Since the first reports on COVID-19 pneumonia, emphasis has been given to the role and impact of thoracic imaging for accurate assessment of lung compromise and timely detection of complications. Chest CT has gained an important role in this setting, because chest radiograph is of limited value for COVID-19 diagnosis, with a reported sensitivity of only 69% [95% CI 56–80%] . Lung US has an already established accuracy and reliability in diagnosing many lung pathologies, but up to now, only few studies have been published on its applicability in COVID-19. For pneumonia and ARDS, it is an excellent method of diagnosis and monitoring and has been found to detect lesions not seen on the chest radiograph, especially those localized in the retro-cardiac or juxta-diaphragmatic region [21,22,23].
Preliminary lung US studies on adult patients with COVID-19 identified numerous B-lines and subpleural pulmonary consolidations in an asymmetric multilobar distribution, involving mainly the lower lobes. What seems to be characteristic of COVID-19 in a positive epidemiological context is the bilateral patchy distribution of multiform clusters alternating with spared areas. Those findings were highly consistent with the findings on CT [24,25,26,27,28,29]. The first few case reports and case series published in children with COVID-19 showed lung US findings similar to what was described in adults [30,31,32].
Our findings are in accordance with previous small pediatric reports and adult studies and we also found an apparently good consistency between lung US and CT in terms of findings and topography. Confluent B-lines were later seen as consolidations on a chest CT performed the following day only in two specific zones. We cannot say whether it was a misdiagnosis or an early sonographic identification of organizing pneumonia. This could be clarified only by a study aiming to compare lung US and CT when both are performed simultaneously. Even though some comparisons were made between CT and lung US, this study was not designed to pair up these imaging modalities due to ethical concerns about unnecessarily exposing children to ionizing radiation.
As far as we know, our study was the first in pediatric COVID-19 population to analyze lung US aeration scores. Most of our patients that classified as moderate and severe/critical, had major abnormalities on lung US, and consequently higher lung US aeration scores. Due to the small sample size, we do not have statistical power to confirm the lung US aeration score as a disease severity predictor, but this preliminary result is an important finding that suggests that this score might be an additional tool to help clinicians in risk stratification and resource allocation. Two patients with moderate disease and normal US had an obstructive airway disease reversed with corticosteroids and bronchodilators.
Despite the apparent good association with disease severity, five of our children had significant lung US abnormalities, elevated scores, but few or no respiratory symptoms. All of them were under 6 months of age and four also had elevated D-dimers, which is believed to be a severity marker of disease in adult patients . One of them, a 2 week-old newborn with fever, also had a chest CT that confirmed lung involvement. The reason why these young infants with confirmed lung involvement on imaging and elevated D-dimer have such a mild disease is yet to be clarified as many other clinical aspects of COVID-19 in children.
There are several reasons why we believe lung US may be a promising tool in COVID-19, especially in the pediatric population. First, although other viruses such as the respiratory syncytial virus and parainfluenza virus cause pneumonia lesions that are mostly distributed along the bronchial tree, studies addressing CT findings in children with COVID-19 showed that the periphery of the lung in the subpleural region is the most commonly affected area [34, 35]. Of 43 patients with CT abnormalities due to COVID-19 reported by Ma et al. , 95% had a predominance of lesions in the subpleural area and in the lower lung lobes (65%), especially in the posterior segment (78%). Given that the subpleural area seems to be the target for COVID-19, lung US may assume a key role in the early detection of these lung involvements as it easily identifies infections extending to the visceral pleura . Second, chest CT should be followed by complex decontamination procedures and requires transporting sometimes critically ill patients to the radiology suite, while lung US can be performed at the bedside and given its smaller size, would be easier to decontaminate . Of note, two of our critically ill patients were too unstable to leave the PICU and initial lung imaging assessments were made by lung US. Third, while US is radiation-free, CT scan exposes pediatric patients to harmful ionizing radiation during a time at which they are believed to be most at risk of harm .
Limitations of this imaging modality still exist, including an inability to visualize centrally located consolidation, inability to differentiate consolidation from atelectasis, and possibly some degree of overdiagnosis, as US can detect even small consolidations of unlikely significance [36,37,38]. Moreover, the aforementioned assumption from adult studies that lung US in COVID-19 may have characteristic features has not yet been extrapolated to children, as they more frequently have lower respiratory tract disease caused by a variety of viruses that may have a similar pattern. Further studies are needed to better understand this.
Our study also has some limitations. It is a descriptive study with a small number of patients included; however, given the scarcity of available data on lung US findings in COVID-19 pediatric patients, the information provided by our study is relevant and may provide a better understanding of this topic. Sonographers were not blinded to clinical information, because lung US assessment is performed on a regular basis as an extension of the physical examination in our institution. Also, even though the lung US score applied in our patients have been used in many lung pathologies, it is not specific to COVID-19 lung disease as the one proposed by Soldati et al. . Lastly, we could not reliably compare lung US with the gold standard chest CT, since not all patients had both exams or had them performed at the same time. Despite these limitations, as far as we know our study on point-of-care lung US findings in children with COVID-19 is the first to include a more extensive number of pediatric patients. We believe our report adds important information regarding the utility of lung US in pediatric population with COVID-19.