Etiology

Historically, Haemophilus influenzae type b (Chapter 186) accounted for more than half of all cases of bacterial arthritis in infants and young children. Since the development of the conjugate, it is now a rare cause; Staphylococcus aureus (Chapter 174.1) has emerged as the most common infection in all age groups. Methicillin-resistant S. aureus accounts for a high proportion (>25%) of community S. aureus isolates in many areas of the USA and throughout the world. Group A streptococcus (Chapter 176) and Streptococcus pneumoniae (pneumococcus) (Chapter 175) historically cause 10-20%; S. pneumoniae is most likely in the first 2 years of life. Kingella kingae is recognized as a relatively common etiology with improved culture and polymerase chain reaction (PCR) methods in children <5 yr old (Chapter 676). In sexually active adolescents, gonococcus (Chapter 185) is a common cause of septic arthritis and tenosynovitis, usually of small joints or as a monoarticular infection of a large joint (knee). Neisseria meningitidis (Chapter 184) can cause either a septic arthritis that occurs in the first few days of illness or a reactive arthritis that is typically seen several days after antibiotics have been initiated. Group B streptococcus (Chapter 177) is an important cause of septic arthritis in neonates.

Fungal infections usually occur as part of multisystem disseminated disease; Candida arthritis can complicate systemic infection in neonates with or without indwelling vascular catheters. Primary viral infections of joints are rare, but arthritis accompanies many viral (parvovirus, mumps, rubella live vaccines) syndromes, suggesting an immune-mediated pathogenesis.

A microbial etiology is confirmed in about 65% of cases of septic arthritis. Prior antibiotic therapy and the inhibitory effect of pus on microbial growth might explain the low bacterial yield. Additionally, some cases treated as bacterial arthritis are actually postinfectious (gastrointestinal or genitourinary) reactive arthritis (Chapter 151) rather than primary infection. Lyme disease produces an arthritis more like a rheumatologic disorder and not typically suppurative.

Epidemiology

Septic arthritis is more common in young children. Half of all cases occur by 2 yr of age and three fourths of all cases occur by 5 yr of age. Adolescents and neonates are at risk of gonococcal septic arthritis.

The majority of infections in otherwise healthy children are of hematogenous origin. Infection of joints can follow penetrating injuries or procedures such as trauma, arthroscopy, prosthetic joint surgery, intra-articular steroid injection, and orthopedic surgery, although this is uncommon. Immunocompromised patients and those with rheumatologic joint disease are also at increased risk of joint infection.

Pathogenesis

Septic arthritis primarily occurs as a result of hematogenous seeding of the synovial space. Less often, organisms enter the joint space by direct inoculation or extension from a contiguous focus. The synovial membrane has a rich vascular supply and lacks a basement membrane, providing an ideal environment for hematogenous seeding. The presence of bacterial products (endotoxin or other toxins) within the joint space stimulates cytokine production (tumor necrosis factor-α, interleukin-1) within the joint, triggering an inflammatory cascade. The cytokines stimulate chemotaxis of neutrophils into the joint space, where proteolytic enzymes and elastases are released by neutrophils, damaging the cartilage. Proteolytic enzymes released from the synovial cells and chondrocytes also contribute to destruction of cartilage and synovium. Bacterial hyaluronidase breaks down the hyaluronic acid in the synovial fluid, making the fluid less viscous and diminishing its ability to lubricate and protect the joint cartilage. Damage to the cartilage can occur through increased friction, especially for weight-bearing joints. The increased pressure within the joint space from accumulation of purulent material can compromise the vascular supply and induce pressure necrosis of the cartilage. Synovial and cartilage destruction results from a combination of proteolytic enzymes and mechanical factors.

Clinical Manifestations

Most septic arthritides are monoarticular. The signs and symptoms of septic arthritis depend on the age of the patient. Early signs and symptoms may be subtle, particularly in neonates. Septic arthritis in neonates and young infants is often associated with adjacent osteomyelitis caused by transphyseal spread of infection, although osteomyelitis contiguous with an infected joint can be seen at any age (Chapter 676).

Older infants and children might have fever and pain, with localizing signs such as swelling, erythema, and warmth of the affected joint. With involvement of joints of the pelvis and lower extremities, limp or refusal to walk is often seen.

Erythema and edema of the skin and soft tissue overlying the site of infection are seen earlier in septic arthritis than in osteomyelitis, because the bulging infected synovium is usually more superficial, whereas the metaphysis is located more deeply. Septic arthritis of the hip is an exception because of the deep location of the hip joint.

Joints of the lower extremity constitute 75% of all cases of septic arthritis (Table 677-1). The elbow, wrist, and shoulder joints are involved in about 25% of cases, and small joints are uncommonly infected. Suppurative infections of the hip, shoulder, elbow, and ankle in older infants and children may be associated with an adjacent osteomyelitis of the proximal femur, proximal humerus, proximal radius, and distal tibia because the metaphysis extends intra-articularly.

Table 677-1 ANATOMIC DISTRIBUTION OF SEPTIC ARTHRITIS