Infectious Disease · Joint Infections
The facts most likely to be tested
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Staphylococcus aureus is the most common causative organism in native-joint septic arthritis in most age groups, whereas coagulase-negative staphylococci (e.g., Staphylococcus epidermidis) are a leading cause in patients with prosthetic joints.
Arthrocentesis with synovial fluid analysis is the key diagnostic test and should ideally be performed before starting empiric antibiotics, without delaying treatment in a septic patient.
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 classically the most common cause of septic arthritis in sexually active young adults, often presenting as a migratory polyarthralgia or tenosynovitis; it is typically treated with ceftriaxone (CDC 2021) plus joint aspiration, and surgical washout is usually unnecessary.
Septic arthritis is a medical emergency that warrants urgent joint drainage (needle aspiration, arthroscopic, or open 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 when the Gram stain is unrevealing (vancomycin alone when it shows Gram-positive cocci), then narrowed to culture results.
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A 67-year-old man with rheumatoid arthritis treated with methotrexate and low-dose prednisone presents to the emergency department with a 2-day history of pain and swelling in his right knee that is worse than his usual flares. He reports fever and chills; his temperature is 38.9°C (102°F). Physical examination reveals a warm, erythematous, and tender knee with a large effusion and limited range of motion due to pain. He has no rash, tenosynovitis, or urethral symptoms. Synovial fluid analysis reveals a white blood cell count of 85,000/mm³ with 92% neutrophils, no crystals, and Gram-positive cocci in clusters on Gram stain.
What is the most appropriate next step in management?
Urgent joint drainage and empiric intravenous vancomycin
Acute monoarticular arthritis with fever, a synovial WBC of 85,000/mm³ with neutrophil predominance, no crystals, and Gram-positive cocci in clusters indicates bacterial septic arthritis, most likely from Staphylococcus aureus; rheumatoid arthritis and immunosuppression raise his risk. Management is empiric IV antibiotics with MRSA coverage (vancomycin) plus urgent joint drainage, by arthroscopic or open washout or by repeated needle aspiration, to limit cartilage destruction. This differs from gonococcal arthritis in a sexually active young adult, which is typically treated with ceftriaxone (CDC 2021) plus joint aspiration and usually does not need surgical washout.
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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 diagnostic test of choice; synovial fluid WBC count >50,000/µL with >90% neutrophils is highly suggestive, but lower counts do not exclude infection and Gram stain and culture establish the diagnosis.
Treatment
Warrants urgent joint drainage (needle aspiration, arthroscopic, or open) and empiric IV vancomycin plus a third-generation cephalosporin (ceftriaxone); 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 most age groups, although Kingella kingae is common in children under 4 years. 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 should ideally be performed before starting antibiotics. Synovial fluid analysis showing WBC >50,000/µL with >90% neutrophils is highly suggestive. Gram stain and culture are essential to guide definitive therapy.
Treatment
Prompt joint drainage (serial needle aspiration, arthroscopic, or open washout) is a cornerstone of management; no method has proven superior for most adult native joints, while the pediatric hip typically needs surgical drainage. Empiric IV vancomycin plus a third-generation cephalosporin (ceftriaxone) is first-line, covering MRSA as well as Gram-negative bacilli and Neisseria gonorrhoeae, and is narrowed to Gram stain and culture results. Suspected gonococcal arthritis is typically treated with ceftriaxone 1 g IV daily (CDC 2021) plus joint aspiration. 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