Key Facts

PsA is a heterogeneous condition within the spondyloarthritis family. It can cause significant joint damage if untreated. Skin disease severity does not correlate with arthritis severity.

Epidemiology

• Prevalence: ~0.1-0.2% of general population; 20-30% of psoriasis patients
• Equal M:F for peripheral; M>F for axial
• Peak onset: 30-50 years (usually 5-10 years after psoriasis onset)
• Can present before psoriasis in 15-20%

Aetiology

• Genetic: HLA-B27 (axial), HLA-Cw6 (psoriasis), IL-23R, TRAF3IP2
• Environmental: trauma (Koebner phenomenon), infection, stress
• Obesity: increases risk and reduces treatment response

Pathophysiology

• IL-23/IL-17 axis is central (unlike RA which is more TNF/IL-6 driven)
• Enthesitis is a hallmark: inflammation at tendon/ligament-bone interface
• New bone formation (periostitis, enthesophytes) alongside erosion
• Synovitis with angiogenesis and pannus formation
• Nail involvement: nail matrix is anatomically continuous with DIP joint entheses

Five Clinical Patterns

1. Asymmetric oligoarthritis (40-50%): most common; few large/small joints; dactylitis
2. Symmetric polyarthritis (25-30%): mimics RA; MCP, PIP, wrist
3. DIP predominant (5-10%): classic but uncommon; strong nail association
4. Spondylitis/sacroiliitis (5-20%): inflammatory back pain; may be asymmetric sacroiliitis
5. Arthritis mutilans (<5%): severe destructive, resorptive arthropathy; telescoping digits ("opera glass" fingers)

Key Features

• Dactylitis (30-50%): diffuse swelling of entire digit ("sausage finger/toe")
• Enthesitis (30-50%): Achilles, plantar fascia, lateral epicondyle
• Nail changes (80-90% with PsA): pitting, onycholysis, subungual hyperkeratosis, oil drop sign
• Skin psoriasis: scalp, natal cleft, elbows, knees; severity does not correlate with arthritis

Extra-Articular

• Anterior uveitis: less common than AS
• IBD: associated
• Cardiovascular risk: increased (metabolic syndrome common)

Red Flags

• Rapidly progressive destructive arthritis → arthritis mutilans
• Significant functional limitation → early biologic therapy consideration
• New eye symptoms → uveitis; urgent ophthalmology

Bloods

• RF: negative in 85-90% (helps distinguish from RA)
• Anti-CCP: usually negative
• ESR/CRP: elevated in 40-50%
• Urate: may be elevated (psoriasis increases urate turnover)
• FBC: exclude other causes

Imaging

• X-ray hands/feet: erosions + periostitis (new bone formation) = characteristic; "pencil-in-cup" deformity, osteolysis, ankylosis, periosteal reaction
• X-ray pelvis: sacroiliitis (may be asymmetric, unlike AS)
• MRI SIJ: if axial disease suspected; bone marrow oedema
• Ultrasound: enthesitis, synovitis, dactylitis assessment

Classification

• CASPAR criteria: established inflammatory joint disease + ≥3 points from:
  • Current psoriasis (2 pts) or history (1 pt) or family history (1 pt)
  • Nail dystrophy (1 pt)
  • Negative RF (1 pt)
  • Dactylitis (1 pt)
  • Radiographic juxta-articular new bone formation (1 pt)

Non-pharmacological

• Exercise, physiotherapy, weight management
• Occupational therapy for hand involvement
• Dermatology co-management for skin disease

Pharmacological

Peripheral PsA:

• First-line DMARD: methotrexate 15-25mg weekly (treats both skin and joints)
• Alternatives: sulfasalazine, leflunomide (hydroxychloroquine generally avoided – can worsen psoriasis)
• Biologic DMARDs (NICE criteria: DAS28-equivalent ≥3.2 or oligoarthritis with significant impact despite csDMARDs):
  • Anti-TNF: adalimumab, etanercept, infliximab, certolizumab, golimumab
  • IL-17A inhibitor: secukinumab 150-300mg SC monthly; ixekizumab
  • IL-12/23 inhibitor: ustekinumab 45-90mg SC
  • IL-23 inhibitor: guselkumab
• Apremilast (PDE4 inhibitor) 30mg BD: oral option after DMARD failure (NICE TA433)
• JAK inhibitors: tofacitinib 5mg BD, upadacitinib 15mg OD

Axial PsA:

• NSAIDs first-line (as per axSpA)
• Anti-TNF or IL-17 inhibitor if NSAID failure
• csDMARDs NOT effective for axial disease

Enthesitis/Dactylitis:

• NSAIDs, local corticosteroid injection
• Biologics if refractory

Referral Criteria

• Suspected PsA → rheumatology
• Psoriasis patients with joint symptoms → rheumatology assessment
• Inadequate DMARD response → biologic therapy

• 20-30% develop significant erosive disease
• Arthritis mutilans in <5% – devastating but rare
• DAS28 remission achievable in 40-60% with modern therapy
• Biologics significantly reduce radiographic progression
• Cardiovascular mortality increased (HR ~1.4); aggressive CV risk management needed
• Functional outcomes: generally better than RA if treated early
• Skin and joint disease: often discordant; skin may remit while joints progress (or vice versa)

PsA vs RA

CASPAR Classification Criteria

Score ≥3 with inflammatory joint disease = PsA (98% specificity)