Psoriatic arthritis: a clinical entity distinct from psoriasis?

Psoriasis and psoriatic arthritis are complex and heterogeneous clinical entities, whose presentations comprise multiple combinations of subtypes. There are doubts even if they are distinct entities or merely variants of the same disease. Epidemiologically, psoriasis can be considered a common disease because it affects about 2% of the world population. Regarding psoriatic arthritis, there is no consensus in the literature about its true incidence and prevalence in the general population. Genetic, immune, and environmental factors interact culminating in skin and joint manifestations of psoriatic disease. The central role of activated T lymphocytes in the pathogenesis of both psoriasis and psoriatic joints is now recognized. Furthermore, proinflammatory cytokines can be found in increased concentrations in both skin and synovium of patients with psoriatic arthritis. Since 1964, when the relationship between psoriasis and psoriatic arthritis was recognized, many studies have been conducted to better understand the common mechanism of both diseases. The HLA has already been considered the center of the psoriatic arthritis immunopathogenesis; today, TNF-α plays such a role. This paper is a review of various factors associating psoriasis and psoriatic arthritis leading to the hypothesis of a single disease with multiple presentations.