Community acquired pneumonia (CAP) represents the single largest cause of paediatric morbidity and mortality worldwide. 1, 2 The diagnosis of pneumonia is essentially based on medical history and clinical examination, which has poor sensitivity and specificity 3, 4 and do not allow to identify the causative agent responsible for the infection. 5 Although viruses represent the most frequent cause of CAP, the majority of children with suspected or confirmed CAP are still treated with empirical and often unnecessary antibiotics 6 , contributing to the spread of antibiotic resistance, one of the biggest medical emergencies of modern medicine. In the Centers for Disease Control and Prevention Etiology of Pneumonia in the Community cohort, only 15% of hospitalized children with radiographic pneumonia had a detectable bacterial aetiology; however, 88% received antibiotics. 7 A recently published among Pakistani children younger than 5 years of age with pneumonia with tachypnea randomized at amoxicillin vs placebo, the number of children with pneumonia and tachypnea who would have needed to be treated with amoxicillin to prevent one treatment failure was 44, suggesting that a significant number of CAP is of viral origin and does not requires antibiotic. 8 However, current guidelines do not help the physician on how to approach to an optimised strategy for prevention, diagnosis and treatment of disease for each single child with CAP, based on his or her unique characteristics, but mainly suggest a general approach to paediatric CAP. 9 To date, both clinical findings 10 and laboratory results [10] [11] [12] [13] [14] [15] [16] failed to accurately distinguish viral, bacterial and atypical pneumonia. Even studies that report differences in laboratory biomarkers could not determine any reliable thresholds for differentiating bacterial pneumonia from viral pneumonia 13 , since normal tests do not always exclude bacterial CAP. 9 Chest x-ray (CXR) is not necessary to confirm the diagnosis of CAP in milder cases, who are treated as outpatients and is also associated with risk of radiation exposure. 17 Moreover, CXR cannot reliably establish the microbial diagnosis of CAP 18, [19] [20] [21] [22] and the interpretation of radiographic images varies significantly among the observers. 23 All rights reserved. No reuse allowed without permission.
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The copyright holder for this preprint this version posted November 4, 2020. ; https://doi.org/10.1101/2020. 10.31.20223867 doi: medRxiv preprint In recent years, lung ultrasound (LUS) use has been widely studied as an alternative diagnostic tool for CAP of both bacterial and viral origin, proving to have high specificity and sensitivity for the diagnosis and as regards the follow-up in children with pneumonia. 21, [24] [25] [26] Moreover, LUS has several advantages over X-ray, particularly useful for the paediatric population: radiation-free, lower cost, possibility of follow-up examinations, ability to monitor treatment, easy accessibility in all settings (including poor countries), fast, and can be used immediately as a point-of-care method.
LUS results are immediately available to the clinician, who must decide about the initial empirical treatment. 19, [21] [22] [23] [24] [25] [26] The new challenge of LUS is to determine the aetiology of CAP. First studies primarily aimed to assess the role of LUS in CAP suggested that smaller consolidations may be associated with a viral aetiology 27 , but only one study prospectively assessed the role of LUS in defining the aetiology of CAP. 19 However, although this study found that small subpleural consolidations and/or an increased number of B-lines (interstitial syndrome) are characteristics of viral pneumonia, the authors did not assessed important LUS features (such as the bronchograms, type and location of vertical artifacts, type of effusions).
Therefore, due to the growing role of LUS in the evaluation of patients with respiratory conditions and the limits of available data, we carried out this prospective study aiming to assess the role of LUS in supporting the etiological diagnosis of CAP in children.
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We prospectively analysed patients aged from 1 month to 17 years admitted between March 2018 and April 2020 who were hospitalized for CAP and had pneumonic infiltrates detected with LUS.
For all patients included in the study, history, clinical parameters, microbiological data, and ultrasound data were collected. The study was approved by the Ethics Committee of Fondazione Policlinic Universitario A. Gemelli IRCCS, Rome, Italy. Informed consent was obtained by all participants and/or their legal guardians (for children younger than 16 years). There is no identifying information or image in the article.
