Discussion
This is the first study characterizing LUS findings of pediatric asthma patients at their baseline state of health. Our study showed that almost 20% of children diagnosed with asthma had positive LUS findings when well. The most frequent findings seen were small consolidations and localized B-lines. These findings may be present at baseline and are not to be attributed to, for example, a co-existing pneumonia or viral upper respiratory tract infection.
An important question to address is how our described findings may impact the evaluation of an asthmatic patient with LUS. The findings we have identified (mostly localized B-Lines in one intercostal space and small consolidations less than 1cm) are similar to what was described in previous studies, commonly seen in patients with mild bronchiolitis, atelectasis and viral pneumonia12,18,30. Therefore, using LUS to try and differentiate these conditions from baseline asthma findings is not possible.
The findings on LUS in our cohort of patients were objectively small and often localized to one lung zone, effecting no more than 2 rib spaces. LUS scores were also quite low (signifying mild changes) in most of the positive LUS cases. Thus, more severe LUS findings including consolidations >1cm and B-lines involving more than 2 intercostal spaces or effecting more than one lung zone, are atypical for a pediatric asthmatic patient at baseline and should be considered clinically significant, alerting the clinician to other possible lung parenchyma changes outside of what is typically seen in pediatric asthmatic patients.
Positive LUS findings in this study were more frequently observed on the right anterior and lateral lung zones.(Figure 3). These findings may correspond to the right middle lobe, where atelectasis is commonly seen in asthmatic patients31. We hypothesizese that these LUS findings are from sub-segmental mucus plugging and atelectasis with the known hypersecretory issues in asthma30,32-34
As a secondary objective, we explored the association of having severe or poorly controlled asthma with a positive LUS. The majority of our population had relatively mild asthma with a median of 96%, and only 17% had an ACQ score that is considered elevated. There was no significant association between having positive LUS and poor asthma control, high ISAAC score, or FEV1 value. However, there was a trend toward more frequent LUS abnormalities in patients already followed in the respiratory clinic for asthma. These previously known asthmatics had significantly lower spirometry FEV1% predicted and were more likely to be on maintenance therapy which included inhaled corticosteroid combined and long-acting beta agonist than those who presented to the clinic for asthma diagnosis with MCT (Table 3). Our LUS correlations with asthma control or severity are only exploratory and further research should be done explore these possible associations.
There are some limitations to this study. First, the sample size was relatively small (yet adequately powered), and our recruited asthmatics had overall mild disease. A larger and more severe asthma population may detect significant relationships between different clinical variables and likelihood of having a positive LUS. Second, sonographers attempted to perform LUS before spirometry maneuvers, but some known asthmatics had their scans completed after spirometry testing, which routinely included the administration of Ventolin. This was not the case for any of the MCT patients, as the scans were always done before the test. Both spirometry maneuvers and beta agonists may have altered findings on LUS. Three different sonographers performed the LUS evaluations, and ultrasound images can often be influenced by operator specific practices. However, the blinded interpretation by a single expert makes this operator bias less of an issue, and prior studies show good interrater reliability between sonographers and interpreters19,20. Four the ISAAC score was used in our study as a surrogate measure to asthma severity, however it is not in itself a true marker of severe asthma; the score was only used as a proxy in an attempt to explore the association of LUS findings with more severe asthma. Finally, reliance upon parental and patient questionnaires to gauge asthma severity and control may have introduced recall bias, as asthma symptoms and severity are often under-recognized by patients and parents.