Psoriatic spondylitis or ankylosing spondylitis with psoriasis: same or different?

Purpose of review Of the four musculoskeletal domains of psoriatic arthritis (PsA), the axial domain is the least studied. With the advent of targeted therapy that is efficacious in some but not all manifestations of spondyloarthritis (SpA), there is interest in understanding the similarities and differences between axial PsA (axPsA) and ankylosing spondylitis. Moreover, there is also interest in evaluating the axial domain in PsA clinical trials, domain that has traditionally been ignored in such studies. This review aims to summarize the current understanding of the differences between ankylosing spondylitis and axPsA. Recent findings Recent observational studies have shown that axPsA forms part of the SpA spectrum, flanked by peripheral PsA on one side and ankylosing spondylitis on the other. Thus, axial disease is more severe in ankylosing spondylitis, whereas peripheral disease is more severe in axPsA. However, the overall disease burden and impact is similar. The expression of axPsA is influenced by age, disease duration, sex and HLA-B*27 status. Summary axPsA has not been properly defined hampering research into pathogenesis, disease impact and treatment. Research to define prevalence, clinical features, impact, similarities and differences with other axial SpA, and treatment is an unmet need.