Axial involvement in psoriatic arthritis: is it unique?

Axial involvement in psoriatic arthritis (PsA) has been a major feature of the disease since the original description by Wright and Moll. However, despite over 50 years of study, there is still no accepted definition of axial PsA, nor validated classification criteria. Numerous observational studies have described a phenotype of axial involvement that differs from classical ankylosing spondylitis (AS or axial spondyloarthritis) both clinically and radiographically, and in the frequency of the HLA-B27 antigen. These differences are important clinically, as axial PsA may be less prominent than AS, and in terms of treatment. This short review discusses these issues and offers some clarification for clinicians.