📚 Rheumatology — L7: Septic & Reactive Arthritis

📚 Lecture Overview

This lecture covers two major forms of inflammatory joint disease: reactive arthritis and septic arthritis, detailing their etiologies, clinical presentations, and management strategies. Understanding these conditions is critical for clinicians to differentiate between immune-mediated joint inflammation following an infection and direct bacterial invasion of the joint space. Mastering these topics ensures timely interventions that prevent permanent joint destruction and systemic complications.

🎯 Key Concepts & Definitions

• Reactive Arthritis: An acute, sterile inflammatory arthritis arising days to weeks after an antecedent gastrointestinal or genitourinary infection; classified as a spondyloarthritis (SpA).
• Septic Arthritis: A dangerous, destructive form of acute arthritis caused by direct microbial invasion (bacterial, fungal, or mycobacterial) of the joint space, typically via hematogenous spread.
• Enthesitis: Inflammation at the site where ligaments, tendons, joint capsules, or fascia insert into the bone, commonly seen in reactive arthritis.
• Keratoderma Blennorrhagica: Hyperkeratotic skin lesions occurring on the soles and palms, associated with reactive arthritis.
• Circinate Balanitis: Painless superficial genital lesions characteristic of reactive arthritis.
• Arthrocentesis: The clinical procedure of aspirating synovial fluid from a joint to analyze cell counts, perform Gram stains, and check for cultures.

📖 Main Content

Reactive Arthritis

• Definition & Pathogens:
• Arises after an infection, but pathogens cannot be cultured from the affected joints.
• Causative gastrointestinal and urogenital pathogens include Chlamydia trachomatis, Yersinia, Salmonella, Shigella, and Campylobacter, with Escherichia coli, Clostridium difficile, and Chlamydia pneumoniae added later.
• Clinical Features:
• Latency period: Several days to weeks between infection and arthritis.
• Pattern: Typically mono- or oligoarticular, strongly favoring the lower extremities.
• Musculoskeletal signs: Asymmetric peripheral oligoarthritis, enthesitis, and dactylitis.
• Extra-articular signs: Conjunctivitis, anterior uveitis, urethritis, cervicitis, oral mucosal ulcers, keratoderma blennorrhagica, and circinate balanitis.
• Diagnostics:
• Evidence of preceding infection via stool, urine cultures, or vaginal swabs.
• Elevated acute phase reactants (ESR or CRP) and peripheral leukocytosis.
• Strongly associated with positive testing for HLA-B27, which portends a worse prognosis.
• RF and Anti-CCP are usually absent.
• Synovial fluid shows inflammatory synovitis (WBCs 2,000–64,000/mm³, predominantly neutrophils).
• Treatment & Prognosis:
• Initial therapy: NSAIDs are the principal form of treatment; intra-articular or oral glucocorticoids are used if NSAIDs fail.
• Refractory disease (>6 months): Non-biologic DMARDs (sulfasalazine preferred over methotrexate) or TNF inhibitors.
• Most patients remit in 3 to 12 months, but 15 to 20 percent develop chronic persistent arthritis.

Septic Arthritis

• Definition & Sources:
• Direct microbial invasion (most commonly bacterial) of the joint space, representing a closed abscess collection.
• Usually arises from hematogenous spread.
• Predisposing Factors:
• Age > 80 years, diabetes mellitus, rheumatoid arthritis, prosthetic joints, recent joint surgery, skin infections, IV drug abuse, alcoholism, and previous intra-articular corticosteroid injections.
• Clinical Presentation:
• Usually presents acutely with a single swollen and painful joint (monoarticular arthritis).
• The knee is involved in >50% of cases; wrists, ankles, and hips are also common.
• Diagnostics:
• Synovial fluid aspiration is mandatory (purulent fluid, WBC count 50,000 to 150,000 cells/mm³, predominantly neutrophils, depressed glucose, elevated lactic acid).
• Gram stain and culture lead to the correct diagnosis.
• Treatment Principles:
• Systemic antibiotics: Parenteral antibiotics for at least 14 days followed by oral therapy for another 14 days (longer for difficult pathogens like P. aeruginosa). Third-generation cephalosporins (ceftriaxone, cefotaxime, ceftazidime) are standard.
• Intra-articular antibiotics are NOT recommended due to poor efficacy and chemical irritation.
• Joint drainage is mandatory for all patients via needle aspiration, arthroscopy, or open arthrotomy.
• Prognosis:
• Polyarticular septic arthritis due to Staphylococcus aureus or occurring alongside rheumatoid arthritis carries mortality rates as high as 50 percent.

📊 Visual Learning

💡 Important Points to Remember

• Reactive arthritis is sterile; bacteria cannot be cultured from the joint, unlike septic arthritis where bacteria are actively present.
• HLA-B27 positivity is associated with a worse prognosis and higher chronicity in reactive arthritis.
• Classic triad of reactive arthritis (historically Reiter's syndrome): arthritis, urethritis, and conjunctivitis.
• Synovial fluid white blood cell counts are lower in reactive arthritis (2,000–64,000/mm³) compared to septic arthritis (50,000–150,000/mm³).
• Sulfasalazine (SSZ) is preferred over methotrexate as the first-line DMARD for refractory reactive arthritis due to better documented efficacy in peripheral spondyloarthritis.
• Intra-articular antibiotics are contraindicated in septic arthritis because they provoke an intense inflammatory reaction.
• Septic arthritis requires both systemic antibiotics and urgent joint drainage (needle aspiration, arthroscopy, or open surgery).
• Polyarticular septic arthritis caused by Staphylococcus aureus in a patient with underlying rheumatoid arthritis carries a mortality rate up to 50%.
• Common skin finding in reactive arthritis: Keratoderma blennorrhagica (hyperkeratotic lesions on palms and soles).
• Common genital finding in reactive arthritis: Circinate balanitis.

⚠️ Common Exam Questions

• The "Sterile vs. Septic" Trap: Examiners love to ask about a patient presenting with an acute monoarthritis following a diarrheal illness. Students often mistakenly select antibiotics to treat the joint directly, forgetting that reactive arthritis joints are sterile and treated with NSAIDs/DMARDs, whereas septic arthritis requires immediate joint drainage and systemic antimicrobials.
• Synovial Fluid Interpretation: Questions will provide a case with a joint fluid WBC count of 60,000/mm³ and negative gram stain versus 120,000/mm³ with positive gram stain. Memorize the numerical cutoffs to quickly differentiate reactive/inflammatory synovitis from septic joints.
• DMARD Preference: In MCQ trick questions regarding refractory reactive arthritis, methotrexate and sulfasalazine are both options; sulfasalazine is favored first due to evidence supporting its use in peripheral spondyloarthritis.

📝 Quick Review Checklist

I can explain the difference between reactive and septic arthritis
I understand why reactive arthritis joints are sterile while septic joints are not
I can define enthesitis, dactylitis, and keratoderma blennorrhagica
I know the key lab values for synovial fluid analysis in both conditions
I understand the predisposing factors for septic arthritis
I know the treatment algorithm for acute vs. refractory reactive arthritis
I know why intra-articular antibiotics are avoided in septic arthritis
I can list the major complications and prognostic factors for both diseases