ESSENTIALS OF DIAGNOSIS
Acute onset of inflammatory monoarticular arthritis, most often in large weight-bearing joints and wrists.
Common risk factors include previous joint damage and injection drug use.
Infection with causative organism commonly found elsewhere in body.
Joint effusions are usually large; synovial fluid WBC counts > 50,000/mcL (50 × 109/L) are common.
Lyme disease is discussed in Chapter 34.
Nongonococcal acute bacterial arthritis is most often due to hematogenous seeding of the joint; direct inoculation from penetrating trauma is rare. The key risk factors are bacteremia (eg, injection drug use, endocarditis, infection at other sites), damaged joints (eg, rheumatoid arthritis), prosthetic joints, compromised immunity (eg, advanced age, diabetes mellitus, advanced CKD, alcohol use disorder, cirrhosis, or immunosuppressive therapy), and 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 are frequent and important causes of septic arthritis. Gram-negative septic arthritis causes about 10% of cases and is especially common in injection drug users and immunocompromised persons; Escherichia coli and Pseudomonas aeruginosa are the most common pathogens. Pathologic changes include varying degrees of acute inflammation, with synovitis, effusion, abscess formation in synovial or subchondral tissues, and, if treatment is not adequate, articular destruction.
The onset is acute, with pain, swelling, and heat of the affected joint worsening over hours. The knee is most frequently involved; other commonly affected sites are the hip, wrist, shoulder, and ankle. Unusual sites, such as the sternoclavicular or sacroiliac joint, can be involved in injection drug users. Chills and fever are common but are 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 of septic arthritis; risk factors for multiple joint involvement include rheumatoid arthritis, associated endocarditis, and infection with group B streptococci.
Synovial fluid analysis is critical for diagnosis. The leukocyte count of the synovial fluid is always inflammatory (greater than 2000/mcL [2 × 109/L]), usually exceeds 50,000/mcL (50 × 109/L), and often is more than 100,000/mcL (100 × 109/L), with 90% or more polymorphonuclear cells (Table 20–2). Synovial fluid cell count in septic arthritis may be lower in immunocompromised patients, especially those on biologic therapies such as TNF inhibitors. Gram stain of the synovial fluid is positive in 75% of staphylococcal infections and in 50% of gram-negative infections. Synovial fluid cultures are positive in 70–90% of cases; administration of antibiotics prior to arthrocentesis reduces the likelihood of a positive ...