Psoriasis and Psoriatic Arthritis: Flip Sides of the Coin?

Presence (current or past) of psoriasis of the skin is a major criterion to establish the diagnosis of psoriatic arthritis. However, in individual patients, the course of psoriasis and psoriatic arthritis do not seem to correlate. This raises the issue of whether psoriasis and psoriatic arthritis are distinct entities, or parts of the spectrum of a "psoriatic disease". Arguments in favour of both concepts, derived from clinical observations, animal experiments, genetic approaches, and therapeutic studies are reviewed, and the implications for scientists and practicing dermatologists highlighted.