Septic Arthritis

Septic arthritis of the hip in a child constitutes a surgical emergency. Prompt recognition, diagnosis, and treatment of this entity are essential. Bacterial infection of the joint space results in an inflammatory effusion consisting of up to 90% polymorphonuclear cells (PMNs).²¹ The release of proteolytic enzymes by PMNs and bacteria, in conjunction with increased intra-articular pressure, can result in rapid and irreversible hyaline cartilage degradation in as little as 6 hours. If unrecognized or left untreated, this process can result in joint destruction with subsequent severe deformity and devastating lifelong disability. Joint infections can ultimately lead to disseminated infection, systemic bacterial sepsis, multiorgan failure, and death. However, if recognized and treated in a timely fashion with surgical drainage and appropriate antimicrobial therapy, a good outcome with minimal sequelae can be expected. Poor outcomes are most closely associated with delay in diagnosis. Other factors related to poor outcome include patient age younger than 6 months, prematurity, Staphylococcus species infection, and concomitant osteomyelitis.

Septic arthritis may occur as a result of hematogenous seeding, local spread from a contiguous infection, or primary seeding of a joint secondary to surgery or trauma. In the hip, shoulder, ankle, and elbow (90% of cases), septic arthritis often develops after metaphyseal osteomyelitis with subperiosteal erosion, abscess formation, and subsequent joint communication (because of the intra-articular location of the metaphyses in these joints). Premature and immunocompromised children are at greater risk. The most commonly identified causative organisms are Staphylococcus aureus, group A streptococci, Streptococcus pneumococcus, other gram-negative organisms (may be seen in special hosts or after trauma-klebsiellae, salmonellae, kingellae), and Neisseria gonorrhea.²² Before the advent of an effective vaccine, Haemophilus influenzae type B was responsible for up to 40% of cases but has largely disappeared because of successful immunization programs.^23,24 In the neonate, group B b-hemolytic streptococcus and gram-negative bacilli are common causative agents in addition to S. aureus. Gram-negative infections are common among intravenous (IV) drug abusers. Mycobacteria and fungi must be considered in patients with chronic infection.

Diagnosis

Children with septic arthritis will often have a history of recent upper respiratory or local soft-tissue infection, as well a recent course of antibiotics. A high level of suspicion is important when evaluating premature and immunocompromised infants. Septic arthritis is more commonly seen in boys and is most common in children younger than 2 years. Patients may report pain, stiffness, and malaise, but such a history is often impossible to ascertain from small children or infants. Fever, limp, inability or refusal to weight bear, limited and painful range of motion (secondary to capsular stretching), erythema, warmth, tenderness, and swelling are common physical findings. The child may complain of anterior hip, groin, thigh, or knee pain. The child may lie with the hip in external rotation, adduction, and mild flexion to maximize joint volume and decrease capsular stretch. Physical examination of the infant can be challenging and may only demonstrate limited or a lack of active motion in the affected extremity (pseudoparalysis).

Laboratory tests that are frequently ordered in cases of suspected septic arthritis of the hip include C-reactive protein (CRP) level, erythrocyte sedimentation rate (ESR), complete blood cell count (CBC) with differential, blood cultures, and throat culture/rapid strep test. Lyme titers are often added in endemic areas. Plain radiographs of the hip or pelvis are routinely ordered. Ultrasound may be useful to detect joint effusion. Magnetic resonance imaging (MRI) may be useful to detect associated osteomyelitis. The definitive diagnosis of septic arthritis is made by joint aspiration and analysis of joint fluid. Joint fluid white blood cell (WBC) count and cultures establish the diagnosis. Organisms can be detected on joint fluid Gram stain or culture. Cases with negative cultures but increased joint fluid WBC counts (>50,000/mm³) are considered to be presumed septic arthritis or culture-negative septic arthritis.

