We found that LUS and CXR were comparable regarding the evaluation of pleural effusion in patients with CF pulmonary exacerbation. On the other hand, LUS was superior to CXR for the detection of air bronchogram, consolidation, and pleural thickening. CXR was only superior to LUS when the diagnosis of atelectasis was concerned. Furthermore, LUS did not yield an acceptable diagnostic performance for the detection of subpleural opacity/septal thickening.
Progressive lung disease limits the survival of CF patients. Having a proper attitude towards the severity of CF lung disease and monitoring it over time is important in directing clinical care and predicting disease outcomes. Imaging provides information about the regional distribution of CF lung disease; therefore, pulmonary imaging studies are recommended for the follow-up of CF patients. CXR, HRCT, and MRI are currently the available imaging techniques [16, 17]. CXR is routinely used in CF clinics and is usually performed annually. Although CXR is valuable for the long-term monitoring of CF patients, it has limited sensitivity when it comes to early pulmonary abnormalities, such as air trapping and primary bronchiectasis [17]. HRCT can demonstrate the initial abnormalities, as well as the progression of structural changes, while delineating the highest morphologic details [18]. HRCT is the gold standard for the diagnosis of pulmonary lesions; nevertheless, its routine use in CF patients, the majority of whom are young and sensitive to ionizing radiation, and its life-long accumulation, is debatable [18]; yet, low-dose chest CT has been proposed as a solution to reduce radiation exposure in a recent study. It has been reported to yield satisfactory image quality with diagnostic capabilities equivalent to standard CT [19]. On the other hand, CXR also exposes the patients to ionizing radiation. In recent years, ultrasound has been used for the evaluation and monitoring of intensive care unit patients. LUS can be used at bed side and it is also rapid, inexpensive, and non-ionizing [20]. Moreover, LUS is easier to use in infants and children [21]. This is of significance since the percentage of pediatric CF patients is higher than adults in the Iranian population [22], unlike many other countries, such as the European countries, where adults make up the majority of CF patients [23].
Consistent with our findings, Peixoto et al. assessed the applicability of ultrasound for the evaluation of CF lung disease and showed that LUS results were comparable with HRCT [24]. Also, Ciuca et al. suggested LUS as a monitoring tool for CF lung disease and pulmonary exacerbation [25]. In another study by Ciuca et al., LUS showed typical signs of consolidation in 8.5% of CF patients [26]. LUS was able to detect consolidation in 19 (63.3%) patients of our study. The discrepancy between their study and ours may be explained by different degrees of exacerbation severity and the etiology behind this condition in the two studies. Ciuca et al. also showed acceptable correlations between CT findings and LUS regarding the detection of consolidation [26]. Our results were in line with their findings. We also showed a good diagnostic performance for the detection of consolidation by LUS (AUC of 0.900).
LUS has also been used for the diagnosis of pneumonia in previous studies. For instance, Berce et al. showed the applicability of LUS for distinguishing bacterial community-acquired pneumonia (CAP) from CAP of other etiologies [27]. Musolino et al. compared LUS findings in children with complicated CAP to those with uncomplicated CAP. In their study, subpleural parenchymal lesions, pleural effusion, and bronchogram were evaluated with LUS, which shows the efficacy of LUS for the detection of these abnormalities [28]. Moreover, Yilmaz et al. demonstrated the comparability of LUS with CXR for the diagnosis of CAP in children [29]. However, in the current study, LUS was superior to CXR for the detection of the majority of CF pulmonary exacerbation findings.
In another study, Dankoff et al., evaluated LUS findings in children with asthma who had been admitted to the emergency department due to respiratory distress. In this study, B-line, consolidation, and pleural line abnormalities were found in 38%, 30%, and 12% of asthma patients, respectively [30]. Accordingly, LUS appears to be an appropriate tool for the evaluation of lung in asthma exacerbation as well. Ho et al. also found LUS to be a sensitive tool for the diagnosis and follow-up of pneumonia in children [31]).
Our study was not without limitations. One limitation was our small sample size which did not allow us to draw ROC curves for either LUS or CXR for the detection of pleural effusion. With the low prevalence of CF, a multi-center study would have solved the problem, while adding another limitation which would be the potential interobserver variability for LUS evaluations at different centers. Another limitation is that we were not able to perform spirometry to determine the severity of the disease based on FEV1 due to the coronavirus disease 2019 pandemic and the possibility of disease transmission. Furthermore, although dornase alfa was not available for the patients and none of them received this medication, all patients underwent chest physiotherapy at home, which may have interfered with imaging findings.