Arthritis, Nongonococcal Acute Bacterial (Septic)

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 ...