The evaluating physician made the clinical diagnosis of CAP in accordance with the British Thoracic Society guidelines. 18 
We defined as CAP those patients requiring: acute respiratory signs and symptoms, fever > 37.5°C, clinical or radiological evidence of a new pulmonary infiltrates.
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Patients with underlying diseases, including respiratory tract anomalies, immunodeficiency, cerebral palsy, neuromuscular diseases, congenital heart disease, and malignancy were excluded, as well as children who did not undergo LUS at 48 hours after treatment. Patients with detected viral infection on nasopharyngeal were included into the viral CAP group only after the exclusion of bacterial superinfection, according to a comprehensive assessment of clinical, laboratory, radiology and microbiological finding. Children were included into the atypical CAP group when Mycoplasma pneumoniae or Chlamydia pneumoniae were detected on All rights reserved. No reuse allowed without permission.
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The copyright holder for this preprint this version posted November 4, 2020. ; https://doi.org/10.1101/2020.10.31.20223867 doi: medRxiv preprint nasopharyngeal swabs and bacterial superinfection was excluded, and clinical data were compatible with a diagnosis of atypical pneumonia.
Nasopharyngeal swabs were obtained and tested fresh from all patients. MicrobScan, a qualitative diagnostic test developed by Nurex S.r.l. (Sassari, Italy) was used for the detection and In addition, bacterial culture of blood and respiratory samples were performed, when indicated.
LUS was performed with the ultrasound machine Esaote Mylab 40. It was performed within 6 hours from the clinical diagnosis of CAP, unaware of microbiological, laboratory and CXR (when performed) results. Linear probe (12 -6 MHz) was used in preschool children. In older children, we used a curved probe .
Images and clips were stored and archived. All LUS was made by the same physician that made the first one to reduce inter-operator differences.
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The copyright holder for this preprint this version posted November 4, 2020. We compared the ultrasound results with those of the nasopharyngeal swab to verify the concordance of the etiological diagnosis, according to the three main groups "bacterial", "viral", "atypical".
We considered standard of care (SOC) the first line treatments (for either outpatient or inpatient) as per international guidelines 9 (amoxicillin, amoxicillin/clavulanate, ceftriaxone, cefotaxime, ampicillin, and penicillin).
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In order to determine the power of the study, the study was set up as a study of agreement between lung ultrasound and the aetiology of CAP. For atypical pneumonia there is a 60% agreement (then a 40% disagreement); for bacterial pneumonia, the agreement rises to 90% (disagreement 10%). To be conservative we considered the highest percentage of disagreement that is 40%. In this case (which was not certified by peer review) is the author/funder, who has granted medRxiv a license to display the preprint in perpetuity.
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A total of 186 children with suspected CAP (103 male and 83 female) with an average age of six were enrolled in the study (Figure 2 ). The prevalent etiology of CAP was viral, with a slight prevalence in male (55% of the cases). Main epidemiological and clinical characteristics of the study population, assessed according to the aetiological group, are described in detail in Table 1 . As reported, the majority of clinical parameters were not able to discriminate the different aetiological groups.
Main diagnostic and laboratory investigations, assessed according to the aetiological group, are described in detail in Table 2 . Neither laboratory nor CXR findings were able to significantly discriminate the etiological groups since, also in case of statistical significance, the overlap between etiological groups was evident.
Details about LUS features in children with bacterial, viral and atypical pneumonia are described in table 3. Consolidations were found in 100% of bacterial infections, with a significant statistical difference compared to viral infections (P value= 0.001) and to atypical infections (P value=0.004).
We analyzed the size of consolidation according to the etiological group:
-for consolidation <1.5 cm, there is a statistical significance between bacterial and viral infections (P value= <0.001) and between bacterial and atypical infections (P value= 0.004). They were more expressed in viral pneumonia (63.8%); -for consolidation between 1.5 and 4 cm, there is a statistical significance between bacterial and viral infections (P 0.022), and they were more expressed in bacterial pneumonia (55.22%); -for consolidation >4cm, there is a statistical significance between bacterial and viral infections (P value <0.001) and between bacterial and atypical infections (P value <0.001). They were more expressed in bacterial infections (22.39%) and poorly expressed in viral infections (1.54%).
