Pulmonary Mycobacterial (Avium Complex) Infection

Cheryl Ann Sadow, MD - Case Coordinator
John Dewolfe Mackenzie, MD - Radiology Discussion
Frank S David, MD, PhD - Pathology Discussion
Pablo R Ros, MD, MPH - Attending Radiologist
Paul Spirn, MD - Attending Radiologist

March 25, 2002

Presentation

Imaging Findings

PA and lateral radiographs reveal severe emphysema, linear opacity in the left upper lobe, some hilar retraction (perhaps caused by fibrosis), and a few circumscribed nodules in the area of the 4th or 5th rib. In addition, the left apex is abnormal.

Computed tomograms obtained a short time later demonstrate severe emphysema with a cavity and some fluid-density bullae in the left upper lobe. The wall demarcated by lung is thin, perhaps with adjacent pleural thickening. Nodules with central calcification are visible, as are areas of consolidative/nodular opacity (perhaps representing bronchiectasis).

Differential Diagnosis

Diagnosis

Discussion

The left lung was subjected to wedge resection in the areas affected by cavitary lesions. The tissue has a cheesy texture throughout. Staining with hematoxylin and eosin indicated massive necrosis, so images were obtained of AFB-stained smears. The microscopic images show massive colonization of necrotizing granulomas by acid-fast bacilli (red rod-shaped structures). No fungi were found with silver staining.

A DNA sample was sent for analysis in order to definitively diagnose mycobacterium avium complex.

Radiology Discussion

Nontuberculous mycobacteria are not transmitted from person-to-person; they ubiquitous in the environment and are typically transmitted through aerosolized water droplets. There are a number of species, but the most common infectious agent is mycobacterium avium complex (MAC), which is a combination of mycobacterium avium and mycobacterium intercellularae. The two species present the same clinical manifestation and can only be differentiated by DNA analysis.

A pulmonary nontuberculous mycobaterial infection can take one of two forms. The classical form is radiographically indistinguishable from tuberculouosis. It is characterized by nodular opacities in the apices, cavitation, apical pleural thickening, and bronchogenic spread. Pleural effusion and adenopathy are uncommon. Bronchiectasis is the primary finding in the middle lobe on high resolution computed tomography (CT). The classical form is male predominant and typically strikes in the sixth or seventh decade of life. Up to 80-90% of patients with these infections are Caucasian.

Infections that present in the classical form are often secondary to underlying lung disease (COPD, prior TB infection, or interstitial lung disease). Other risk factors include smoking, alcoholism, cardiovascular disease, chronic liver disease, and previous gastrectomy. Classic mycobacterial infection is insidious—symptoms include cough (60-100%), weight loss, fever (10-13%), weakness, and hemoptysis (15-25%), but are often mild or absent.

The second form is non-classical (atypical). Atypical mycobacterial infection is characterized by a haphazard distribution of multiple bilateral nodular and irregular interstitial opacities (<1 cm). It is not necessarily related to any underlying chronic lung disease and is found most often in women.

Nontuberculous mycobacterial infection may also be found incidentally, either as a single pulmonary nodule or as a small cluster of nodules.

MAC has a very different presentation in patients with HIV and AIDS. Inthese patients, the primary presentation is retroperitoneal lymphadenopathy. Evidence of MAC infection is also generally found in the lungs, but it does not usually produce pulmonary symptoms. It is thought that there is a gastrointestinal route of transmission in these immunosuppressed patients.

Following the resident’s presentation, Dr. Spirn offered a few additional words about mycobacterial infection. The use of the words "classical" and "atypical" may be somewhat misleading now. The atypical presentation is actually 4-5 times more common than the classical presentation, and appears mainly in middle-aged or elderly women. It is not certain why this group is more susceptible to infection via aerosolized water. More than half of those diagnosed are only mildly symptomatic—often a chronic cough is the only sign. About 10% of those go on to progressive disease that requires treatment. Their radiologic presentation is that of "classic" bronchogenic spread—"tree and bud" clusters of bronchiolar plugging, nodules, and peribronchiolar infiltration. For diagnosis, it is preferred to see nodules and bronchiectasis in same area.

References

Moore EH. Atypical mycobacterial infection in the lung: CT appearance. Radiology 1993;187(3):777-82.

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