Helliwell P., Coates LC., Gladman D.
• Psoriatic arthritis (PsA) is a distinct form of inflammatory arthritis that is largely, but not always, associated with cutaneous psoriasis, making psoriasis one of the most important biomarkers in rheumatology. • The prevalence of PsA in the general population is estimated to be between 0.5% and 1%, only slightly less common than that of rheumatoid arthritis (RA). Up to 30% of people with psoriasis will develop PsA. • PsA is a member of the spondyloarthritis family of diseases, along with axial spondyloarthritis (including ankylosing spondylitis), reactive arthritis, and the arthropathy of inflammatory bowel disease. • A variety of clinical patterns of PsA are recognized: distal joint disease, oligoarthritis (less than five joints), a symmetrical polyarthritis, and spondylitis. Some patients present with combinations of these features, e.g., oligoarthritis plus spondylitis. In addition, people may have co-existing enthesitis and dactylitis (sausage digit), clinical features that are of considerable help in making the diagnosis. • These clinical patterns are not necessarily stable over time. For example, distal joint disease can evolve into polyarthritis, and axial disease develops in up to 40% of patients who present with other patterns of arthritis involvement. • The most common pattern of joint involvement at presentation is polyarthritis, followed closely by oligoarthritis. Approximately 45% of cases have an oligoarthritic pattern at presentation.