Similarities and differences: disentangling the intersection between axial psoriatic arthritis and axial spondyloarthritis.

Spondyloarthritis is a group of chronic inflammatory diseases that includes axial spondyloarthritis (axSpA) and psoriatic arthritis (PsA). AxSpA primarily affects the axial skeleton, manifesting with hallmark features such as inflammatory back pain and a strong association with human leukocyte antigen-B27. On the other hand, axial involvement in PsA (axial PsA) poses unique challenges in the diagnosis, classification, and management. These challenges stem from a limited understanding of this condition and an absence of a specific definition for its diagnosis. Although shared genetic and environmental contributors are observed, the presence of differences suggests the possibility that axial PsA may, in fact, represent a distinct clinical entity rather than axSpA. The prevailing classification criteria, such as ClASsification criteria for Psoriatic ARthritis for PsA and the Assessment of SpondyloArthritis International Society criteria for axSpA, are insufficient in capturing the full scope of axial PsA. Moreover, treatment paradigms for axial PsA are primarily extrapolated from axSpA due to the lack of targeted trials in this specific population. Biologic disease-modifying anti-rheumatic drugs, encompassing tumor necrosis factor inhibitors, interleukin (IL)-17 inhibitors, and Janus kinase inhibitors, have demonstrated efficacy in axSpA and PsA. However, IL-23 inhibitors have not shown efficacy in axSpA, and currently, no results from randomized controlled trials in axial PsA are available. While axial PsA exhibits features that overlap with axSpA, emerging evidence underscores its distinct pathophysiology and clinical characteristics, highlighting the need for standardized definitions and tailored therapeutic approaches to optimize outcomes. Ongoing studies evaluating therapeutic efficacy and molecular characterization hold promises to enhance understanding and management of axial PsA, thus paving the way for personalized treatment strategies. This review aims to provide an overview of the similarities and differences between axial PsA and axSpA and seeks to disentangle the intersections between these two diseases.