Chest Imaging

Atypical mycobacterial infection

(also called nontuberculous mycobacterial infections (NTMB)), infection caused by various species of the genus Mycobacterium. These organisms are aerobic Gram-positive rods with variable acid-fast staining characteristics. They have been classified into four major groups:

  • photochromogens (e.g. M. kansasii);

  • scotochromogens (e.g. M. gordonae);

  • nonphotochromogens (e.g. M. xenopi, M. avium-intracellulare); and

  • rapid growers (e.g. M. fortuitum, M. chelonei).

    These organisms have been discovered throughout the world in water and soil. Unlike tuberculosis, human to human transmission is rare if it occurs at all, and disease is generally acquired by aerosolized water droplet inhalation. Typically infection occurs in patients with underlying lung disease or altered immune status. With increasing numbers of HIV infected patients more and more cases of NTMB are being reported.

    The diagnosis of actual NTMB infection is difficult since these organisms commonly colonize airways. The American Thoracic Society recommends that for a firm diagnosis the following criteria are met:

  • NTMB isolated from autopsy or biopsy specimens;

  • granulomatous changes in biopsy material along with positive culture for NTMB from respiratory secretions;

  • four or more positive sputum cultures;

  • bronchoscopic material demonstrating M. kansasii since this is seldom a colonizing organism;

  • bronchoscopic washings yielding NTMB in association with positive blood or bone marrow cultures.

    Along with these laboratory findings patients should have clinical signs of disease and an abnormal chest film that improves with appropriate antimycobacterial agents.

    The most common group of patients infected with NTMB are elderly males with underlying lung disease such as chronic obstructive pulmonary disease (COPD). These patients may present with a cough, haemoptysis, weight loss and low-grade fever. Radiographically chest films reveal medium to coarse linear nodular opacities in the apical and posterior segments of upper lobe and superior segment of lower lobe. These findings may slowly worsen or remain unchanged for years. Associated with these abnormalities are bronchiectasis, cavitation, and pleural thickening in a large percentage of patients. Fibrosis may occur with significant volume loss causing displacement of the hila or mediastinum. In time calcification of hilar and mediastinal nodes as well as parenchymal abnormalities may be observed. Free pleural effusions and hilar lymphadenopathy and/or mediastinal lymphadenopathy (Fig.1) are, however, uncommon.

    Another group of patients who develop NTMB are elderly women without prior lung disease. They may present with chronic cough and a chest film demonstrating scattered nodules simulating metastases as well as progressive bronchiectasis. CT is better at revealing these abnormalities.

    In immunocompromised patients, M. avium-intracellulare complex (MAC) is becoming more common. Clinically patients have fever, weight loss and malaise. The chest film is often normal but hilar lymphadenopathy or mediastinal lymphadenopathy and scattered coarse lung nodules may be seen occasionally (Fig.2). Cavitation, effusions and atelectasis are uncommon.

    • Because NTMB causes low-grade infection with slow progression there has been some debate as to whether treatment is indicated. Most believe, however, there is an increased morbidity and mortality with this disease and so therapy is warranted. M. kansasii is fairly easy to treat as it responds to triple antimycobacterial drug therapy. MAC is more resistant although some newer agents such as clarithromycin have proven somewhat effective.

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    Fig.1

    A PA chest film demonstrates heterogeneous opacity in the right midlung with adjacent ipsilateral right hilar adenopathy. These findings should suggest the possibility of tuberculosis but in this case Mycobacterium kansasii was discovered. Atypical mycobacterial disease may cause radiographic findings similar to those seen with tuberculosis.
    Atypical mycobacterial infection, Fig.1
    Atypical mycobacterial infection, Fig.2