Psoriatic arthritis: phenotypic variance and nosology

Psoriatic arthritis (PsA) has been recognized as a distinct entity different from rheumatoid arthritis. Classification and phenotyping of PsA have progressed substantially since the first classification criteria of the disease were published in 1973 by Moll and Wright. The initial disease patterns described by Moll and Wright have been found to overlap and change over time. There has been controversy about whether these should be maintained or whether the phenotype of PsA should include peripheral and axial disease only. PsA is a multifaceted disease that can present as different clinical phenotypes: peripheral arthritis, axial disease, skin and nail disease, dactylitis, and enthesitis. Development of the high-sensitivity and high-specificity CASPAR criteria was the first step to conducting high-quality trials and observational studies in the field.