A 5 week old infant presented to the Emergency Department (ED) with refusal to move right arm for the past 3 days. No significant past medical history; the pregnancy was uncomplicated, and born via Cesarian-section due to failure to descend at full term at 40 weeks. The patient has otherwise been feeding well and moving all of his other extremities. No history of trauma or fever. The arm and shoulder have no erythema, no swelling, however the patient screams in pain whenever you move the arm.
X-rays of the right shoulder and entire right arm were unremarkable. Laboratory tests demonstrated a slightly elevated white blood cell count (WBC), however the erythrocyte sedimentation rate (ESR) and C-reactive protein (CRP) were both elevated. You admit the patient for further work up. The following day (actually a few hours later since was a late admission) the patient underwent a sedated MRI, which revealed humeral osteomyelitis with associated septic arthritis.
What are common causes of osteomyelitis/septic arthritis in a newborn? What is the best tests/imaging to diagnose it?
Osteomyelitis can cause significant morbidity and mortality in a neonate/infant and can be difficult to diagnose. In one small study, Wong and colleagues found that only 10 of the 30 babies studied demonstrated any systemic signs of infection, such as fever. This study also found that more than half of the infants affected were born pre-term, and 70% of the patients with osteomyelitis had extended contact with the healthcare system (eg prolonged stay in the hospital).
A review by Montgomery and colleagues found that Staphylococcus aureus (S. aureus), to be the most common cause of osteomyelitis, with cases of Methicillin resistant S. aureus rising nationally. In infants and children specifically, other common bacteria causing osteomyelitis are Group B Streptococcus, Ecsherichia coli, Kingella kingae usually spread hematogenously.
In a review of septic arthritis with concomitant adjacent osteomyelitis, such as this particular case, it was found that the shoulder was the most likely of all the joints (elbows, hips, knees, ankle) to be infected, as it was in this patient. S. aureus again was the most common organism to cause a simultaneous osteomyelitis with associated septic arthritis.
In osteomyelitis, the WBC is often not a sensitive marker. One study in the journal of Pediatrics found that only 35% of children with osteomyelitis had an elevated WBC. In contrast, ESR and CRP elevations were more sensitive, at 92% and 98% respectively. Combined together, ESR and CRP offered the greatest sensitivity in detecting osteomyelitis. After initiation of treatment, the ESR usually normalized within 24 days and the CRP in 10 days.
The recommended imaging modality for acute osteoarticular infections is magnetic resonance imaging (MRI) with contrast given the superior imaging it provides of bone as well as the soft tissues when compared to other imaging modalities. In follow up after treatment, positron emission tomography (PET) or commuted tomography appears to be better imaging modalities.
Take home points:
-Osteomyelitis/septic arthritis needs a high degree of suspicion for diagnosis given paucity of other symptoms such as fever.
-WBC can be normal, ESR and CRP together are more sensitive.
-Patients can have no other symptoms besides from joint pain.
-Preferred imaging is MRI with contrast.
1. Montgomery NI, Rosenfeld S. Pediatric Osteoarticular Infection Update. J. Pediatr Orthop. 2014.
2.Unkila-Kallio L, Kallio MJ, Eskola J, Peltola H. Serum C-reactive protein, erythrocyte sedimentation rate, and white blood cell count in acute hematogenous osteomyelitis of children. Pediatrics. 1994;93(1):59
3.Wong M, Isaacs D, Howman-Giles R, Uren R. Clinical and diagnostic features of osteomyelitis occurring in the first three months of life. Pediatr Infect Dis J. 1995;14(12):1047-53
Submitted by Steven Hung (@DocHungER), PGY-2
Faculty reviewed by Joan Noelker