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The copyright holder for this preprint this version posted November 4, 2020. ; https://doi.org/10.1101/2020. 10.31.20223867 doi: medRxiv preprint Multiple and bilateral consolidation are infrequent in bacterial infections, making them statistically significant to distinguish bacterial pneumonia from viral and atypical diseases (P < 0.0001).
We also considered air bronchogram, that was highly represented in bacterial (77.61%) and atypical -the presence of fluid bronchogram was more frequently described in case of bacterial pneumonia compared with viral (P <0.001) and atypical pneumonia (P 0.003).
Pleural effusions were significantly more frequent in children with bacterial CAP (P < 0.0001).
We also assessed vertical deep artefacts. They were frequently found in the viral (88.16%) and atypical (88.37%) infections, and moderately found in the bacterial ones (58.21%). Children with viral and atypical pneumonia had significantly more vertical deep artefacts compared with bacterial pneumonia (P <0.001 and P 0.001, respectively), while there were no significant differences between viral and atypical pneumonia (P > 0.05). Also the distribution of the vertical deep artefacts in the three groups, being significantly more diffuse in children with viral (78.57%) and atypical All rights reserved. No reuse allowed without permission.
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The copyright holder for this preprint this version posted November 4, 2020. ; https://doi.org/10.1101/2020.10.31.20223867 doi: medRxiv preprint (70.27%) CAP (P < 0.05), while children with bacterial CAP mainly had vertical artefacts located in continuity with the main consolidation. Tables 4, 5 and 6 shows the multivariable logistic regression analyses applied to compared the three main groups.
We made interobserver agreement on a random set of 10% images. The levels of interobserver agreement were good (values of coefficient between 0.6 and 0.8) for air bronchogram position (deep, superficial) and motion (static, dynamic), while was above 0.8 (confirming a high interobserver agreement) for consolidation size, effusions, presence and distribution of vertical artifacts.
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Our prospective study is one of the largest prospective study assessing the role of detailed LUS features to discriminate bacterial, viral and atypical CAP in children. We found that the size, number and distribution of consolidations, the position and motion of air bronchograms, pleural effusions and distribution of vertical artifacts significantly differ in children with bacterial, viral and atypical CAP. Such differences were particularly relevant when bacterial CAPs were compared with viral and atypical ones. Conversely, clinical parameters, including fever, chest pain and main auscultation features, and laboratory were not able to significantly distinguish between these groups. CXR, despite being still widely used, was the less useful tool in this discrimination. These findings highlight, therefore, the need of new tools to support the etiological classification of pediatric CAP and we proved that several LUS features may easily support clinicians in this.
Given the increasing phenomenon of antibiotic resistance in children, the possibility of knowing the etiology of the infection represents an important step forward. The first decade of LUS studies focused on the role of LUS in detecting pneumonia. A recently performed meta-analysis confirmed high sensitivity (96%) and specificity (93%) of LUS for detecting pneumonia in children. 4 First studies showed specific LUS patterns to diagnose viral lower respiratory tract infections and bronchiolitis in children. [28] [29] [30] [31] . Buonsenso and colleagues 15, 24 showed that specific LUS patterns on diagnosis and after 48 hours of treatments (bronchograms, consolidation size, characteristics of pleural effusion) were predictive of antibiotic response in children with CAP, more than clinical data and laboratory results. Moreover, recent basic science and clinical studies are clarifying the knowledge of and genesis of artifacts generated by the ultrasound-lung interaction. In particular, performing physical analyses on artificial models and using modern deep learning strategies and train a fully convolutional neural network, Demi and colleagues 32, 33, 34 showed that B-lines have different morphologies according to the medical conditions that generate them (interstitial lung diseases, cardiogenic and non-cardiogenic lung edema, interstitial pneumonia and lung contusion). All rights reserved. No reuse allowed without permission.
