22. Septic arthritis

Question 1

What are the infectious microorganisms of septic arthritis?

Answer 1

• S. Aureus, N. Gonorrhea and other bacteria are the most common causes of infectious arthritis
  
  • They can destroy articular cartilage rapidly, thus inflammed joints must be evaluated rapidly to exclude non-infectious processes and determine the appropriate antimicrobial therapy and drainage procedures.
• Various mycobacteria, spirochetes, fungi and viruses can also infect the joints

Question 2

What is the pathomechanism of septic arthritis?

Answer 2

• Bacteria enter the joint from the bloodstream
  
  • From a bordering site of infection (bones, soft tissue)
  • From direct inoculation during surgery, injection, animal or human bite, or, trauma
• In a hematogenous infection, the bacteria escape from synovial capillaries (no limiting basement membrane) into the joint space, which leads to neutrophilic infiltration of synovium within hours
  
  • Neutrophils and bacteria enter the joint space and later, bacteria adhere to the articular cartilage
  • Degradation of the cartilage begins within 48h as a result of:
    
    • Increased intra-articular pressure
    • Release of preoteases and cytokines from chondrocytes and synovial macrophages
    • Invasion of the cartilage by bacteria and inflammatory cells

Question 3

What are the considerations for diagnosis?

Answer 3

• Septic arthritis (infectious arthritis) is different from reactive arthritis (post-infectious arthritis)
• Consider septic arthritis in any acutely inflammed joint as it can destroy it < 24h.
• The knee is affected in 50% of the cases

Question 4

What are the risk factors of septic arthritis?

Answer 4

• Pre-existing joint disease (especially RA)
• DM
• Immunosuppression
• Recent joint surgery, prosthetic joints
• IV drug abuse

Question 5

How do we investigate septic arthritis?

Answer 5

• Urgent joint aspiration for synovial fluid –> microscopy and culture
• Plain radiographs may be normal
• Main differential diagnoses are crystal arthropathies
• Blood cultures may be helpful for guiding choice later

Question 6

What is the treatment of septic arthritis?

Answer 6

• Empirical antibiotic treatment until sensitivities are discovered
  
  • Consider flucloxacillin IV, vancomycin if MRSA or history of MRSA
  • Cefotaxime IV if gonococcal
• If HIV positive, look for atypical mycobacteria and fungi
• Orthopedic advice for arthrocentesis, lavage and debridement