General data
A total of 113 participants with BAF with a mean age of 66.2±9.3 years were included in the present study; 53 participants (46.9%) were men, the mean BMI was 21.6±3.7 kg/m2, and 42 participants (37.2%) were smokers. Thirteen participants (11.5%) had a history of TB, which may be associated with a high prevalence of TB in the region where this study was conducted, and 57 participants (50.4%) had been exposed to biofuels for a mean duration of 17.4±6.2 years. The comorbidities of the participants were as follows: diabetes mellitus (4 participants, 3.5%), ischemic heart disease (22 participants, 19.4%), chronic liver disease (one participant, 0.14%), chronic kidney disease (one participant, 0.14%), and hyperlipidemia (26 participants, 23.0%).
Bronchoscopy results
Under bronchoscopy, participants with flat BAF (Figure 1A) exhibited an unobstructed lumen with normal morphology; absence of evident mucosal hyperemia, edema, and thickening; and a sharp-appearing bronchial carina. In participants with deep-seated retracted BAF (Figure 1B), we observed evident fibrous tissue proliferation, slight distortion of the lumen, stenosis of the affected bronchi (commonly occurring as external compression-type stenosis), slight hyperemia in the mucosa, and a sharp-appearing bronchial carina. Additionally, difficulties were encountered during the biopsy procedures of these participants. Bronchoscopy of participants with black mucosal protruding BAF (Figure 1C) revealed severe distortion of the lumen; evident lumen stenosis often accompanied by bronchial obliteration; evident mucosal hyperemia, edema, and thickening; a broadened and deformed bronchial carina with occasional mucosal ulceration or necrosis; and high bleeding tendency upon contact with the bronchoscope. Significant post-biopsy bleeding was observed (four participants experienced bleeding during bronchial brushing), and bleeding control was relatively difficult. However, effective control was achieved after subsequent treatment. Among the 113 participants with BAF, 31 (27.4%) had flat BAF, 38 (33.6%) had deep-seated retracted BAF, and 44 (38.9%) had black mucosal protruding BAF.
EBUS examination results
EBUS examinations using a mini-probe revealed the following manifestations in the airways of the healthy control and BAF groups: (1) the healthy control group (Figure 2A) showed uniform aeration patterns in the peripheral lung tissue, (2) the flat BAF group (Figure 2B) showed uniform aeration patterns in the lung tissue, (3) the deep-seated retracted BAF group (Figure 2C) showed disordered lung tissue signals with scattered calcification patterns, (4) and the black mucosal protruding BAF group (Figure 2D) showed soft tissue patterns in the lung tissue. Figure 3 shows the ImageJ 18.0-magnified ultrasound images and transbronchial lung biopsy specimens of an airway with BAF.
The magnified ultrasound images of the bronchial walls of participants in various groups revealed the following manifestations: (1) The healthy control group showed a regular circular distribution of the airway wall, occasional shadows on the cartilage rings, uniform mucosa, and uniformly dense submucosal tissue; (2) the flat BAF group showed a decreased mucosal area compared with the healthy control group and uneven thickening of submucosal tissue; (3) the deep-seated retracted BAF group showed a decreased mucosal area compared with the healthy control group, significant thickening of the cartilage layer, and uneven calcification on the airway wall; (4) and the black mucosal protruding BAF group showed a decreased mucosal area, uneven thickening of submucosal tissue, and a disorderly arrangement of cartilage tissue.
Table 1 shows the airway wall indicators of the various BAF subgroups measured after magnification of the ultrasound images using ImageJ 18.0. The maximum wall thicknesses of the flat BAF, deep-seated retracted BAF, and black mucosal protruding BAF groups were 12.5±5.0 mm, 17.3±5.7 mm, and 19.3±5.4 mm, respectively, with multigroup comparisons showing statistically significant differences (F=14.946, P=0.000). Pairwise comparisons showed that the maximum wall thicknesses of the deep-seated retracted BAF and black mucosal protruding BAF groups were significantly higher compared with that of the flat BAF group (P<0.05), but the difference in maximum wall thickness between the deep-seated retracted BAF and black mucosal protruding BAF groups was not statistically significant (P=0.090). The maximum submucosal thicknesses of the flat BAF, deep-seated retracted BAF, and black mucosal protruding BAF groups were 6.6±3.5 mm, 9.8±3.0 mm, and 14.5±5.0 mm, respectively, with multigroup comparisons showing statistically significant differences (F=36.819, P=0.000). Pairwise comparisons showed that differences among the three groups were statistically significant (P<0.05). The proportions of bony tissue area in the flat BAF, deep-seated retracted BAF, and black mucosal protruding BAF groups were 33.3±9.3%, 65.2±8.7%, and 34.9±12.1%, respectively, with multigroup comparisons showing statistically significant differences (F=113.473, P=0.000). Pairwise comparisons revealed that the bony tissue area of the deep-seated retracted BAF group was significantly higher compared with those of the other two groups (P<0.05), but the bony tissue area of the black mucosal protrusion BAF group did not differ significantly from that of the flat BAF group (P=0.508). The proportions of submucosal area in the flat BAF, deep-seated retracted BAF, and black mucosal protruding BAF groups were 64.8±9.1%, 30.4±8.8%, and 58.6±11.7%, respectively, with multigroup comparisons showing statistically significant differences (F=120.031, P=0.000). Pairwise comparisons indicated that differences among the three groups were statistically significant (P<0.05).