Septic arthritis is a bacterial infection of the joint space The etiologies vary by age In the neonatal period and early infancy, GBS and S aureus are the predominant pathogens, with gram-negative bacilli and Candida seen sporadically, especially in hospitalized infants Beyond this time period, S aureus overwhelmingly is the most common pathogen causing septic arthritis Pneumococcus, GAS, gonococcus, and K kingae also are associated with septic arthritis in children Salmonella can be seen in children with hemoglobinopathies Brucella septic arthritis can be associated with consumption of unpasteurized milk products and has a predilection for the sacroiliac joint Neisseria gonorrhea can cause a monoarticular arthritis can be seen in sexually active adolescents Hib is a rare cause of septic arthritis in the modern era Lyme arthritis and tuberculosis arthritis are discussed elsewhere Goals of Treatment The goal of treatment is to rapidly identify children with septic arthritis so that prompt arthrocentesis can be performed and antibiotics administered Clinical outcomes include orthopedic sequelae (e.g., chondrolysis, osteonecrosis) and differentiation of septic arthritis from other orthopedic complaints of childhood Clinical Considerations Clinical recognition: The most common manifestation is limp, as 90% of children with septic arthritis have monoarticular involvement of a lower extremity joint In the child who is limping or refusing to ambulate, occasionally it will be difficult to determine a focal lesion after manipulation of the lower extremity joints and palpation of the long bones Clinicians should be sure to evaluate the sole of the foot for foreign bodies and also to palpate over the sacroiliac joint to assess for tenderness Pain may be referred to other areas (e.g., hip septic arthritis presenting as knee pain) The child with a septic hip often lies quite still with the leg abducted and in external rotation Fever is present in almost two-thirds of children with septic arthritis, but can be absent in adolescents with gonococcal infections or in neonates An erythematous swelling may surround a superficial joint that is infected Although a temperature difference exists between the affected and unaffected sites, it can be difficult to discern in the febrile child Inflammation within the joint distends the capsule and produces pain with movement If a child allows the physician to manipulate an extremity through a full range of motion, septic arthritis is unlikely Triage considerations: Any child with fever and a limp or other joint complaint should be promptly evaluated for septic arthritis The most urgent of locations is septic arthritis of the hip, as this can result in compromised vascular flow to the femoral capitis Associated tachycardia and/or hypotension can imply sepsis and would require fluid resuscitation Clinical assessment: Septic arthritis is diagnosed via aspiration and culture of joint fluid Arthrocentesis is not only diagnostic, it is therapeutic; many children report decreased pain after synovial fluid is aspirated As such, early consultation with orthopedic surgery is critical Most children with pyogenic septic arthritis have synovial fluid white blood cell count (WBC) of >50,000 cells/mm3 and a neutrophilic predominance However, cell counts may be lower with Brucella or tuberculosis arthritis and may exceed 50,000 cells/mm3 with some noninfectious causes of arthritis, such as juvenile idiopathic arthritis A Gram stain should be sent, as some organisms may be seen in the synovial fluid and not grown in culture, even in children without antibiotic pretreatment, as synovial fluid has some bacteriostatic properties Approximately 50% of children with septic arthritis will have positive synovial fluid cultures Culture yield is enhanced for certain pathogens with appropriate specimen handling If Kingella is suspected, synovial fluid should be injected into a blood culture bottle to increase yield If Brucella is suspected, the laboratory should be notified so that the cultures can be kept far beyond the usual protocol, as it often takes weeks for this organism to grow Acid-fast cultures and M tuberculosis PCR should be sent in immunocompromised hosts or children with epidemiologic risk factors for tuberculosis Fungal, anaerobic, and acid-fast cultures (the latter for nontuberculous mycobacteria) should be obtained in children with penetrating trauma, as these infections often are polymicrobial and can be caused by a broad spectrum of pathogens Ancillary laboratory evaluation should include a blood culture, complete blood count, ESR, and CRP The ESR and CRP are the most consistent abnormal laboratory studies and can be used to monitor response to therapy Radiographic studies (e.g., MRI) can be used to evaluate for contiguous osteomyelitis This is particularly common in the first year of life, because at this time, the metaphysis is located within the joint capsule Recognition of osteomyelitis in association with septic arthritis is an important consideration when determining the duration of therapy