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Mycobacterium avium complex lung disease in immunocompetent patients: radiography–CT correlation

Department of Medical Imaging, The Toronto General Hospital, Toronto University, 200 Elizabeth Street, Toronto, Ontario, Canada M5G 2C4

Correspondence: Dr Conrad Wittram, Section of Thoracic Radiology, Massachusetts General Hospital, FND 2, 55 Fruit Street, Boston MA 2114-2696, USA

	Abstract

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This study aimed to describe and correlate the chest radiography and CT findings of Mycobacterium avium complex (MAC) lung disease in immunocompetent patients. 26 patients fulfilled The American Thoracic Society criteria for MAC lung disease and underwent chest radiography and CT within 6 weeks of positive cultures. All abnormalities and predominant lobar involvement were recorded and abnormalities on chest radiography were correlated with those on CT. The images were evaluated by two chest radiologists and decisions were reached by consensus. 21 females and 5 males, with an overall mean age of 69 years, were included in the study. All chest radiographs and CT scans were abnormal. On chest radiography, overinflation was demonstrated in 18 patients. CT scan abnormalities included atelectasis (n=17), bronchiectasis (n=24), cavities (n=13), consolidation (n=3), emphysema (n=11), ground-glass opacity (n=8), linear opacities (n=26), mediastinal lymphadenopathy (n=3), nodules (n=25) and pleural disease (n=15). CT findings were at variance with chest radiography findings in 15 lobes. A new feature from this study is that the majority of patients with MAC lung disease demonstrate overinflation on chest radiography. 19% of cases had predominant upper lobe disease, indistinguishable from post-primary Mycobacterium tuberculosis infection. 77% of cases demonstrated the major imaging criteria of MAC lung disease. These are ill defined nodules, bronchiectasis, predominant middle lobe and/or lingular abnormalities, with or without overinflation. We believe that these characteristic radiological signs will assist the physician in the diagnostic work-up of patients with MAC lung disease.

	Introduction

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The non-tuberculous Mycobacterium avium complex (MAC) is ubiquitous in the environment and has been isolated from water, soil and animals. Water is the likely source of human MAC infection. Recently, there has been a dramatic increase in the prevalence of MAC infections. This is thought to be owing to better clinical recognition and increased culturing for pulmonary disease [1]. There is evidence that bronchiectasis and bronchiolectasis are caused by MAC infection [2]. Lung disease caused by MAC infection can occur in patients with AIDS [3], pre-existing lung disease [4] and in patients without pre-existing lung disease [2, 5–11].

The appearances of MAC lung disease on chest radiography were considered indistinguishable from those owing to Mycobacterium tuberculosis [12–14]. In 1989 Prince et al [5] were the first to recognize MAC infection in patients without pre-existing lung disease, predominantly in elderly women. The most common radiological pattern was multiple pulmonary nodules [5]. In 1992 Reich et al [8] reported that MAC lung disease can present with a lingular and middle lobe pattern on chest radiography. CT studies of MAC lung disease have demonstrated nodular opacities and bronchiectasis [11, 15, 16] with middle lobe and lingular bronchiectasis being suggestive of MAC lung disease [4, 10, 11].

There are no prior studies that correlate chest radiography and CT abnormalities of patients with MAC lung disease. Chest radiographs are the initial medical imaging assessment of patients with suspected lung disease. They are inexpensive, readily available and can often provide sufficient information for diagnosis and management. To assist in the interpretation of chest radiograph manifestations of MAC lung disease we undertook this study to describe and correlate chest radiography and CT abnormalities in a group of immunocompetent patients with MAC lung disease.

	Materials and methods

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We reviewed the previous 75 months of microbiology records to obtain all patients with positive MAC cultures. As the immune status of a patient may affect the imaging manifestations of MAC lung disease, all immunodeficient patients with HIV, hypogammaglobulinaemia or leukaemia, and those on steroid therapy, were excluded. 60 immunocompetent patients fulfilled The American Thoracic Society criteria for MAC lung disease [1]. 19 patients were excluded because of coexisting infection (n=12), bronchogenic carcinoma (n=4), sarcoidosis (n=2) and desquamative interstitial pneumonitis (n=1) as these diseases would confound the radiological signs. 26 patients were identified who had both a chest radiograph and CT scan within 6 weeks of positive MAC cultures in order to image active infection. 21 of the 26 patients were female, with an age range of 48–81 years (mean 69 years), and 5 were male, with an age range of 51–84 years (mean 72 years). Patients were not on chemotherapy for MAC at the time of imaging. The case notes of the 26 patients did not document a previous medical history of pulmonary Mycobacterium tuberculosis infection.

