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) or prosthetic joints

  • Compromised immunity (eg, advanced age, diabetes mellitus, advanced chronic kidney disease, alcoholism, 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 about 50% of all cases

• Methicillin-resistant S aureus (MRSA) and group B streptococcus have become increasingly frequent and important causes of septic arthritis

• Gram-negative septic arthritis is seen in injection drug users and in other immunocompromised patients

• Staphylococcus epidermidis is the usual organism in prosthetic joint septic arthritis

Clinical Findings

Symptoms and Signs

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

• Unusual sites, such as the sternoclavicular or sacroiliac joint, can be involved in injection drug users

• 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 greatly aggravated by walking

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

• Polyarticular septic arthritis is uncommon except in patients with rheumatoid arthritis or with group B streptococcal infections

• Manifestations of prosthetic joint infection are 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

    • 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 monarthritis of the knee, but the synovial fluid is Gram stain and culture negative

• Acute rheumatic fever and rheumatoid arthritis commonly involve several joints

• Still disease may mimic septic arthritis, but laboratory evidence of infection is absent