Originally published at Pediatric EM Morsels on August 28, 2015. Reposted with permission.

Follow Dr. Sean M. Fox on twitter @PedEMMorsels

We have discussed several entities that may lead to a child limping. We have covered osteomyelitis, plantar punctures, and toddler’s fractures.  We have also touched upon Osgood Schlatter’s Disease, SCFE, osteosarcoma and even Growing Pains. Now let us review a topic that always crosses our minds when considering the painful extremity: Septic Arthritis.

Septic Arthritis: Basics

• Septic arthritis is an infection in the joint space and synovial fluid.
• Can occur by hematogenous spread of bacteria or direct inoculation.
  • Often associated with adjacent osteomyelitis. [Nduaguba, 2015]
• High Risk populations = children less than 2 years of age, immunocompromised, and patients without functional spleens
• Complications:
  • Capsule damage
  • Chronic arthritis
  • Osteonecrosis
  • Growth Arrest
  • Sepsis

Septic Arthritis: Presentation

• Fever
  • Although no/low fever noted in up to 20% of cases!
• Joint pain, swelling, and erythema
  • Pain with passive range of motion!
  • Limps or refuses to bear weight on limb.
• 80% of cases in children involve the lower limbs
  • Knee involved in 40% of cases
  • Hip involved in 20% of cases

Septic Arthritis: The Bugs

• Staph aureus = most common organism across all ages
  • MRSA has become more prevalent [Young, 2011]
  • Group B Strep is 2nd most common
• Special Population considerations:
  • Infants:
    • E. Coli
  • Young Children (<4 years)
    • Klingella kingae (notoriously difficult to culture)
    • Hemophilus influenza B has become less prevalent since HiB vaccination.
  • Immunocompromised:
    • Klingella kingae
    • Streptococcus pneumoniae (especially with HIV infection)
  • Sickle Cell Disease:
    • Salmonella (although, S. Aureus is still most common)
  • Sexually Active:
    • N. Gonorrhea – most common cause of polyarticular infections in sexually active patients

Septic Arthritis vs. Toxic Synovitis

• Despite the name, toxic synovitis is the self-limited, benign inflammation of the joint that gets treated symptomatically.
• Unfortunately, the presentation of toxic synovitis can be difficult to differentiate from septic arthritis, particularly when involving the hip joint.
  • Atraumatic 
  • Acute pain
  • Limp / refuses to bear weight
  • Fever
• The treatment strategies and potential outcomes are quite different for the two conditions, so differentiating between them is critical… although challenging. (again, your job isn’t easy)

Septic Arthritis: Kocher’s Criteria

• In 1999, Kocher et al published retrospective data from cases that presented to their facility from 1979-1996 due to “acutely irritable hip.”
• Through a logistic regression analysis of 168 patients, they devised a probabilityalgorithm to help differentiate between septic arthritis and toxic synovitis.
• There was no single lab test that was able to differentiate between the two entities. [Kocher, 1999]

• Kocher’s Criteria: [Kocher, 1999]

  • Predictors associated with risk of Septic Arthritis
    • Fever
    • Non-weight-bearing
    • ESR = 40 or more
    • Serum WBC = 12,000 or more
  • Probability of Septic Arthritis based on number of Predictors
    • 0 Predictors – <0.2 %
    • 1 Predictor – 3.0%
    • 2 Predictors – 40.0%
    • 3 Predictors – 93.1%
    • 4 Predictors – 99.6%

• Use this information wisely… not blindly.
  • May not apply to your patient.
    • Not hip pain?
    • Any underlying high-risk factors?
    • Clinical Decision Rules typically have diminished performance in different populations other than the derivation group. [Kocher, 2004]
  • Must balance the risk of false-positives vs false-negatives.
    • At what point does risk of missing septic arthritis outweigh the morbidity of joint aspiration? [Kocher, 1999].
      • 0 or 1 Predictors – close follow-up / observation
      • 2 Predictors – Aspiration via fluoroscopy/ultrasound
      • 3 or 4 Predictors – Aspiration in OR with likely arthrotomy and drainage.

Morals of the Morsel

• Septic Arthritis needs to be higher on your differential than Toxic Synovitis.
• Appreciate the diagnostic challenge inherent in the evaluation.
• NO SINGLE TEST WILL DIAGNOSE OR RULE-OUT SEPTIC ARTHRITIS. [Dodwell, 2013]

• Anticipate what tool (ex, Kocher Criteria) your consultants will likely use, but know their limitations.
• 2 Predictors is more reassuring than 3, but still comes with increased risk.
  • Having Fever and being Non-Weight Bearing with normal labs can still be associated with Septic Arthritis!
• Your pretest probability has to be taken into account, like always.
• Don’t forget to give some analgesics!  
  • The child who is now weight-bearing after NSAIDs just became less concerning and it may be better to arrange close followed-up rather than ordering a bunch of non-specific lab tests.

References