Psoriatic arthritis

Psoriatic arthritis (PsA) is a chronic inflammatory arthritis associated with psoriasis, classified within the spondyloarthritis (SpA) spectrum. It affects approximately 20–30% of people with psoriasis and presents with a heterogeneous clinical phenotype including peripheral arthritis, axial disease, enthesitis, dactylitis, and nail involvement. PsA can lead to significant joint damage and functional impairment if untreated. Early diagnosis and treat-to-target strategies improve long-term outcomes.

Australian Epidemiology

Psoriasis affects approximately 2–3% of the Australian population (approximately 700,000 individuals). PsA develops in 20–30% of these, giving a prevalence of approximately 150,000–200,000 Australians. PsA affects men and women equally and typically presents between ages 30–55 years. It is associated with significant cardiovascular comorbidity, metabolic syndrome, and reduced quality of life. Diagnosis is often delayed by 5–7 years from symptom onset.

Immune Mechanisms

PsA pathogenesis involves dysregulation of innate and adaptive immunity, particularly the IL-17/IL-23 and TNF-alpha axes. Activated dendritic cells and macrophages in psoriatic skin and synovium drive IL-17A, IL-22, and TNF-alpha production. IL-17A promotes keratinocyte proliferation (causing psoriasis) and osteoclast activation (causing bone erosion). The IL-23/Th17 axis is central, explaining the efficacy of IL-17 and IL-23 inhibitors.

Genetic Factors

HLA-C*06:02 is the strongest genetic risk factor for psoriasis. HLA-B27 is associated with axial PsA and dactylitis. PSORS1 locus, IL12B, IL23R, and TNF gene polymorphisms contribute to susceptibility. Environmental triggers include streptococcal infection, trauma (Koebner phenomenon), stress, and medications (lithium, beta-blockers, antimalarials).

Entheseal Origin

The enthesis (tendon/ligament-to-bone insertion) is a primary site of inflammation in PsA. Entheseal stress responses activate resident immune cells, initiating synovial and bone inflammation. This entheseal origin distinguishes PsA from rheumatoid arthritis and explains the high frequency of enthesitis and dactylitis.

Five Clinical Patterns (Moll and Wright)

• Oligoarticular asymmetric arthritis — most common (70%); affects fewer than 5 joints; knees, ankles, PIPs, DIPs
• Polyarticular symmetric arthritis — resembles RA; small joints of hands and feet; seronegative
• DIP joint predominant arthritis — distal interphalangeal joints, often with nail disease
• Axial PsA — inflammatory back pain, sacroiliitis; may occur without peripheral disease
• Arthritis mutilans — rare, severe destructive arthritis with telescoping digits

Additional Features

• Enthesitis — Achilles tendon, plantar fascia, lateral epicondyle
• Dactylitis — diffuse swelling of entire digit ("sausage finger/toe")
• Nail disease — pitting, onycholysis, subungual hyperkeratosis (present in 80%)
• Skin psoriasis — may precede, coincide with, or follow arthritis onset
• Uveitis — less common than in ankylosing spondylitis

• Essential
  Inflammatory Markers (ESR, CRP)

  May be elevated; correlate with disease activity. Normal markers do not exclude PsA (especially in oligoarticular disease).
• Essential
  Rheumatoid Factor and Anti-CCP

  Usually negative (seronegative). RF may be positive in 5–10% — important to distinguish from RA as treatment differs.
• Essential
  X-rays (Hands, Feet, Pelvis, Spine)

  Pencil-in-cup deformity, DIP erosions, bone proliferation (periostitis), sacroiliitis. Baseline and surveillance imaging.
• Essential
  CASPAR Criteria Assessment

  Classification of Psoriatic Arthritis criteria: psoriasis, nail dystrophy, dactylitis, juxta-articular new bone formation, negative RF. Score ≥3 with inflammatory arthritis is diagnostic.
• Available
  MRI (Sacroiliac Joints, Spine, Peripheral Joints)

  Superior sensitivity for early sacroiliitis, enthesitis, and synovitis. Useful for treatment decisions and monitoring.
• Available
  Musculoskeletal Ultrasound

  Detects subclinical synovitis, enthesitis, and tendon involvement. Useful in clinical assessment and guided procedures.
• Available
  HLA-B27

  Positive in ~20% of PsA (especially axial disease). Aids diagnosis of axial involvement.

Use DAPSA (Disease Activity in PSoriatic Arthritis) or MDA (Minimal Disease Activity) criteria to guide treat-to-target strategy. MDA requires 5/7 of: TJC ≤1, SJC ≤1, PASI ≤1 or BSA ≤3%, patient pain VAS ≤15, patient global VAS ≤20, HAQ ≤0.5, enthesitis ≤1.

Non-Pharmacological Management

Regular exercise, physiotherapy, and occupational therapy are essential. Low-impact aerobic exercise (swimming, cycling) preserves joint function. Skin management with dermatology input. Cardiovascular risk factor modification — weight management, smoking cessation, blood pressure and lipid control. Psychological support for chronic disease and body image concerns.

NSAIDs

First-line for mild peripheral arthritis and axial disease. Provide symptomatic relief but do not modify disease progression. Indomethacin, naproxen, or celecoxib commonly used. Continuous dosing preferred for axial disease. Gastroprotection with PPI recommended for long-term use.

Conventional Synthetic DMARDs (csDMARDs)

Methotrexate is preferred first-line csDMARD for peripheral PsA, particularly with significant skin involvement. Leflunomide is an alternative. Sulfasalazine has modest efficacy for peripheral arthritis. csDMARDs have limited efficacy for axial disease. Combination csDMARD use is generally not recommended in PsA.