Infectious Disease · Joint Infections
The facts most likely to be tested
Staphylococcus aureus is the most common causative organism in all age groups, including patients with prosthetic joints.
Arthrocentesis with synovial fluid analysis is the gold standard diagnostic test and must be performed prior to initiating empiric antibiotics.
Synovial fluid with a white blood cell count greater than 50,000 cells/mm³ with a predominance of neutrophils is highly suggestive of septic arthritis.
Neisseria gonorrhoeae is the most common cause of septic arthritis in sexually active young adults, often presenting as a migratory polyarthralgia or tenosynovitis.
Septic arthritis is a medical emergency that requires urgent surgical drainage and joint lavage to prevent permanent cartilage destruction and joint deformity.
Patients with rheumatoid arthritis are at significantly higher risk for developing septic arthritis, which often presents with a monoarticular flare in a previously stable joint.
Empiric antibiotic therapy for suspected septic arthritis should cover Staphylococcus aureus and Gram-negative bacilli using vancomycin plus a third-generation cephalosporin.
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A 24-year-old female presents to the emergency department with a 3-day history of pain and swelling in her right knee. She reports a recent history of fever and chills. Physical examination reveals a warm, erythematous, and tender knee with limited range of motion due to pain. She is sexually active and reports inconsistent condom use. Synovial fluid analysis reveals a white blood cell count of 65,000/mm³ with 90% neutrophils.
What is the most appropriate next step in management?
Urgent surgical drainage and initiation of intravenous antibiotics
The patient presents with classic signs of septic arthritis, which is a medical emergency requiring immediate joint decompression and systemic antibiotics to prevent irreversible joint damage.
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Etiology / Epidemiology
Most common in Staphylococcus aureus; risk factors include prosthetic joints, rheumatoid arthritis, and age >80.
Clinical Manifestations
Presents with monoarticular joint pain, swelling, and fever; patient will exhibit refusal to move the joint.
Diagnosis
Arthrocentesis is the gold standard; synovial fluid WBC count >50,000/µL with >90% neutrophils is diagnostic.
Treatment
Requires surgical drainage and IV vancomycin; delay in treatment leads to permanent joint destruction.
Prognosis
High risk of permanent cartilage damage within 24-48 hours; mortality rate is 10-15%.
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Epidemiology & Etiology
Hematogenous spread is the most common route of infection. Staphylococcus aureus is the primary pathogen in all age groups. Patients with prosthetic joints or rheumatoid arthritis are at significantly higher risk for bacterial seeding.
Pertinent Anatomy
The knee is the most commonly affected joint in adults. The hip is a frequent site in children and requires urgent evaluation to prevent avascular necrosis of the femoral head.
Pathophysiology
Bacteria invade the synovial space, triggering an intense inflammatory response. Proteolytic enzymes released by neutrophils cause rapid cartilage degradation. If untreated, the infection leads to septic shock and systemic inflammatory response syndrome.
Clinical Manifestations
Patients present with acute onset of monoarticular pain, warmth, and erythema. The joint is held in a position of maximal capsular volume to minimize pain. Systemic signs of sepsis like hypotension or altered mental status are late, ominous findings.
Diagnosis
Arthrocentesis must be performed before starting antibiotics. Synovial fluid analysis showing WBC >50,000/µL with >90% neutrophils is highly suggestive. Gram stain and culture are mandatory to guide definitive therapy.
Treatment
Immediate surgical drainage (arthroscopic or open) is the definitive management. Empiric IV vancomycin is the first-line choice to cover MRSA. Avoid intra-articular corticosteroids as they exacerbate the infection.
Prognosis
Rapid intervention is critical to prevent permanent joint ankylosis. Monitor for osteomyelitis if the infection persists despite drainage. Mortality is significant in elderly patients with comorbidities.
Differential Diagnosis
Gout: presence of negatively birefringent crystals
Pseudogout: presence of positively birefringent crystals
Reactive Arthritis: associated with urethritis or conjunctivitis
Lyme Arthritis: history of erythema migrans or tick exposure
Osteomyelitis: pain localized to bone rather than joint space