Nontuberculous mycobacterial pulmonary disease (NTM pulmonary disease) is the most common type of nontuberculous mycobacterial disease, and is analogous to pulmonary tuberculosis in clinical presentation.
Etiology
Nontuberculous mycobacteria (NTM) are a large group of mycobacteria other than Mycobacterium tuberculosis complex that includes Mycobacterium tuberculosis, Mycobacterium bovis, Mycobacterium africanum, and Mycobacterium microti, and only few NTM are pathogenic to humans. Therefore, NTM are opportunistic pathogens. Nontuberculous mycobacterial disease (NTM disease) is an infection of the human body by NTM, resulting in lesions in related tissues and organs. The lungs are the most affected organ.
Risk factors
Individuals with underlying lung diseases are susceptible to NTM pulmonary disease, mostly secondary to chronic lung diseases such as bronchiectasis, silicosis, and tuberculosis. Gastroesophageal reflux, rheumatoid arthritis, malnutrition, and HIV infection are risk factors for NTM disease.
Long-term use of immunosuppressants, including glucocorticoids, immunosuppressants used after organ transplantation or for autoimmune diseases, and tumor necrosis factor-α inhibitors, azithromycin, and proton pump inhibitors are also risk factors for NTM disease.
NTM is widely found in soil and water. Its distribution in nature is affected by various factors such as temperature and humidity, and the distribution has regional differences. NTM infections are more common in hot and humid areas and coastal regions.
Transmission of NTM
It was previously believed that humans or animals could be infected with NTM from the environment, but animal-to-human and human-to-human transmission are generally absent. However, in recent years, through whole-genome sequencing analysis of Mycobacterium abscessus strains infecting patients with cystic fibrosis, it has been found that these strains have high homology, indicating that Mycobacterium abscessus disease can spread from humans to humans, especially in patients with cystic fibrosis, possibly through aerosols or contaminants, which should attract great attention.
Pathology
NTM pulmonary disease is similar to tuberculosis in pathology, but in NTM pulmonary disease, epithelioid cell granuloma is more common in histology, with no significant caseous necrosis. The proliferation of collagen fibers and the mostly hyaline degeneration are the main characteristics that distinguish NTM pulmonary disease from tuberculosis in histology.
Etiological detection
Specimens can be prepared with sputum, induced sputum, bronchoalveolar lavage fluid, lung biopsy, lymph node biopsy, and pleural effusion for detection. The acid-fast staining smear examination is positive, but it is impossible to distinguish Mycobacterium tuberculosis from nontuberculous mycobacteria. Mycobacterial isolation and culture and species identification, metagenomic next-generation sequencing (mNGS), and targeted next-generation sequencing (tNGS) can be used for bacterial identification.
Clinical Manifestations
Clinical manifestations usually include recurrent cough, expectoration, and hemoptysis, persisting for several months or years.
The manifestations of imaging are diverse and lack specificity. There are mainly fibrocavitary bronchiectasis and nodular bronchiectasis.
Diagnosis
If patients have respiratory symptoms and/or systemic symptoms, chest imaging examination shows cavitary lesions, multifocal bronchiectasis, or multiple small nodular lesions, other lung diseases are excluded, and there is at least one of diagnostic considerations, a clinical diagnosis can be established.
Diagnostic considerations include:
- Two sputum NTM cultures showing the same pathogen
- One strongly positive NTM culture in bronchoalveolar lavage fluid (BALF)
- One positive NTM culture in BALF, and strongly positive acid-fast bacillus smear
- The pathological changes of lung tissue biopsy through bronchoscopy or other approaches in line with the characteristic changes of NTM pulmonary disease (granulomatous inflammation or positive acid-fast staining), and positive NTM culture
- The pathological changes of lung tissue biopsy through bronchoscopy or other approaches in line with the characteristic changes of NTM pulmonary disease (granulomatous inflammation or positive acid-fast staining), and at least one positive NTM culture in sputum specimens and/or BALF
Treatment
Currently, there are no specific drugs for treatment of NTM pulmonary disease and no standard treatment regimens. Moreover, most NTM are resistant to anti-tuberculosis drugs. Therefore, anti-tuberculosis drugs in combination with other antibiotics can be administered, usually 3 - 5 drugs. Generally, after the acid-fast bacillus turns negative in NTM pulmonary disease, treatment still needs to continue for 18 - 24 months, at least 12 months, which is significantly different from the treatment regimen for pulmonary tuberculosis.