Diagnostic Considerations
The differential diagnosis of a painful monoarthritis is rather extensive. In contrast to children with septic arthritis (SA), children with transient synovitis appear well and are usually afebrile with just a mild limp. [10] The American College of Radiology has established guidelines for the assessment of a limping child. [11]
Differentiating septic arthritis from transient synovitis of the hip is particularly important given the need for urgent surgical intervention for the former condition. Kocher assessed children who underwent joint aspiration for suspected septic arthritis. Septic arthritis was defined as having a pathogen isolated and/or having > 50,000 WBC/mL in the synovial fluid; other children were defined as having transient synovitis. Kocher identified four clinical and lab predictors that could discriminate between these 2 conditions: (1) inability to bear weight; (2) Peripheral WBC of greater than 12 × 109/L; (3) Sedimentation rate ≥ 40 mm/hr; and (4) Fever > 38.5°C. If all four predictors were present, there was a probability of septic arthritis of 99.6%; the probability was 93% if 3 predictors were present. [12] In a subsequent validation study, 3 predictors had a probability of 73%. [13]
In adolescents, a slipped capital femoral epiphysis may manifest as a painful hip, thigh, or knee. Most patients are afebrile and the onset of pain may be preceded by minor trauma.
Legg-Calve-Perthes disease, which is most common in boys, afflicts children aged 4-8 years. In contrast to SA, the pain is subacute, with a more indolent onset, and these children do not have fever.
One study demonstrated that children with SA were less likely to have knee involvement, a history of a tick bite, or a fever than were children with Lyme disease. [2] Additionally, median values of inflammatory markers were higher among those with SA; however, a large overlap was noted between the groups.
Aside from gonococcal arthritis or SA in the neonate, polyarthritis is not typically caused by bacteria in the joints. The differential for polyarthritis in children is broad and includes Lyme disease, acute rheumatic fever, serum sickness, Kawasaki disease, systemic lupus erythematosus, and Henoch-Schönlein purpura.
In a 2014 report from New Zealand, Mistry concluded that serological inflammatory markers and white cell count (WCC) on presentation differ significantly between children with acute rheumatic fever (ARF) and septic arthritis. Children with ARF displayed significantly higher erythrocyte sedimentation rate (ESR), higher serum C-reactive protein, and lower serum WCC than children with septic arthritis on presentation to hospital. [14]
A study by Horton et al found that only 35% of cases of C difficile infection-associated reactive arthritis were correctly diagnosed by treating health care professionals and that five affected children (19%) were treated for presumed culture-negative septic hip arthritis despite having prior postantibiotic diarrhea and/or other involved joints. [15]
Differential Diagnoses
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Lyme Disease
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Emergency room photograph of an infant with septic arthritis of the left hip. The child holds his hip rigidly in the classic position of flexion, abduction, and external rotation, a position that maximizes capsular volume. The patient is relatively comfortable as long as the hip joint remains immobile in this position.
