Adult TBM frequently occurs in middle-aged and elderly patients , consistent with the results in this paper. The course of adult TBM varies widely (from several months to more than ten years). The different courses of disease may be related to different etiologies. TBM has a wide range of etiologies. Congenital TBM results from abnormalities of airway maturation, most seen in children born with esophageal atresia and esophageal tracheal fistula [24, 25]. Adult TBM occurs in the normally developed trachea due to chronic airway inflammation, mechanical causes and congenital causes [12-21]. In this study, the etiologies of TBM included COPD, tracheobronchial tuberculosis, bronchiectasis, relapsing polychondritis, and chronic bronchitis. However, the main etiologies in our hospital were COPD and tracheobronchial tuberculosis, which differed from those in Western countries.
COPD is one of the main causes of TBM in adults in some countries, but COPD and TBM are likely to be confused with each other because of similar symptoms and pulmonary function . A study demonstrated evidence of TBM on expiratory phase CT scan in 53% of patients with COPD . Smoking and chronic airway inflammation can lead to structural changes in the airway and result in bronchial cartilage deficiency . Pathogenesis of TBM from chronic bronchitis is like that of COPD. In China, the overall prevalence of spirometry-defined COPD is 13.7% in a national cross-sectional study by Chen-Wang , indicating that there are approximately 100 million patients with COPD in China. The possible reason is that there are a considerable number of patients with TBM secondary to COPD. However, there are few case reports about TBM secondary to COPD in China due to insufficient attention. TBM should be suspected if the patient with COPD has poor or no response to rational bronchodilators and glucocorticoids. More attention should be paid to dynamic multidetector CT with 3-dimensional imaging and flow-volume curve. If the patient is in good condition, bronchoscopy can be performed to assist diagnosis.
Tracheobronchial tuberculosis was another main cause of TBM in this study. The two patients had no history of other diseases , except tracheobronchial tuberculosis, one of the most common concurrent diseases in China. When Mycobacterium tuberculosis directly involves the bronchial wall, bronchial cartilage will be destroyed and the bronchial wall will become thickened . Tracheobronchial tuberculosis can be divided into active type and fibrous type , but its exact cause remains unclear, which may be related to the location of the lesion in the tracheobronchial tract, the range and depth of the lesion, missed diagnosis and delayed treatment. Finally, destruction of bronchial cartilage and fibrosis of the bronchial wall led to TBM. Tuberculosis cases in eight countries account for two thirds of the total globally , and the top three are India (26%), Indonesia (8.5%) and China (8.4%) . The number of tuberculosis cases reported in China has shown a continuous downward trend, but there are nearly 800,000 new cases of tuberculosis every year in China with a large population base . Tracheobronchial tuberculosis is present in 10-40% of patients with active pulmonary tuberculosis . It is surmised that there are an abundant number of patients with TBM secondary to tracheobronchial tuberculosis. However, the morbidity of TBM secondary to tracheobronchial tuberculosis is substantially undervalued due to pulmonologists' insufficient recognition of TBM in China. Therefore, a long-term prospective multicenter study on the morbidity of TBM secondary to tracheobronchial tuberculosis is needed.
Relapsing polychondritis is a rare systemic disease of unknown etiology, characterized by recurrent episodes of inflammation with the systemic destruction of cartilage tissues . In a study involving 337 cases of relapsing polychondritis, Kent et al  found that 55% of them have respiratory tract symptoms. Case 1 was diagnosed with relapsing polychondritis according to the criteria proposed by Diamiani and Levine .
Etiologies of TBM in Western countries were summarized in previous reports, mainly including COPD, asthma, Mounier-Kuhn syndrome, and relapsing polychondritis. In China, COPD and tracheobronchial tuberculosis are the major etiologies of TBM. The morbidity of TBM is substantially undervalued due to insufficient recognition and epidemiological survey results.