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The copyright holder for this preprint this version posted November 4, 2020. ; https://doi.org/10.1101/2020. 10.31.20223867 doi: medRxiv preprint These data, all together suggested that a more detailed analyses of LUS features ("ultrasonomic") may represent a new and promising tool to personalize the diagnosis, treatment, follow-up and care of children with CAP. In this context, recognizing bacterial pneumonia from a viral or atypical using LUS at patient's bed-side would allow us to offer a more focused approach and treatment to each child, responding to a modern medical concept that each patient is a unique one and requires a personalized approach. To date, few studies have described LUS findings able to define the etiology of pediatric. In particular, Berce et al 19 pneumonia. 21, [28] [29] [30] 35 These findings were comparable with our study. However, our study has evidenced more important LUS features not previously assessed. We found that air bronchograms were more common in bacteria and atypical CAP but, importantly, fluid bronchograms were almost exclusively described in bacterial cases. Also, complicated pleural effusions were never described in viral CAPs. Vertical artefacts, which gained more interest during the last year and in particular since LUS has been routinely used in COVID-19 pneumonia 21,35-38 , also played a significant role, since in bacterial CAP were mainly located in proximity of the main consolidation, while in the others were mostly diffuse and bilateral.
Considering the well-known advantages of LUS (a low-cost, easily reproducible, non-invasive tool that does not cause pain and damage induced by radiations), all the ultrasound findings obtained by our study can have an important impact on the daily pediatric clinical practice. LUS, particularly when used in adjunction to clinical examination by experienced operators, can support the etiological of pneumonia and offer a personalized approach to patients. If further confirmed, this approach can also support antibiotic stewardship programs. All rights reserved. No reuse allowed without permission.
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The copyright holder for this preprint this version posted November 4, 2020. ; https://doi.org/10.1101/2020.10.31.20223867 doi: medRxiv preprint Importantly, the detailed analyses offered by our study represent a modern view of LUS, as recently highlighted by international experts 39 : during the last decades, LUS moved from being a diagnostic tool (qualitative approach) to a monitoring tool for lung aeration quantification (i.e.,quantitative approach). In this new vision, a more attention to several LUS features (vertical artifacts, consolidations, echogenicity, bronchograms, and dynamic data) need to be taken in consideration.
Artificial intelligence softwares, since we are entering the era of the digital medicine (a process particularly speed-up by the COVID-19 pandemic), will also support this view. 40 Our study has limitations to address. The gold standard for the diagnosis of CAP is the chest CT scan, however its routine use in children is not ethical in children so we did not do the CT.
Therefore, the diagnosis of CAP for the inclusion of patients in our study was made on a clinical bases according to the available guidelines. The stratification of patients according to the etiology cannot be 100% accurate, since the definitive microbiological diagnosis in non-mechanically ventilated children is difficult to establish, since bronchoalveolar lavages are invasive procedures.
However, to date all studies assessing the microbiological diagnoses of CAP, including vaccineprobe study 41 , have the same limitation that is difficult to address. Nevertheless, we used a comprehensive assessment of each child including clinical, laboratory, imaging and microbiological data, including last generation molecular assays. On this regard, a limit is that we did not use quantitative assessments of nasopharyngeal PCR results that, according to recent studies in adult patients, have been positively associated with a diagnosis of CAP other than simple colonization. 42
In conclusion, our study provides a detailed analysis of LUS features able to predict the etiology CAP in children. These findings may help the physicians to better manage a child with CAP and to offer personalized approach, from diagnosis to treatment and follow-up. However, further studies on pediatric CAP are necessary to confirm our findings. All rights reserved. No reuse allowed without permission.
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The copyright holder for this preprint this version posted November 4, 2020. ; https://doi.org/10.1101/2020.10.31.20223867 doi: medRxiv preprint