Flexible bronchoscopy is considered the gold standard for the evaluation of the presence and severity of TBM. Rigid bronchoscopy tends to underestimate the degree of the tracheal collapse because it “stents” the airway51,86–88.
There was moderate agreement regarding the use of standard chest CT because it may not capture the dynamic changes of the tracheobronchial lumen that occur during the respiratory cycle (especially with cough). Inspiratory and expiratory dynamic multi-detector chest CT scans were considered superior because they are fast, relatively inexpensive, and provide additional and detailed information about the airways, the lung parenchyma, and the thoracic vasculature51,87,89,90. However, at the moment they may not be available in every hospital.
Maximal expiratory flow-volume curves (MEFVCs) can be often diagnostic of tracheomalacia91, showing a characteristic flattening of the initial portion of the MEFVC (Figure 3A). Its disadvantage is that it can only be performed in children over 4-5 years of age, and it becomes less sensitive in older children and adolescents44,92. Infants can be evaluated with the raised-volume rapid thoracoabdominal compression (RVRTC) technique, which allows the performance of MEFVCs without the patient’s cooperation but is labor intensive and usually requires sedation93. An alternative easy, non-invasive technique to evaluate flow-limitation is with tidal flow volume loops94 (Figure 3B). However, it is not well standardized, and it may miss mild/moderate TBM.