Pathophysiology
Clinical meaning
Psoriatic arthritis (PsA) is a seronegative inflammatory arthropathy occurring in approximately 30% of psoriasis patients, driven by the IL-23/IL-17 axis with TNF-alpha as a key amplifying cytokine. PsA manifests in five clinical patterns: asymmetric oligoarthritis (most common), symmetric polyarthritis (mimics RA), distal interphalangeal predominant, spondylitis/sacroiliitis, and arthritis mutilans (most destructive). Unique features distinguishing PsA from RA include enthesitis (inflammation at tendon/ligament insertions into bone), dactylitis (diffuse digit swelling from combined synovitis, tenosynovitis, and enthesitis), and concurrent new bone formation (periostitis, enthesophytes) alongside erosive disease. The CASPAR criteria are used for classification: inflammatory articular disease plus โฅ 3 points from current psoriasis (2 pts), personal/family history of psoriasis (1 pt), dactylitis (1 pt), juxta-articular new bone formation (1 pt), RF negativity (1 pt), nail dystrophy (1 pt). Treatment follows a treat-to-target strategy using the GRAPPA recommendations: NSAIDs for mild disease, conventional DMARDs (methotrexate, leflunomide, sulfasalazine) for peripheral arthritis, and biologics (TNF inhibitors, IL-17 inhibitors, IL-23 inhibitors) or targeted synthetic DMARDs (apremilast, JAK inhibitors) for moderate-severe or DMARD-refractory disease. Notably, methotrexate treats skin but has limited evidence for axial disease or enthesitis.