Treatment

Poor outcome is most closely associated with delay in diagnosis with subsequent delay in treatment. Once septic arthritis is diagnosed, prompt treatment with adequate surgical drainage and empiric IV antimicrobial therapy is cardinal. Ideally, synovial and blood cultures are obtained before treatment to increase the likelihood of identifying an organism. Open surgical irrigation and debridement to remove the microorganisms, host and bacterial enzymes, and particulate debris is indicated for most patients. However, arthroscopy has been used successfully in the shoulder, elbow, knee, and ankle, but it has not been widely utilizedin the hip. Empiric IV antimicrobial therapy should be initiated as soon as cultures are obtained. Empiric coverage must include an anti-staphylococcal agent, either a b-lactamase–resistant penicillin or a first-generation cephalosporin. Cefazolin is chosen as the drug for initial empiric therapy because it is effective against S. aureus, group A streptococcus, and S. pneumococcus, which should account for nearly all of the organisms among normal hosts with acute hematogenous osteomyelitis. Gram-negative coverage is indicated in the neonate (causative organisms may also include group B streptococci and gram-negative bacilli) and adolescents (to cover gonococcus). In children who are allergic to penicillin, clindamycin can be used. Ceftriaxone should be considered for coverage of gonococcal or Lyme arthritis. The antimicrobial regimen is adjusted according to culture speciation; if another organism is identified as the causative agent, an infectious disease consultation should be sought. IV antibiotics are continued for 72 hours. If the child shows evidence of clinical improvement (afebrile, decreased localized swelling, decreased/no pain, increased range of motion) and lyme titers are negative, conversion to oral antibiotics can be considered. The criteria for oral antibiotics include diagnosis within 4 days of the onset of symptoms, no concurrent osteomyelitis, and an ability to tolerate and be rigorously compliant with taking oral antibiotics. Patients treated with cefazolin may be given cephalexin 100 mg/kg/day divided in 4 daily doses (maximum dose, 4 g/day), and those treated with ceftriaxone can receive cefixime 8 mg/kg/day every 12 to 24 hours (maximum dose, 400 mg/day) for 21 days. The median duration of IV antibiotics is 5 to 15 days, and the total duration of antibiotics has been 4 to 6 weeks in uncomplicated cases.

Imaging

Ultrasonography has been demonstrated good accuracy in detecting an effusion associated with septic arthritis of the hip in children. Zamzam²⁵ retrospectively studied 81 children with septic arthritis and 73 children with transient synovitis, finding sensitivity of 86.4% and specificity of 89.7%. Dörr and colleagues²⁶ also found ultrasound to be useful in detecting effusion in 204 acutely irritable hips. In addition, they found certain characteristics, such as synovial hypertrophy and capsular thickening, to be helpful in differentiation between septic arthritis and transient synovitis. In a prospective analysis of 500 children with a painful hip or limp, Miralles and researchers²⁷ found ultrasonography to be useful in the detection of effusion. In addition, these authors showed that transient synovitis had resolution of the effusion at repeat ultrasound 2 weeks later.

MRI has been shown to be useful in detecting associated osteomyelitis but may also be useful in differentiating septic arthritis from transient synovitis. In a retrospective case series of 49 children with transient synovitis and 18 children with septic arthritis, Yang and investigators²⁸ found that patients with transient synovitis were more likely to have contralateral effusion and absence of signal intensity in the surrounding soft tissue and bone marrow.

Laboratory Tests

Clinicians have empirically utilized different labora-tory variables to help distinguish between septic arthritis and transient synovitis. Bennett and Namnyak²⁹ emphasize leukocyte count in children older than 3 years. Morrey and coauthors²⁰ and Klein and resear-chers²¹ accent increased ESR. Chen and colleagues³⁰ found both leukocytosis and increased ESR to be common. However, because these retrospective case series of children with septic arthritis did not have a comparison group of children with transient synovitis, they were unable to identify differences between the groups and were unable to determine the diagnostic performance of assorted variables.

On comparing children with septic arthritis and transient synovitis in retrospective case series, poor diagnostic utility has been found for various screening clinical, laboratory, and radiographic data because of substantial overlap between the two groups. Molteni²² retrospectively reviewed 97 patients with so-called nonspecific arthritis and 37 patients with septic arthritis of various joints including the hip, knee, ankle elbow, and wrist. Diagnoses were established presumptively, with joint fluid analysis of 12 of 97 patients with nonspecific arthritis and of 32 of 37 patients with septic arthritis. Fifty percent of the patients with septic arthritis had negative cultures and joint fluid WBC count varied from 5 to 385 3 1000/mm³