Psoriasis and psoriatic arthritis: separate or one and the same?

The presence and severity of skin and joint symptoms in patients with psoriasis and psoriatic arthritis frequently do not correspond, a discrepancy that has raised the question of whether they represent two related but different disease processes. The fact that some agents seem to work preferentially in one state over the other reinforces this idea. However, there are also several agents with combined efficacy against cutaneous and articular inflammation that appear to support the existence of a common aetiology. Here we review the clinical, epidemiological and genetic evidence for and against a common pathogenesis for the two diseases. We then discuss the cellular and molecular targets of their selected therapies and how they potentially implicate effector pathways as a common immunopathogenic mechanism. Finally, we examine a recently proposed model of psoriasis pathogenesis involving type 1 interferon-producing plasmacytoid dendritic cells and how it may provide further clues to the aetiological links between psoriasis and psoriatic arthritis.