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KEY FEATURES

Essentials of Diagnosis

  • Acute onset of inflammatory monarticular arthritis, most often in large weight-bearing joints and wrists

  • Common risk factors include previous joint damage or injection drug use

  • Infection with causative organisms commonly found elsewhere in body

  • Joint effusions are usually large; synovial fluid white blood cell counts commonly > 50,000/mcL (50 × 109/L)

General Considerations

  • Most often due to hematogenous seeding of the joint; direct inoculation from penetrating trauma is rare

  • Key risk factors are

    • Bacteremia (eg, injection drug use, endocarditis, infection at other sites)

    • Damaged (eg, from rheumatoid arthritis [RA]) or prosthetic joints

    • Compromised immunity (eg, advanced age, diabetes mellitus, advanced chronic kidney disease, alcohol use disorder, cirrhosis, and immunosuppressive therapy)

    • Loss of skin integrity (eg, cutaneous ulcer or psoriasis)

  • Staphylococcus aureus is the most common cause of nongonococcal septic arthritis, accounting for ∼50% of all cases

  • Methicillin-resistant S aureus (MRSA) and group B Streptococcus are frequent and important causes of septic arthritis

  • Gram-negative septic arthritis

    • Causes ∼10% of cases

    • Especially common in persons who use injection drugs and in those who are immunocompromised

    • Escherichia coli and Pseudomonas aeruginosa are most common pathogens

CLINICAL FINDINGS

Symptoms and Signs

  • Acute onset, with pain, swelling, and heat of affected joint—most frequently the knee—and worsening over hours

  • Other common sites are the hip, wrist, shoulder, and ankle

  • Unusual sites, such as the sternoclavicular or sacroiliac joint, can be involved in those who use injection drugs

  • Chills and fever are common (but absent in up to 20% of patients)

  • Infection of the hip usually does not produce apparent swelling but results in groin pain aggravated by walking

  • More than one joint is involved in 15% of cases; risk factors include RA, associated endocarditis, and infection with group B streptococci

  • Prosthetic joint infection

    • Influenced by whether the infection is early (< 3 months after surgery), delayed (3–12 months after surgery), or late (> 12 months after surgery)

    • Early infections

      • Acute redness and swelling

      • Usually caused by S aureus and gram-negative organisms

    • Delayed infections

      • Often present with subtle manifestations

      • Pain is common but only 50% will have fever

      • Most commonly caused by less virulent organisms, such as coagulase-negative Staphylococcus, Proprionibacterium acnes, and enterococci

    • Late infections

      • Acute pain, swelling, and fever

      • Often caused by hematogenous seeding of S aureus, gram-negative bacilli, and hemolytic streptococci

Differential Diagnosis

  • Gout and pseudogout are excluded by the failure to find crystals on synovial fluid analysis

  • Chronic Lyme disease commonly manifests as inflammatory monoarthritis of the knee, but the synovial fluid is Gram stain- and culture-negative

  • Acute rheumatic fever commonly involves an inflammatory migratory oligoarthritis

  • RA

    • An uncommon initial presentation is an acute inflammatory monoarthritis ("pseudoseptic")

    • Septic arthritis ...

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