Psoriatic arthritis
Psoriasis is a chronic skin condition affecting about 3% of Europeans and North Americans. About 15% of people afflicted with psoriasis will develop psoriatic arthritis—cutaneous risk factors for this are psoriasis of the nails, scalp, and flexures. Since most cases of arthritis develop in people with psoriasis, new screening tools, both clinical and imaging, are available. Some genetic factors may also explain susceptibility and severity. Historically, five clinical subgroups have been described but these may be simplified to axial and peripheral involvement, the latter dividing into oligo- and polyarticular patterns. The importance of these clinical subdivisions is still under debate and research but it is clear that there is marked heterogeneity in all manifestations of this disease. In recent times the importance of extra-articular features has gained prominence such that the metabolic syndrome and cardiovascular morbidity are now seen as important features of ’psoriatic disease’. The diverse changes seen in bone on imaging reflect both the underlying pathogenic mechanisms and the ways in which the disease progresses. Recent work with animal models and immunohistochemistry has further advanced our understanding of these features. In the biologic era renewed interest in psoriatic arthritis has stimulated research into outcome assessment and permitted clearer understanding of how these new drugs work on the different aspects of the disease. In addition, improved recognition of the impact of the disease on the person has stimulated the development of new patient-reported outcome tools.