Epidemiology of psoriasis and psoriatic arthritis.

Psoriasis is widely diffused in the World, with the exception of a few populations, such as the natives from Alaska and Australia, where it is unknown. Its average prevalence is about 3-4%. This is probably an underestimate, for it is mostly based on self-reports. In fact, on the one hand minimal psoriasis, e.g. nail disease, could remain undiagnosed; on the other, precise classification criteria for psoriatic arthritis (PsA) are lacking and the skin disease is often of elusive nature. The frequency of PsA may be higher than commonly believed, as suggested by recent studies reporting a prevalence of up to 0.42%. There are no major differences in the frequency of psoriasis between sexes, nor specific time trends. Indirect data suggest that PsA may be more frequent in the old than in the new World, a point that could be clarified only by standardized international studies. In practice, both psoriasis and PsA are relatively common conditions, with major impact on the patients'quality of life, and requiring appropriate intervention strategies. An important advance should be the adoption of univocal definitions of psoriasis and PsA, including guidelines for patterns of skin and joint involvement.