Introduction

It is important to acknowledge that osteomyelitis is a serious disease with a mortality rate of up to 5% of affected cases [1]. This pathology is however difficult to diagnose due to the absence of specific signs and symptoms. Early diagnosis is essential to avoid complications, to properly direct antibiotic treatment and to reduce the need for surgical intervention [2]. Medical imaging tests, such as radiography, ultrasound, magnetic resonance imaging (MRI), single-photon emission computed tomography (SPECT) and positron emission tomography (PET), are often used to diagnose osteomyelitis. In a systematic review of imaging tests for the detection of osteomyelitis, it revealed in adults that MRI, SPECT and PET had similar diagnostic performance, whilst CT, radiography (Rx) and scintigraphy all had a generally inferior diagnostic accuracy. In children, there was not enough evidence to reach a clear conclusion [3].

In the case of a critically ill infant or child, the main challenge is achieving an accurate diagnosis of osteomyelitis at the bedside, without having to transfer the patient to the radiology department. It is accepted that sonography and MRI imaging have the best sensitivity for detection of acute osteomyelitis and septic arthritis in children [4]. With ultrasound, the first sign that presents, even before the periosteal reaction, is swelling and oedema of the deep soft tissues surrounding the bone, as well as thickening and increased echogenicity of subcutaneous fat, and loss of differentiation between fat and dermis. Ultrasound could be used to monitor the evolution of the disease and to detect collections [5].

Clinical Case

We present the case of a 2-month-old infant with rib osteomyelitis detected by ultrasound, in the context of sepsis after abdominal surgery. One month prior to this finding, his parents presented at the emergency department for significant irritability and abdominal distension. Upon admission, a poor general condition was observed with hemodynamic instability and severe lactic acidosis. In the abdominal ultrasound, intestinal loop distention and abundant free fluid were observed. Laparotomy was performed, finding intestinal malrotation with volvulus. One month after the surgical correction, the patient began to have a fever and irritability, so it was necessary to proceed with empirical antibiotic therapy. Methicillin-sensitive Staphylococcus aureus (MSSA) was isolated in several hemocultures and the central venous catheter tip. Catheter-related bacteremia was diagnosed, and antibiotic therapy was indicated according to the antibiogram. Coinciding with bacteremia, the patient developed pulmonary oedema and respiratory failure, due to fluid overload. In the pulmonary ultrasound performed at the bedside, B lines were observed, compatible with interstitial oedema due to increased pulmonary fluid. Diuretics and high-flow oxygen therapy were added to the therapy. In the following days, a progressive improvement was observed in the ultrasound findings as well as in the respiratory failure. However, in one of the ultrasounds performed, a 3 × 3 cm round extraparenchymal collection with a heterogeneous appearance in the right costophrenic sinus was detected (Fig. 1, 2, 3, video 1, video 2, video 3). No alteration was observed in skin or subcutaneous tissue or in the contralateral hemithorax (video 4). The chest radiograph confirmed the image of lung consolidation accompanied by a lytic lesion at the costochondral junction of the right seventh rib, with an associated periosteal reaction (Fig. 4). Chest CT confirmed the presence of a hypodense collection at the costochondral junction of the right seventh rib with pleural thickening and an increase in soft tissues, compatible with osteomyelitis (Fig. 5). As a result, antibiotic therapy was maintained, and echocardiography was performed to rule out endocarditis lesions. The follow-up thoracic ultrasounds showed a progressive decrease in the heterogeneous extraparenchymal collection in the right costophrenic sinus (Fig. 6, Video 5), which subsequently disappeared.

Fig. 1
figure 1

Ultrasound image of the lesion adjacent to the right sixth rib. Note the imprint it produces on the pleural line (star). Arrow: ultrasound image of the lung with B lines

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Fig. 2
figure 2

Ultrasound image of the lesion located in between the diaphragm (star) with the liver below it (arrow) and the pleura (cross). Note the diaphragm flattening due to the lesion

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Fig. 3
figure 3

Lesion measurements by ultrasound

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Fig. 4
figure 4

Chest radiograph showing radiographic image of lytic lesion in the right seventh rib and right basal round consolidation lesion

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Fig. 5
figure 5

Abdomen CT showing paracostal osteolytic lesion (arrow)

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Fig. 6
figure 6

Thoracic ultrasound a week after ultrasound diagnosis with reduction of size

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Discussion

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 haematogenous route, with Staphylococcus aureus being the most common causative microorganism in all ages [5]. Severity of the course of osteomyelitis in children has been reported to be rising, possibly due to more frequent presence of more aggressive MRSA and PVL gene (Panton-Valentine leukocidin), where prompt and accurately targeted therapy is vital for favourable outcome [8].

In the diagnosis of acute osteomyelitis, imaging tests play an important role. A study comparing different diagnostic methods showed that magnetic resonance and bone scintigraphy are most valuable methods at the onset of the illness [9].

Conventional radiographs are diagnostic in less than 20% of acute osteomyelitis in the early stages of the disease; at the same time, they are useful in distinguishing conditions as trauma or potentially tumour. The sensitivity of conventional X-ray increases in late diagnosis. Sensitivity and specificity of ultrasound modality have been stated around 50% [9].

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. [10] found that typical ultrasound patterns of osteomyelitis precede to radiographic images by several days in most cases. In the first 48–72 h 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 2 and 4 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 [10]. Therefore, ultrasound is a technique of considerable significance and use in osteomyelitis. Tayal et al. [11] found that the use of ultrasound in this instance changes the management of patients in approximately half of the cases.

Ultrasound seems to be a safer, advantageous, quick and easy to use method compared to an MRI despite its relatively low specificity and sensitivity. Sensitivity and specificity of the ultrasound might be potentially modifiable factor by operator experience. Since the ultrasound imaging has become frequently used method in the intensive care units or emergency department settings, operators’ experience has been broadening. The advantages of ultrasound have been broadly recognised by medical society. The ultrasound technique is easy to use, an investigation is possible to repeat at any time and as many times as patient tolerates, allows comparison of both sides, does not introduce a burden for a patient (e.g. radiation or pain) and is very cost-effective.

As we show in this case, ultrasound can be useful for an early detection of osteomyelitis in children although an MRI will still be required for a more thorough assessment. Ultrasound is able to detect very early in course of the illness:

  • Soft tissue swelling seen as areas of fluid collection and hypervascularity around the affected bone on colour Doppler.

  • Periostal thickening and subperiostal collection seen as periostal elevation with an underlying fluid collection.

  • The collection might be assessed using dynamic nature of ultrasound and allow for needle aspiration if required.

  • Scanning the cortical bone surface and periosteum can be helpful in distinguishing between infection, tumour and trauma [12].

In conclusion, there is no doubt that ultrasound detects signs of acute osteomyelitis in children earlier than standard radiography, hence is very useful as a screening method. Being rapid, cheap, easily available and accurate diagnostic modality, it is very useful guide allowing to plan more thorough investigations and target the therapy with minimal burden to a patient. The article contributes to the broadening evidence that ultrasound is becoming the method of choice for assessment of patients, especially in intensive care or emergency medicine settings bringing faster and more accurate diagnostic, hopping to improve patients experience and outcome.

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 haematogenous cause. As Marin et al. [13] show, 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 [14].