Pulmonary Mycobacterium avium complex (MAC) infection is a type of non-tuberculous mycobacterial (NTM) infection. It is relatively common and continues to pose significant therapeutic challenges. In addition, the role of MAC in pulmonary pathology remains controversial in many instances.
Mycobacterium avium complex (MAC) infections often occur in patients with preexisting pulmonary disease or those with depressed immunity. However, it is also seen frequently in otherwise healthy patients, with a predilection for older women who deliberately suppress the cough reflex (Lady Windermere syndrome) 1-3.
A number of patient groups have been associated with increased risk of pulmonary MAC. They include 2-3:
- elderly, white, thin women: nodular bronchiectatic form (see below)
- middle-aged or elderly males who are smokers or alcoholics: upper lobe cavitary form (see below)
- immunocompromised patients, e.g. AIDS
- patients with cystic fibrosis: MAC isolated in up to 13% of patients
- patients with alpha-1-antitrypsin deficiency
- other causes of bronchiectasis
- chronic obstructive pulmonary disease (COPD)
Isolation of MAC from a patient's lung is not pathognomonic of infection, as colonisation is common, and thus microbiology needs to be correlated with clinical and radiographic appearances 2-3.
Pulmonary MAC infection is typically insidious, with a chronic cough usually productive of purulent sputum being most common. Haemoptysis and constitutional symptoms are not typical 2.
Mycobacterium avium and Mycobacterium intracellulare are now considered together, and referred to as Mycobacterium avium complex (MAC) or Mycobacterium avium-intracellulare complex (MAIC). They cannot be distinguished on the grounds of human pathologic manifestation or imaging features, and are treated similarly, although M. avium has a predilection for chickens whereas M. intracellulare prefers rabbits 2-3.
They are ubiquitous organisms, found in both fresh and salt water, but do not tend to cause human disease. Patients with MAC infection are, unlike those with pulmonary tuberculosis, not contagious 2.
- hot tub lung: granulomatous pneumonitis from exposure to aerosolised Mycobacterium avium complex (MAC) organisms in contaminated water (may not necessarily imply infection) 4
Three main forms of pulmonary MAC infections are recognised 3,5,6:
- upper lobe cavitary form/cavitary form (classic infection)
- nodular bronchiectatic form/bronchiectatic form (non-classic infection)
- mixed form
Bronchiectasis with associated centrilobular nodules is the dominant feature in the former, which unlike pulmonary tuberculosis does not have a predilection for the upper lobes. In elderly white females the right middle lobe and lingula are particularly affected.
In upper lobe cavitary form, thin-walled cavities with overall volume loss and fibrosis are the dominant feature, often also with features of endobronchial spread with tree-in-bud opacities seen elsewhere.
Bronchiectasis, seen as tram-track opacities and ring shadows, may be evident. Patchy airspace opacities are also common. Pleural effusions are uncommon 2. Upper zone cavities may also be seen with associated volume loss and scarring 3.
The most common findings of MAC infections include 1-2:
- bronchiectasis and bronchial wall thickening: most common findings
- ground-glass opacities
- small centrilobular nodules and tree-in-bud appearance
- patchy consolidation
- predilection for the right middle lobe and lingula is seen particularly in elderly white women
- pleural thickening may be seen, usually adjacent to parenchymal change
- upper lobe cavitation may also be seen, although it is more characteristic of pulmonary tuberculosis
Treatment and prognosis
Many treatment regimes have been published, with no clear gold-standard evident, although as is the case with pulmonary TB, multi-drug therapy is ideal to avoid resistance 2.
In patients who are unable to tolerate medical management, and who have adequate respiratory reserve, resection of affect portions of the lung may be undertaken. Complications of surgery include bronchopleural fistulas, haemoptysis and empyema 2.
In patients in whom isolates of MAC are not clearly pathogenic, follow-up is required, keeping in mind that evidence of radiographic progression may take a number of years to be convincing 3.
Prognosis depends on the form of disease. In the upper lobe cavitary form, lung destruction is usually progressive and can lead to respiratory failure and death if successful treatment is not instituted.
In patients with the nodular bronchiectatic form (Lady Windermere syndrome) the disease is much more indolent, however eventually this form may also lead to enough parenchymal damage to result in respiratory failure and death 3.
Mycobacterium tuberculosis pulmonary infection:
- bronchiectasis is less commonly the dominant feature 1
- changes usually in the upper lobes 1
- for associated bronchiectasis: see other causes of bronchiectasis
- 1. Primack SL, Logan PM, Hartman TE et-al. Pulmonary tuberculosis and Mycobacterium avium-intracellulare: a comparison of CT findings. Radiology. 1995;194 (2): 413-7. Radiology (abstract) - Pubmed citation
- 2. Field SK, Fisher D, Cowie RL. Mycobacterium avium complex pulmonary disease in patients without HIV infection. Chest. 2004;126 (2): 566-81. doi:10.1378/chest.126.2.566 - Pubmed citation
- 3. Müller NL, Franquet T, Lee KS et-al. Imaging of pulmonary infections. Lippincott Williams & Wilkins. (2007) ISBN:078177232X. Read it at Google Books - Find it at Amazon
- 4. Hartman TE, Jensen E, Tazelaar HD et-al. CT findings of granulomatous pneumonitis secondary to Mycobacterium avium-intracellulare inhalation: "hot tub lung". AJR Am J Roentgenol. 2007;188 (4): 1050-3. doi:10.2214/AJR.06.0546 - Pubmed citation
- 5. Kim TS, Koh WJ, Han J et-al. Hypothesis on the evolution of cavitary lesions in nontuberculous mycobacterial pulmonary infection: thin-section CT and histopathologic correlation. AJR Am J Roentgenol. 2005;184 (4): 1247-52. doi:10.2214/ajr.184.4.01841247 - Pubmed citation
- 6. Martinez S, McAdams HP, Batchu CS. The many faces of pulmonary nontuberculous mycobacterial infection. AJR Am J Roentgenol. 2007;189 (1): 177-86. doi:10.2214/AJR.07.2074 - Pubmed citation