Key Points

• The knee is the most common source of infectious arthritic pain.

• Septic arthritis is a surgical emergency.

• Staphylococcus aureus and Neisseria gonorrhoeae are the two most common causes.

Acute bacterial arthritis is one of the few rheumatologic emergencies. Failure by the primary care physician to diagnose this entity and to initiate prompt antibiotic therapy results in significant morbidity (functional disability, joint destruction) and at least 5% mortality. Even in patients with known rheumatologic disease, a bacterial infection may cause an acutely inflamed joint. The three mechanisms of a septic joint are (1) hematogenous spread from a distant location, such as a urinary tract infection (UTI) or pneumonia; (2) contiguous spread from a wound infection, abscess, or osteomyelitis; and (3) direct introduction of bacteria through trauma, surgery, or arthrocentesis. Concern about the third mechanism should not prevent the family physician from ruling out bacterial infection by performing synovial fluid analysis in an inflamed joint if the diagnosis is unclear. As for lumbar puncture, if the physician thinks it should, arthro-centesis probably should be done.

From 80% to 90% of acute bacterial articular infections are monoarticular. In adults with nongonococcal bacterial arthritis, the most common sites are the knee (50%), hip (20%), shoulders (8%), ankles (7%), wrists (7%), elbow (6%), other (5%), and more than one joint (usually two; 12%) (Brusch, 2005). In children, the most common sites are the knee (40%), hip (28%), ankle (14%), shoulder (4%), wrist (3%), elbow (11%), other (3%), and more than one joint (7%) (Baker and Schumacher, 1993). About 20% of patients are afebrile. Septic joints are normally painful, swollen, red, and warm. Diabetes mellitus, malignancy, chronic liver disease, and other rheumatic diseases (e.g., RA, SLE) increase the risk of a septic joint and probably its severity. Other risk factors for a septic joint include advanced age, intravenous drug abuse, HIV infection, and having a prosthetic joint. Almost half of adults with septic arthritis are older than 60 years, and the condition usually affects an arthritic hip, knee, or shoulder. Septic arthritis in older adults causes a fever in only 10% of patients and a marked leukocytosis in only one third, although ESR elevation is usually marked. Joint and blood cultures are usually positive.

Most polyarticular disease is seen in immunosuppressed patients or those with underlying rheumatic disease. The causative organism is usually Staphylococcus aureus. The mortality rate in patients with polyarticular septic joints approaches 40% (Youssef and York, 1994).

Most cases of septic arthritis are secondary to hematogenous spread of infection. In drug abusers, the causative organism is usually S. aureus or gram-negative organisms and affects predominantly the joints of the axial skeleton (hip, shoulder, vertebrae, symphysis pubis, costochondral, sternoclavicular, sacroiliac). Iatrogenic septic arthritis is usually caused by S. aureus, Staphylococcus epidermidis, and gram-negative organisms. This complication might be difficult to recognize because the joint is already symptomatic (which prompted the arthroscopy or arthrocentesis) before infection. Bacterial infection after arthroscopy is 0.04% to 4% and after arthrocen-tesis, 0.01%. Septic arthritis complicating RA is polyarticular in 50% of patients, usually caused by S. aureus, and arises from pulmonary or UTIs, infected rheumatoid nodules, or foot infections. Prosthetic infections are from direct inoculation or hematogenous spread; the prosthesis often must be removed.

Septic arthritis in children normally involves the lower extremities (knee, hip, and ankle). An infant or child with a septic joint often presents with not moving the infected joint and being generally irritable. Septic arthritis can complicate otitis media, an umbilical catheter, meningitis, or osteomyelitis. S. aureus and group B streptococci are the most common organisms in infants and children, except ages 6 months to 2 years, when Haemophilus influenzae and Kingella kingae organisms predominate. H. influenzae septic arthritis is seen especially in partially immunized children.

The most common form of acute bacterial arthritis is disseminated gonococcal infection, which causes a migratory polyarthritis and tenosynovitis, affecting predominantly the small joints of the hands, wrists, elbow, ankles, and knees. Papules and vesicles are often apparent on the trunk and extremities, including the palms and soles. Patients usually do not have symptoms of urethritis, cervicitis, or pharyngitis. If gonococcal arthritis is suspected, empiric treatment should be initiated immediately while culture results are pending. Neisseria meningitidis can cause a similar arthritis-rash syndrome following an illness, ranging from a mild upper respiratory infection to a frank meningitis. In contrast to gonococcal infections, the meningococcus might cause oral mucosal lesions as well as skin lesions.

Acute gonococcal arthritis can be confirmed by Gram stain in only 25% of patients and by culture in 50%; nongono-coccal bacterial arthritis can be confirmed in 50% and 90% of patients, respectively. It is therefore important to make a clinical diagnosis rather than rely solely on laboratory studies. Fever is often absent or of low grade. Blood cultures are also positive only approximately half the time but might be positive when synovial fluid cultures fail to identify an organism. Synovial fluid analysis usually shows WBC count over 50,000/mm3, with over 90% PMNs. Crystals can coexist with bacterial infections, and their presence should not rule out bacterial infection. Plain radiography might detect an osteomyelitis and should be done as a baseline study, because destruction can be seen on radiographs 10 to 14 days later. Air in the joint suggests an anaerobic infection, which accounts for 1% of septic joints.

The duration of appropriate intravenous (IV) antibiotic treatment depends on the presumptive or culture-identified causative organism. Initial therapy depends on the Gram stain result from synovial fluid. If gram-positive cocci are seen, IV vancomycin should be started empirically. If gramnegative bacilli are seen, a third-generation IV cephalosporin should be initiated. If Gram stain is negative, IV vancomycin should be considered. Once sensitivity data return, antibiotic therapy can be narrowed appropriately. Duration varies, but often the patient will receive 2 weeks of IV therapy followed by 2 weeks of oral therapy. Intra-articular antibiotic injections are unnecessary. A joint might need repeated needle aspirations or tidal lavage with arthroscopy to sterilize the joint space (Klippel, 2001).

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