Review Question - QID 218700

Given the patient’s age and the moderately elevated laboratory findings combined with a concerning physical exam, this patient has most likely developed septic arthritis from Kingella kingae, a Gram-negative, facultatively anaerobic coccobacillus, shown in Figure B (Answer 2).

While Staphylococcus aureus remains the most common organism responsible for pediatric septic arthritis, these infections typically present with a more severe clinical course and significant elevations in inflammatory markers. Kingella kingae most commonly affects children under 4 years, oftentimes presents with less severe elevations in CRP and ESR, and is now recognized as a major causative organism in pediatric joint infections. It carries a less pronounced clinical picture compared with other causative organisms, and as a result of its indolent nature and comparatively milder signs and symptoms, Kingella-associated septic arthritis lends itself to underdiagnosis. Therefore, Kingella must always be on the differential in the setting of suspected pediatric septic arthritis. Kingella-specific genetic targets such as the rtxA toxin gene have allowed for increased detection rates via nucleic amplification assays (PCR/RT-PCR), demonstrating that Kingella is a much more prevalent etiological agent than was previously suspected. Urgent surgical debridement is critical to timely source control and preservation of the articular cartilage.

Williams et al. published a 2014 retrospective study reporting their experience with PCR detection of Kingella kingae in patients with septic arthritis. Using PCR as the gold standard, the authors reviewed the presentation, diagnosis, and management of Kingella-associated septic arthritis. The authors found that of the 27 cases of PCR-positive septic arthritis, 22 cases (81.5%) were culture-negative. Additionally, laboratory findings (WBC, CRP, and ESR) in Kingella-positive cases were not significantly different from those in Kingella-negative cases, the latter of which were culture-negative 70.7% of the time. This article highlighted the increased prevalence of Kingella as an etiological agent for pediatric septic arthritis, as well as the diagnostic difficulties due to subtle clinical presentations, equivocal blood test parameters, and poor culture yields.

Wong et al. published a 2020 systematic review of previous studies to determine the proportion of K. kingae in bacteriologically proven musculoskeletal infections among the pediatric population. Of the 1070 patients aged younger than 48 months, K. kingae was the organism identified in 47.6% of infections. 65% of cases involved joint infections, followed by 18.4% osteoarticular infection (concomitant bone and joint involvement), with isolated bone and spine at 11.9% and 3.5%, respectively. The authors emphasize that a high index of suspicion is needed due to the subtle presentation of K. kingae musculoskeletal infection in young children younger than 48 months.

Figure A is a Gram stain micrograph of a Staphylococcus species. Figure B is a Gram stain micrograph of Kingella kingae. Figure C is a Gram stain micrograph of a Streptococcus species. Figure D is a Gram stain micrograph of a Neisseria species. Figure E is an acid-fast stain of a Mycobacterium species.

Incorrect Answers:
Answer 1: While Staphylococcus aureus remains the most common etiologic agent for septic arthritis across all ages, younger children are at higher risk of Kingella-associated septic arthritis, which presents with mildly elevated inflammatory markers and a more insidious presentation.
Answer 3: Septic arthritis secondary to Streptococcal species is more often seen in unvaccinated patients, however, it is not as common as Kingella in this patient's age group.
Answer 4: Neisseria species are most often responsible for gonococcal septic arthritis in sexually active adolescents and young adults (N. gonorrheae) and bacterial meningitis (N. meningitides). It is not classically associated with septic arthritis in young children.
Answer 5: Mycobacterium species are most well-known for causing tuberculosis infections (M. tuberculosis) and leprosy (M. leprae). It is not classically associated with septic arthritis in young children.