Key Facts

Psoriatic arthritis is a seronegative spondyloarthropathy with heterogeneous presentation. Early recognition and treatment are essential to prevent irreversible joint damage.

Epidemiology

• Affects 20–30% of psoriasis patients
• Equal sex distribution (unlike RA which is female-predominant)
• Peak onset: 30–50 years
• Skin disease precedes arthritis in ~75%; arthritis precedes skin in ~15%; simultaneous in ~10%

Aetiology

• Genetic: HLA-B27 (axial disease ~50%), HLA-Cw6 (skin psoriasis)
• Environmental: trauma, infection may trigger
• Shared IL-23/IL-17 pathway with psoriasis

Pathophysiology

• IL-23 → Th17 cells → IL-17A → synovial inflammation + bone erosion + new bone formation
• Entheseal inflammation is the primary site of disease (distinguishes from RA)
• Both erosive AND proliferative bone changes (unlike RA which is purely erosive)
• Dactylitis: combined synovitis + tenosynovitis + enthesitis of entire digit

Moll & Wright Classification

1. Asymmetric oligoarthritis (~40%): <5 joints; often large joints + dactylitis
2. Symmetric polyarthritis (~25%): resembles RA; RF negative
3. DIP-predominant (~15%): distal interphalangeal joints — characteristic; often with nail disease
4. Spondylitis/sacroiliitis (~5%): axial involvement; often asymmetric sacroiliitis
5. Arthritis mutilans (<5%): severe destructive; telescoping digits (opera-glass hand)

Key Features

• Dactylitis: sausage-shaped digit swelling
• Enthesitis: Achilles tendon, plantar fascia insertion tenderness
• Nail disease: pitting, onycholysis (strongly associated with DIP arthritis)
• Morning stiffness: typically >30 minutes

Red Flags

• Rapidly progressive joint destruction
• Axial symptoms (inflammatory back pain)
• Severe functional impairment

Bloods

• RF: negative in ~95% (seronegative spondyloarthropathy)
• Anti-CCP: negative (helps distinguish from RA)
• CRP/ESR: may be elevated
• FBC, LFTs, U&Es: baseline before DMARD therapy

Imaging

• X-ray hands/feet: pencil-in-cup deformity, periostitis, juxta-articular new bone, joint space narrowing, ankylosis
• MRI: early synovitis, bone marrow oedema, enthesitis — more sensitive than X-ray
• USS: synovitis, enthesitis, tenosynovitis — used in clinic
• MRI sacroiliac joints: if axial disease suspected

Screening

• PEST questionnaire: 5-question screening tool for PsA in psoriasis patients (≥3 = positive screen)

Non-Pharmacological

• Physiotherapy, occupational therapy
• Exercise: maintain joint mobility and muscle strength
• Patient education, psychological support

Pharmacological

Peripheral arthritis:

• NSAIDs: first-line symptomatic relief (e.g. naproxen 500mg BD)
• csDMARDs: methotrexate 15–25mg weekly (first-line); sulfasalazine; leflunomide
• Biologics: if inadequate response to ≥2 csDMARDs
  • Anti-TNF: adalimumab 40mg every 2 weeks, etanercept
  • Anti-IL-17: secukinumab 150–300mg monthly
  • Anti-IL-12/23: ustekinumab
  • JAK inhibitors: tofacitinib 5mg BD, upadacitinib
  • PDE4 inhibitor: apremilast 30mg BD

Axial disease:

• NSAIDs first-line
• Biologics (anti-TNF or anti-IL-17) if inadequate response — csDMARDs NOT effective for axial disease

Enthesitis:

• NSAIDs; local corticosteroid injection (avoid Achilles tendon injection); biologics if persistent

Referral Criteria

• Rheumatology: all suspected PsA for confirmation and treatment
• Early referral: within 6 weeks of suspected inflammatory arthritis (NICE NG100)

• Progressive joint damage in 40–60% within 10 years without treatment
• Early DMARD therapy improves outcomes
• Biologics: 60–70% achieve ACR50 response
• Arthritis mutilans: severe disability but rare (<5%)
• Mortality: increased cardiovascular risk (as with psoriasis)
• Functional outcomes improved with treat-to-target approach

CASPAR Criteria (Score ≥3 = PsA)

PsA vs RA Comparison