Psoriatic Arthritis

Overview

Psoriatic arthritis (PsA) is a chronic, immune-mediated inflammatory arthritis associated with psoriasis, affecting up to 30% of patients with skin psoriasis. It commonly presents in adults aged 30–50 years, with equal gender distribution. The disease is highly variable, involving peripheral joints, axial skeleton, entheses, and skin/nails. Delay in diagnosis is common and can result in irreversible joint damage.

Anatomy and Physiology

Aetiology and Risk Factors

• Genetics: HLA-B27 (axial), HLA-Cw6 (psoriasis)
• Family history of psoriasis or PsA
• Psoriasis: particularly widespread, involving scalp, groin or nail involvement
• Environmental triggers: trauma (Koebner phenomenon), infections
• Obesity and smoking increase severity and risk

Pathophysiology

• Genetic predisposition → dysregulated immune response
• Trigger (trauma/infection) leads to activation of dendritic cells, Th17 cells, and production of cytokines (IL-17, IL-23, TNF-α)
• Leads to synovitis, enthesitis, osteitis, and bone proliferation and erosion
  • Skin: Psoriasis involves the epidermis and dermis—keratinocyte hyperproliferation and inflammation
  • Joints: PsA affects synovial joints (peripheral and axial)
  • Entheses: Common sites include Achilles tendon, plantar fascia, patellar tendon
  • Nail unit: Nail matrix and bed involvement results in pitting, onycholysis

Clinical Manifestations

• Peripheral arthritis:
• Axial disease: Sacroiliitis, spondylitis (more asymmetric than AS). Can involve cervical or thoracic spine initially (unlike the classic SIJ in ankylosing spondylitis)
• Enthesitis: Achilles, plantar fascia, costochondral junctions
• Dactylitis
• Psoriasis 
• Nails: Pitting, onycholysis, subungual hyperkeratosis
• Anterior uveitis (less common)

Diagnosis

CASPAR Criteria (ClASsification criteria for Psoriatic ARthritis):

Inflammatory articular disease (joint, spine, or entheseal) plus ≥3 points:

• Psoriasis (current = 2 pts; personal/family history = 1 pt)
• Nail dystrophy (1 pt)
• Negative RF (1 pt)
• Dactylitis (current or history, 1 pt)
• Juxta-articular new bone formation on imaging (1 pt)

Investigations:

• RF and anti-CCP: typically negative (seronegative)
• CRP/ESR: may be elevated
• Imaging:
  • X-ray: joint space narrowing, erosions, new bone formation, pencil-in-cup deformity
  • MRI: enthesitis, synovitis, sacroiliitis
• Ultrasound: detects enthesitis and synovitis

Differential Diagnoses

Condition	Differentiating Feature
Rheumatoid arthritis	Symmetric, RF/anti-CCP positive, no nail/skin psoriasis
Osteoarthritis	DIP involvement but no dactylitis, no erosions
Gout	Monoarticular, crystals on aspiration
Ankylosing spondylitis	Younger males, bilateral sacroiliitis, no skin psoriasis

Treatment

Mild Disease

• NSAIDs
• Intra-articular corticosteroids

Moderate to Severe Disease (or with poor prognostic markers)

• csDMARDs
  • Traditional: Methotrexate, sulfasalazine, leflunomide (esp. for peripheral arthritis)
  • Newer: Ampremilast (more so for skin) or Deucravacitinib
• bDMARDs:
  • TNF inhibitors: adalimumab, etanercept
  • IL-17 inhibitors: secukinumab, ixekizumab
  • IL-17 A/F inhibitor: Bimezikumab
  • IL-12/23 inhibitor: ustekinumab
  • IL-23 inhibitors: guselkumab
• tsDMARDs: JAK inhibitors (e.g. upadacitinib, tofacitinib)

Lifestyle & Supportive

• Weight loss, smoking cessation
• Physical therapy, patient education

Complications and Prognosis

Complications

• Joint damage and deformity (may occur within 2 years of onset)
• Reduced function and quality of life
• Chronic pain and disability
• Increased cardiovascular risk (due to systemic inflammation)

Poor prognostic factors:

• High CRP
• Polyarticular disease
• Dactylitis
• Nail disease
• Early erosive changes

References

1. Mease PJ, Gladman DD, Papp KA, et al. Prevalence of rheumatologist-diagnosed psoriatic arthritis in patients with psoriasis in European/North American dermatology clinics. J Am Acad Dermatol. 2013;69(5):729–735.
2. Gossec L, Baraliakos X, Kerschbaumer A, et al. EULAR recommendations for the management of psoriatic arthritis with pharmacological therapies: 2019 update. Ann Rheum Dis. 2020;79(6):700–712.
3. Taylor W, Gladman D, Helliwell P, et al. Classification criteria for psoriatic arthritis: development of new criteria from a large international study. Arthritis Rheum. 2006;54(8):2665–2673.
4. Ogdie A, Weiss P. The epidemiology of psoriatic arthritis. Rheum Dis Clin North Am. 2015;41(4):545–568.