpatients' rheumatology clinics from two tertiary hospitals (December 2018-July 2020) were included. Demographic, radiologic and clinical characteristics were recorded and compared between the two groups. For comorbidities (coronary disease, cerebrovascular accidents, hypertension, diabetes mellitus, dyslipidaemia, depression, osteoporosis, and malignancies), adjustments were made for relevant confounders. AS patients were younger (p=0.05) and were diagnosed at a younger age (p=0.002), more frequently of male gender (p=0.04), had lower BMI (p=0.006) and they were more frequently HLA-B27-positive (p=0.006). In AS patients, peripheral arthritis, dactylitis and nail involvement were less common (p=0.001 for all), in contrast to eye (p=0.001) and bowel involvement (p=0.004). Frequency of radiologic abnormalities in the spine was similar between the two groups while sacroiliitis was more often bilateral in AS and unilateral in axial-PsA. Comorbidities, including cardiovascular-related ones, were comparable between AS and axial-PsA, apart from depression which was more frequent in axial-PsA (p=0.07 in logistic regression). AS and axial-PsA have certain clinical and radiologic differences. Comorbidities were comparable, while depression was more common in axial-PsA.