Septic arthritis

Septic arthritis is an infection of the joint space that affects 20,000 people in the US per year. It is more common in male, elderly, and immunosuppressed patients. It is also particularly prevalent in prosthetic joints, affecting up to 10% of them. Bacteria invade the joint either from the bloodstream from another infected site or by direct inoculation of the joint. Joints with structural damage (such as from rheumatoid arthritis) are more susceptible to infection. Once the joint is infected, destruction occurs quickly (within a few days). The cartilage becomes damaged by bacteria directly, however influx of leukocytes into the joint and the subsequent inflammation also causes more destruction. Large joint effusions can prevent perfusion of the joint resulting in ischemic necrosis of the bone. Even if treated quickly, half of adults who get septic arthritis may have chronic complications such as pain or decreased range of motion

• Staph aureus is the most common cause overall (80% cases) and carries the highest mortality rate (50%)
• Neisseria gonorrhoeae is the most common cause in young/sexually active patients and has a very low mortality rate. It is more common in women.
• Strep species (viridans, pneumoniae, group B), and gram negative rods are less common
• Polymicrobial or anaerobic infections are usually secondary to another process such as trauma or other spread of infection from somewhere else
• Viruses and fungi can cause joint infections in rare cases

• Symptoms: severe joint pain (75% cases) fever/chills, inability to range, acute swelling and erythema around joint
• Relevant history that increases the risk of septic joint: prosthetic joint, sexual history/history of STIs, IV drug use or other risk factors for bacteremia, structural joint disease such as rheumatoid arthritis or trauma, immunosuppression such as diabetes or cancer, recent instrumentation of the joint, recent tick bite or other symptoms of lyme disease
• Most common sites of infection (in order): knee (50%), hip (20%), shoulder (15%), ankle, wrist, elbow
  • If more than one joint is affected, the causative organism is more likely to be N. gonnorrhoeae, a virus, or lyme disease (or non infectious)
• Exam: significant tenderness, warmth, erythema, joint effusions, decreased range of motion

• Synovial fluid aspiration for microscopy, Gram stain and culture under sterile conditions
  • Joint fluid may appear more yellow (from PMNs) and be less stringy/viscous (due to destruction of hyaluronic acid, which is what causes synovial fluid to be viscous)
  • WBC count >20,000/mm3 with PMN predominance (>75%)
    • WBC count can be lower, especially in gonococcal infection
  • Cultures is the only way to definitively diagnose septic arthritis, however they are not very sensitive
• Likely will have an elevated ESR or CRP
• Blood cultures should be sent to evaluate for bacteremia
• If gonococcal infection is suspected, consider sending genital cultures
• Imaging: X-rays are not generally helpful, but can evaluate for underlying osteomyelitis (same with CT or MRI)

• Antibiotics
  • Generally based off culture results and local resistance patterns; vancomycin and cephalosporins are commonly used
  • At least 4 weeks of IV antibiotics in Staph aureus, less in other types
  • For gonococcal infections, treat empirically for chlamydia as well (azithromycin or doxycycline)
  • Consider repeated joint aspirations for fluid re-accumulation in rare cases
• Surgical irrigation may be required