Chest and lung ultrasound in childhood: applications, role, value and limitations

Chest and, in particular, lung ultrasound (LUS) has been an established tool in children for decades. Sonographic assessment of the heart (echocardiography) was actually one of the first applications of ultrasound (US). Other chest US uses that have been recognized for a long time and became part of many standard US textbooks include an assessment of the thymus and mediastinum, pleural effusions, consolidations, diaphragmatic movements and sometimes tumours in adults and children^(1–9). With time, other applications have been gradually introduced, such as chest wall assessment, including the ribs (especially the cartilaginous portion e.g. for rib anomalies), sternum (e.g. fractures), the musculoskeletal system (e.g. agenesis of the pectoralis muscle) and US of thoracic soft tissue masses, such as lymph nodes, vascular malformations or the breasts^(5,9,10). Chest US can be also used to assess the thoracic vessels for catheter-related thrombosis or embolism. There are multiple accessible sonographic windows for all these applications. In neonates, the non-ossified parts of the thoracic cage allow for further sonographic windows.

Over the last decade, pulmonary assessment has become the passionate focus of new sonographic applications, initially through the adult sub-specialties of intensive care and emergency medicine. Various LUS artifacts, previously dismissed for the most part by radiologists, have become valuable in the ability to draw conclusions about underlying lung conditions. Numerous phenomena have been described and various new terms and signs have been defined, such as A and B lines, seashore sign, sliding sign, bat sign, comet-tail artifact and lung point etc.^{(4,7,11–13)}

The drive by our clinical colleagues was likely through increased availability of portable US machines and the ability to rapidly scan at the bedside, so-called point of care ultrasound (POCUS). Conventional imaging techniques were not always accessible or readily available, and LUS became an alternative option where the confidence in the diagnostic accuracy of the standard chest radiograph for certain conditions, such as childhood pneumonia, is lower than desired^(14,15). Numerous recent studies show the applicability and findings of LUS in neonatal respiratory distress syndrome, transitory tachypnoea of the newborn (TTN, former called “wet lung”) and other lung diseases^(16–25). Furthermore, LUS has been shown useful for finding small consolidations in the lung periphery, depicting small effusions, and even diagnosing pneumothorax^{(2,3,5,9,26,27)}.

However, with all these advances there is one intrinsic problem. Most of these recent studies looked at a pre-selected patient group, trying to answer a specific clinical question, with intrinsic bias through patient selection. Only very limited systemic, unbiased blinded and randomized studies are available compared to the gold standard, i.e. CT of the lung. A recent meta-analysis of LUS in neonatal RDS also confirms this bias, demonstrating that whatever US may show with a very high sensitivity if performed properly covering the entire chest, the specificity remains low when taking all possible lung conditions into account^(28–30). Selection bias and study design (non-random or non-sequential participants) may lead to an over-estimation of diagnostic accuracy. In general, tachypnoea in a premature or a full-term newborn remains a diagnostic challenge for our clinical colleagues, at times maintaining a differential diagnosis rather than being specific, thus it is understandable that new imaging options allowing for a rapid bedside differentiation are pushed – sometimes overestimating their reliability, but still helpful in the individual situation with respect to the clinical symptoms & information.

Examples and causes for potential misreading are as follows:

All these aspects have to be considered when performing and interpreting a chest US or LUS. It is difficult to state definitively that LUS can replace all chest radiographs or CTs. LUS can potentially reduce the number of chest radiographs, both as a follow-up or as a first-line investigation supporting the initial clinical suspicion, helping to evaluate management before and during treatment, or supplementing other imaging techniques. POCUS has arguably even greater potential in remote areas or developing countries. Nevertheless, before its potential can be completely judged, superior clinical research needs to be performed, with better-designed, unbiased, randomized studies, greater statistical power, clearly outlined definitions of conditions and clinical questions to be answered, and looking at efficacy and treatment/ prognosis implications. It might be that a redefinition of some conditions may be needed in the future, e.g. “consolidation” and “atelectasis”, due to the latest LUS findings. The natural history of respiratory tract infections may also need to be reconsidered, in that some small pneumonic consolidation may be far more common than known previously; it could represent almost a normal or incidental manifestation of a lower respiratory tract infection, and thus does not need to be considered as serious as it traditionally has been. It could be also argued whether we do need to look for it as often as we do.

In summary, LUS has its important role, but should be applied with caution considering the restrictions and limitations. LUS must be performed skillfully and adequately, then helping to minimize radiation burden by diagnosing many conditions and reducing the need for plain films – though sometimes differential diagnosis will be difficult and bias from clinical expectation may occur.