Difficult to Treat Psoriatic Arthritis: The Road So Far.

Psoriatic Arthritis (PsA) is a multifaceted, immune-mediated disease marked by chronic musculoskeletal inflammation (peripheral arthritis and axial disease, dactylitis, and enthesitis), extra-musculoskeletal manifestations (psoriasis, nail involvement, Inflammatory Bowel Disease [IBD], and uveitis) and multi-comorbidity (cardiovascular disease, metabolic syndrome, mental health disorders, and fibromyalgia). Immunological and non-immunological factors have led, despite the progress made in the understanding, treatment and management of PsA, to a minority of patients being able to achieve satisfactory outcomes. Following the establishment of the definition for difficult to treat rheumatoid arthritis, efforts are underway for difficult to treat PsA (D2T PsA). Defining D2T PsA and its predictors is crucial for advancing clinical trials, treatment strategies, and patient care. Proposed definitions and criteria for D2T PsA vary, but the few available data indicate that extensive psoriasis, axial involvement, obesity, female gender, and comorbidities like IBD, depression, and fibromyalgia are involved. Concerns are also raised for the lack of a universally accepted index for disease activity measurement and for the inclusion of a time-related criterion in the definition of D2T. Moreover, the potential need for distinction between D2T and refractory-to-treatment PsA has also been suggested. In this narrative review, we summarise the current knowledge on the D2T PsA field, highlighting the gaps and the necessity of the "D2T" concept, providing further considerations on the matter.