In the presents study, morphological features of bronchiectasis were evaluated using HRCT images, comparing the patients with NTM and those with IPF. In the NTM patients, the inner diameter and outer diameter fluctuated greatly depending on the generation of the bronchus, while in the IPF patients, the inner diameter and outer diameter decreased linearly toward the distal side. In contrast, in the NTM patients, WA% and T/D ratio increased linearly toward the distal side, whereas, in the IPF patients, WA% and T/D ratio fluctuated between generations of the bronchi. In summary, the changes seen in dilated bronchi differed between NTM and IPF, with NTM having a larger lumen and thicker wall possibly due to edematous changes of the bronchial wall, and IPF having a larger lumen and thinner airway wall due to traction along with the fibrosis of the surrounding alveoli. (Fig. 2). To the best of our knowledge, this is the first report that evaluated morphological features of bronchiectasis using HRCT images.
Lung disease, which accounts for 80–90% of NTM, is a chronic progressive disease, causing bronchiectasis . Its characteristic pathologic findings include extensive granulomas affecting the airways. Peribronchial granulomas of the trachea, bronchi, and bronchioles can cause airway narrowing, and at the same time, granulomas can disrupt the muscular layer of the airway and cause bronchiectasis [13, 14]. The increased thickness of the tracheal wall seen in NTM may also reflect ulceration of the bronchial wall and edematous changes due to infiltration of inflammatory cells into the periphery of the bronchi.
On the other hand, traction bronchiectasis is a characteristic finding in fibrotic lung diseases such as IPF. Pathologically, it is a secondary dilatation of the bronchi and bronchioles as a result of contraction of the lung tissue around the airways due to inflammation, fibrosis and scarring . The presence of traction bronchiectasis in IPF has been reported to correlate with the abundance of fibroblastic foci , which is also a predictor of poor prognosis . In addition, in the UIP pattern seen in IPF, fibrosis has been found to begin in the alveolar structures and continue from the peripheral lobular margins into the more proximal airways .
In this study, we observed that the IPF patients had dilated bronchi with stretched and thinned walls, which were consistent with the pathological findings. However, there was no significant difference in the coefficient of variation between the distal and proximal sides, probably because we measured bronchiectasis that had already been completed after a long period of time. If the measurements had been taken at a relatively early stage of the disease, there might have been a difference between distal and proximal bronchiectatic findings.
In the present study, we included patients with NTM as a representative of chronic lower respiratory tract infections. However, NTM is caused by intracellular organisms and pathologically characterized by granulomas, which is different from most bacterial infection . It remains unclear whether the results of this study can be applicable to chronic lower respiratory tract infections caused by other pathogens, such as Haemophilus influenzae and Pseudomonas aeruginosa.
There have been a few reports about the distribution of bronchiectasis, and the spatial heterogeneity of the disease makes it difficult to reach a consensus. Reid described that, in cylindrical bronchiectasis, the average numbers of bronchial subdivisions were 7.5 on bronchogram but 16 in histopathology . Recently, Ikezoe and colleagues reported that thickening of the airway wall and dilation of the lumen were observed in the 7th to 17th generation in IPF patients . They also noted that the airway lumen was dilated due to the presence of markedly non-uniform strain along the bronchioles. This may be comparable with the coefficient of variation used as a measure of variability in our study.
There are several limitations to our study. Firstly, the study was conducted at only one institution and the total number of patients was small. The existence and effect of chance errors cannot be denied. Secondly, the automatic extraction of the airway by the image workstations is based on the CT values, which makes it difficult to distinguish between anatomical airway structures and transient appearance of secretions or other materials when their CT values are similar. Thirdly, we evaluated the HRCT data taken at a single time point in the patients with established bronchiectasis. Due to the lack of longitudinal data, it remains unclear whether the results of the present study can be applicable to the airways with developing bronchiectasis.