Psoriatic arthritis: one or more diseases?

Psoriatic arthritis (PsA) is a common, debilitating auto-immune disease with diverse clinical features. In this paper, published evidence is examined, which addresses the issues that (a) PsA exists; and (b) PsA can or cannot be viewed as a distinct rheumatic disease from other spondyloarthritides. Evidence derived from epidemiological, clinical, genetic and immunohistological studies is included. Summarizing the evidence, it is clear that PsA does indeed exist, with the prevalence of rheumatic disease in patients with psoriasis (Ps) higher than would be expected. Certain clinical features also occur more commonly in PsA, although none can differentiate consistently from other arthropathies. Both genetic and immunohistological studies suggest that PsA, both oligo- and polyarticular disease, can be clearly separated from rheumatoid arthritis and that it belongs to the family of spondyloarthritides. The presence of Ps may confer a more severe clinical phenotype with poor radiological outcome. It may be that, with time, a specific genetic marker or diagnostic feature will emerge; additional, more detailed pathogenic studies are required. In the meanwhile, particularly with new treatments being evaluated, it is important to continue to develop specific classification or diagnostic criteria and to define both clinical and laboratory-based outcome measures.