Key Facts

Reactive arthritis is an immune-mediated response to a distant infection. The joint itself is sterile. Identifying and treating the triggering infection (particularly Chlamydia) is important but does not alter the course of the arthritis.

Epidemiology

• Incidence: ~30-40 per 100,000 following STI; ~5-14 per 100,000 following GI infection
• Most common in young adults (20-40 years)
• Post-STI form: M>>F; Post-GI form: M=F
• 1-4% of patients with Chlamydia, Salmonella, or Shigella develop reactive arthritis

Aetiology

Genitourinary triggers:

• Chlamydia trachomatis (most common overall)
• Ureaplasma urealyticum

Gastrointestinal triggers:

• Salmonella, Shigella, Campylobacter, Yersinia enterocolitica, C. difficile

Pathophysiology

• Molecular mimicry: bacterial antigens share epitopes with self-antigens in HLA-B27+ individuals
• Bacterial components (LPS, DNA, RNA) found in synovial fluid but viable organisms are absent
• IL-23/IL-17 axis involvement (as with other spondyloarthropathies)
• Enthesitis is a primary pathological feature

Musculoskeletal

• Asymmetric oligoarthritis: typically knee and ankle (lower limb predominant)
• Onset 1-4 weeks after triggering infection
• Dactylitis: sausage digit
• Enthesitis: Achilles tendinitis, plantar fasciitis
• Sacroiliitis/spondylitis: inflammatory back pain (20-30%)

Extra-Articular

• Eyes: conjunctivitis (30-50%); anterior uveitis (less common but more serious)
• Urogenital: urethritis (even post-GI infection), prostatitis
• Skin: keratoderma blennorrhagicum (waxy papules on soles/palms – resembles pustular psoriasis), circinate balanitis (painless lesions on glans penis)
• Oral: painless oral ulcers
• Nails: nail dystrophy (resembles psoriatic nail disease)

Red Flags

• Hot, swollen joint + fever → exclude septic arthritis (aspirate urgently)
• Acute anterior uveitis → ophthalmology referral
• Cardiac conduction abnormalities (rare – aortitis)

Bloods

• ESR/CRP: elevated
• HLA-B27: positive in 50-80% (associated with severity and chronicity; NOT diagnostic)
• FBC: leucocytosis
• RF, anti-CCP: negative (excludes RA)

Infection Screen

• NAAT for Chlamydia: first-void urine or urethral/endocervical swab
• Stool culture: if GI symptoms preceded arthritis
• Blood cultures: if febrile (exclude sepsis)

Joint Aspiration

• Inflammatory fluid: elevated WCC (usually 10,000-50,000), sterile culture
• No crystals (excludes gout/pseudogout)
• Gram stain and culture to exclude septic arthritis

Imaging

• X-ray: usually normal early; periostitis and enthesophytes if chronic
• MRI SIJ: if axial symptoms
• Ultrasound: synovitis, enthesitis, dactylitis

Acute

• NSAIDs: naproxen 500mg BD or indomethacin 50mg TDS (first-line)
• Intra-articular corticosteroid: for persistent monoarthritis
• Rest and physiotherapy: maintain joint range of motion

Infection Treatment

• Chlamydia: doxycycline 100mg BD for 7 days (treat patient and partner)
• Treatment of GI infection: usually self-limiting; antibiotics do not alter arthritis course
• Prolonged antibiotics for arthritis itself: NOT recommended (no evidence of benefit)

Chronic/Refractory (>6 months)

• Sulfasalazine 500mg-1g BD: first-line DMARD for chronic reactive arthritis
• Methotrexate 15-25mg weekly: if sulfasalazine fails
• Anti-TNF: adalimumab, etanercept for severe, refractory cases
• Intra-articular steroids: ongoing for individual joint flares

Eye

• Conjunctivitis: usually self-limiting; lubricant eye drops
• Anterior uveitis: urgent ophthalmology; topical corticosteroid drops + mydriatic

Referral Criteria

• Persistent arthritis >6 weeks → rheumatology
• Acute anterior uveitis → ophthalmology
• STI confirmed → GUM clinic referral

• Self-limiting in 60-80%: resolves within 3-12 months
• Chronic or relapsing: 20-30%; more likely if HLA-B27 positive
• Progression to AS: 15-30% of HLA-B27 positive patients over 10-20 years
• Functional outcomes: majority have good functional outcome
• Recurrence: 15-50% have at least one recurrence
• Poor prognostic factors: HLA-B27, hip involvement, raised ESR, poor response to NSAIDs

Reactive Arthritis vs Gonococcal Arthritis

Spondyloarthritis Shared Features