🔑 Key Learning

  • A group of HLA-B27-associated arthritides that are Rheumatoid Factor negative
  • Includes Reactive Arthritis, Psoriatic Arthritis, Enteropathic Arthritis, and Ankylosing Spondylitis
  • Shared features: asymmetrical inflammatory arthritis, enthesitis, sacroiliitis, and extra-articular involvement

🧬 Pathophysiology

  • Immune-mediated disorders with strong genetic association (HLA-B27)
  • Inflammatory arthritis affecting axial or peripheral skeleton
  • Common sites: SI joints, spine, entheses

👀 Clinical Features (Shared)

  • Asymmetrical oligoarthritis (esp. lower limbs)
  • Sacroiliitis → lower back pain and stiffness
  • Enthesitis (e.g. Achilles, plantar fascia)
  • Dactylitis ("sausage digits")
  • Extra-articular features vary by subtype

🔍 Subtypes

🔁 Reactive Arthritis

  • Clinical features - A triad of:
    • Conjunctivitis
    • Urethritis
    • Arthritis
    • Memory aid: Patients with ReA can’t see, can’t pee and can’t climb a tree.
  • Causes:
    • Reactive arthritis is typically preceded by an infection of the GI or GU tracts which is a key clue in exam MCQs
      • Post-STI (especially men) – Chlamydia trachomatis
        • This is SARA – sexually acquired reactive arthritis 
      • Post-dysentery – campylobacter, Shigella, Salmonella, Yersinia 
  • Investigations: STI screen, stool cultures
  • ManagementL
    • 1st line: NSAIDs ± antibiotics if indicated
    • DMARDs for chronic disease (rarely required)

🎨 Psoriatic Arthritis

  • ~10–20% of patients with cutaneous psoriasis will develop psoriatic arthritis 
  • Clinical Features:
    • DIP arthritis (differentiates from RA which spares DIPs)
    • Dactylitis
    • Enthesitis
    • Nail pitting
  • 🔎 XR: DIP erosions, "pencil-in-cup" deformity
  • 💊 Manage as RA: DMARDs 1st line - methotrexate

💩 Enteropathic Arthritis

  • Occurs with Crohn’s or UC
    • Important clue in exam MCQs: History of IBD or abdominal pain/diarrhoea/PR bleeding etc. 
  • Types: peripheral arthritis or axial (sacroiliitis)
  • May have enthesitis, uveitis, or oral ulcers
  • 💊 Treat underlying IBD + NSAIDs/DMARDs as needed

🪵 Ankylosing Spondylitis

  • Young males, insidious onset back pain and significant morning stiffness
    • Pain is worse at night, improves with exercise
  • Reduced spinal mobility, positive Schober’s test
  • Extra-articular: The A's of Ank Spond:
    • Anterior uveitis
    • AVN block on ECG (1st degree HB)
    • Aortic regurgitation - early diastolic murmur
    • Amyloidosis A
    • Apical lung fibrosis
  • Investigations
    • 1st Line: plain XR of sacroiliac joint - sacroiliitis, squaring of the vertebra, syndesmophytes and a bamboo spine 
    • 2nd line: If XR is negative but clinical suspicion - MRI L/S spine
    • Other:
      • Spirometry – restrictive pattern (fibrosis)
      • As above, HLA-B27 is NOT used in the diagnostic work up
  • Management
    • Pharmacological
      • 1st Line: NSAIDs
      • DMARDs (inc. TNF-inhibitors) are only effective for peripheral joint arthritis (not back pain/stiffness). 
    • Exercise is extremely important

📝 Exam Clues & Clinchers

  • Conjunctivitis + urethritis + asymmetrical arthritis + Hx STI/diarrhoea ➝  Reactive arthritis
  • Dactylitis + nail pitting + DIP joint pain ➝ Psoriatic arthritis
  • Crohn’s/UC + asymmetric lower limb arthritis ➝ Enteropathic arthritis
  • Young man with back stiffness improving with activity  ➝  Ankylosing spondylitis