Acute osteomyelitis is usually unifocal and prefers the metaphysis of the long bones (femur, tibia and humerus, in this order). Rib osteomyelitis is very infrequently reported. Typically, it has an incidence of approximately eight cases per 100,000 children per year, with a higher prevalence in those under 5 years old. However, in the recent years, an increase in the number of incidences has been observed (6): the number of cases of acute osteomyelitis has risen by 2.8 times in the last 20 years (7). Most infections are produced by the hematogenous route, with S. aureus being the most common causative microorganism in all ages (5). In the diagnosis of acute osteomyelitis, imaging tests play an important role. Magnetic resonance (MR) imaging is generally the best test in these cases, due to its high sensitivity and specificity. However, this technique has some drawbacks such as less availability, having to transfer the children to the Radiology Department and the requirement of sedation during the procedure. For all these reasons, it is usually restricted to cases of torpid evolution. Other relevant imaging tests are bedside ultrasound, plain radiography, CT and bone scintigraphy (4). CT was used in our case as it has good specificity and adequate sensitivity (69,7% sensitivity, 95% CI 40,1% to 88,7%; 90,2% specificity (57,6% to 98,4%) (5).
The use of ultrasound has gained increasing prominence due to its usefulness in diagnosis and monitoring given the ease of use at the patient's bedside. The most frequent early sonographic findings are intra-articular fluid collections and/or subperiosteal abscess formation that precedes radiographic changes by several days. It can detect superficial cortical erosion and even intramedullary. Point of care ultrasound (POCUS) is a very helpful tool for establishing the correct diagnosis in osteomyelitis and for reducing the frequency of additional imaging studies. In a pediatric study, Riebel et al (8) found that typical ultrasound patterns of osteomyelitis precede to radiographic images by several days in most cases. In the first 48–72 hours after infection, ultrasound can detect soft tissue inflammation followed by periosteal elevation due to fluid accumulation, subsequent subperiosteal collection and, finally, erosion of the cortical bone that appears between two and four weeks of evolution. If necessary, it can be helpful in guiding the puncture of a bone or abscess. With Doppler ultrasound, an increase in vascular flow can be found in the early phases, a common finding in the diagnosis of osteomyelitis (8). Therefore, ultrasound is a technique of considerable significance and use in osteomyelitis. Tayal et al (9) found that the use of ultrasound in this instance changes the management of patients in approximately half of the cases.
In our case, the performance of daily pulmonary ultrasounds to monitor fluid management and assess fluid overload, led us to the incidental finding of costal osteomyelitis of probable hematogenous cause. As Marin et al (10) shows, there are many lesions that are not clinically evident, and in these cases, ultrasound may improve the accuracy of the clinical examination.
Our ultrasound finding in the absence of specific clinical data led us to a change in the duration of the antibiotic treatment already established, and therefore a likely improvement in the patient's prognosis (11).