Chest distress and cough are the main symptoms of patients with TBM , consistent with our findings, and they are common in obstructive airway disease, leading to misdiagnosis of TBM as COPD or asthma. Although chest distress and cough are not the specific clinical symptoms of TBM, they may be indications of TBM. In clinical practice, therefore, it is needed to consider whether patients with chest distress and cough as the main clinical manifestations suffer from COPD or asthma, and to pay attention to TBM.
The diagnosis of symptomatic TBM can be complex. The clinical presentation of TBM is nonspecific and like that of other common respiratory diseases. It is important to have a high degree of clinical suspicion especially when the symptoms are caused by COPD and asthma or conventional treatment fails. For TBM, pulmonary function test is neither sensitive nor specific, and main airway obstruction shown in flow-volume curve may be a hint of TBM. Multidetector CT with 3-dimensional imaging is a noninvasive method for the diagnosis of TBM, and bronchoscopy is regarded as the golden standard for diagnosis [4, 33]. Therefore, TBM is mainly diagnosed by bronchoscopy or dynamic multidetector CT with 3-dimensional imaging. Chest distress and cough, inspiratory wheezing rale in auscultation and flow-volume curve are beneficial supplement and hint for the diagnosis of large airway obstruction.
Therapeutic methods of TBM mainly include medical management, stent insertion and surgical treatment. Airway collapse impairs airway clearance [34, 35]. Chest physiotherapy, gastroesophageal control, and nebulization of normal saline or hypertonic saline are effective management means in a clearance regimen , and almost all patients with mild to severe TBM will benefit from airway clearance of mucus . Corticosteroids, regardless of routine, active or continuous type, should be avoided because of the risks of cartilage degradation and progressive tracheomalacia . Patients may not benefit from the use of bronchodilators, because bronchodilator can relax airway smooth muscle, resulting in aggravation of airway collapse [38, 39]. Continuous positive airway pressure (CPAP) and bilevel positive airway pressure (BIPAP) are helpful for patients with TBM-induced decreased lung compliance and airway obstruction . However, noninvasive ventilation is generally not considered a therapeutic method, but a "bridge" to surgical intervention, or the last treatment for patients who cannot tolerate long-term airway stents . Tracheobronchial stenting can improve dyspnea, lung function, and quality of life in patients with airway obstruction, which, however, are associated with a higher risk of granulation tissue formation as well as stent fatigue and fracture, migration or blockage [23, 41]. Therefore, tracheobronchial stents should only be used when tracheobronchoplasty is not feasible or is contraindicated [23, 42, 43]. Surgical stabilization of the airway is the definitive treatment for patients with TBM. Thoracoscopic aortopexy, and anterior and posterior tracheobronchopexy are successful surgical treatments [36, 44]. In our study, all patients were given symptomatic treatment, and their symptoms were improved to some extent. Some patients chose conservative treatment, and three patients were inserted with tracheal stents. Despite severe symptoms, some patients refused tracheobronchial stenting because of worry about the side effects. None of the 10 patients received surgical treatment.
Almost all symptoms of patients can be improved after relaxing airway, tracheobronchial stenting or tracheobronchoplasty. There are diverse etiologies of TBM in adults. Different primary diseases have different outcome and survival time. A report by Hong G et al  indicated that the 5-year overall survival rate of a group patients with relapsing polychondritis is 75%. The long-term outcomes of TBM in adults with different etiologies are absent.
The main limitation of this study is that the number of cases was only ten, and some etiologies were not summarized. However, Shanghai is a large city in eastern China with a population of more than 20 million and is representative of East Asia. Patients in Shanghai, in addition to residents, are from everywhere of Chinese mainland. The etiologies of TBM in this study included COPD (Case 2, 6 and 10), tracheobronchial tuberculosis (Case 3 and 4) and relapsing polychondritis (Case 1). China has a large population and the incidence of COPD and tracheobronchial tuberculosis is higher, indicating that the number of patients with COPD and tracheobronchial tuberculosis are large. The results of this study still have considerable clinical value. Large-scale epidemiological investigation about TBM remains to be conducted in the future.