All 26 patients were imaged on either a General Electric Highlight Advantage (General Electric, Milwaukee, WI) or a CTi scanner (General Electric, Milwaukee, WI). 18 patients were imaged with high resolution CT (HRCT) images with 1 mm collimation at 10 mm intervals, 3 patients with 10 mm collimated contiguous images, 1 patient with 7 mm thick contiguous images and 4 patients with our haemoptysis protocol. The haemoptysis protocol includes HRCT images at 10 mm intervals from the lung apex to the right upper lobe bronchus, 5 mm thick contiguous images between the right upper lobe bronchus and the right inferior pulmonary vein and HRCT images at 10 mm intervals from the right inferior pulmonary vein to the lung bases. No patient received contrast medium. All images were hard copied on mediastinal and lung parenchyma window settings.

The chest radiographs and CT scans of all 26 patients were reviewed by two chest radiologists. For each patient, the posteroanterior and lateral chest radiographs were read before the CT scans. For the purpose of recording location of abnormalities, the lingular segments are regarded as a lobe. All chest radiograph and CT scan abnormalities were recorded [17, 18]. Nodules were defined as a round opacity up to 3 cm in maximum diameter [18]. Atelectasis was defined as a loss of segmental or subsegmental lung volume resulting in discoid or linear opacities [17]. Pleural effusion or thickening on chest radiography was defined as the blunting or loss of the normal lateral or posterior costophrenic angle, and on CT as abnormal soft tissue opacity demonstrated on the inside of the ribs [19]. The features used for the diagnosis of overinflation on the posteroranterior chest radiograph included either flattening of a hemidiaphragm (where the highest level of the dome is less than 1.5 cm above a straight line drawn between the costophrenic and the vertebrophrenic junctions) or the location of the right hemidiaphragm at or below the anterior aspect of the seventh rib [20]. An assessment was made of the predominant distribution of disease on the chest radiograph and CT scans; up to three lobes were recorded with none recorded if there was no such predominance. Following these readings the chest radiograph abnormalities were correlated with the CT scan abnormalities, and similarities and differences between the two modalities were recorded. Decisions were reached by consensus.

	Results

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All chest radiographs and CT scans were abnormal (Tables 1–3; Figures 1–3). On chest radiography, predominant upper lobe involvement, excluding lingular segments, was present in four male patients and two female patients. Predominant middle lobe and/or lingular involvement was present in six female patients. Predominant middle lobe and/or lingular, with either upper or lower lobe, involvement was present in 10 female patients. Predominant lower lobe involvement was present in one female patient. Three patients had no predominant distribution of disease on chest radiography.

View larger version (150K): [in this window] [in a new window]   Figure 1. 74-year-old female with Mycobacterium avium complex lung disease. Posteroanterior chest radiograph demonstrates overinflated lungs. Ill defined nodular opacities are demonstrated affecting the middle lobe; atelectasis and ill defined nodules affect the lingula. There is blunting of the left costophrenic sulcus indicating a minor amount of pleural disease.

View larger version (58K): [in this window] [in a new window]   Figure 2. 61-year-old female with Mycobacterium avium complex lung disease. (a) Selected view of lateral chest radiograph demonstrates atelectasis and bronchiectasis (arrows) affecting the lingula and middle lobe. (b)Correlative high resolution CT scan demonstrates atelectasis and bronchiectasis affecting the lingula and middle lobe. Peripheral, ill defined small nodular opacities are noted in the right and left lower lobes (arrows).

View larger version (64K): [in this window] [in a new window]   Figure 3. 58-year-old male with Mycobacterium avium complex lung disease. (a) Selected view of left upper lobe demonstrates an ill defined round opacity (arrow) with a cavity (curved arrow). (b) High resolution CT image demonstrates a 17 mm diameter thick-walled cavitary region of consolidation affecting the left upper lobe. A nodule affects the right upper lobe posteriorly. Centrilobular emphysema is demonstrated affecting both lungs.

The chest radiographs of each case were compared with the CT scan regarding identification and distribution of abnormalities. It was found that CT added nine abnormal lobes thought to be normal on chest radiography. CT demonstrated six normal lobes felt to be abnormal on chest radiography and agreed with lobar distribution in 15 patients. The cavities were single with ill defined margins, and wall thickness on CT ranged between 2 mm and 2 cm. Bronchiectasis was present on the chest radiograph of 12 patients and the CT scan of 24 patients. On CT, four patients had one lobe involved, four patients had two lobes, four patients had three lobes, seven patients had four lobes, three patients had five lobes and two patients had six lobes (including lingula as a lobe). On CT, the middle lobe was involved with bronchiectasis in 19 cases (18 female, 1 male), right upper lobe in 16 cases (15 female, 1 male), lingular in 13 cases (12 female, 1 male), right lower lobe in 12 cases (all female), left upper lobe in 11 cases (9 female, 2 male) and left lower lobe in 8 cases (7 female, 1 male). Exclusive middle lobe and/or lingular bronchiectasis was present in 5 patients (3 female, 2 male).

	Discussion

Top Abstract Introduction Materials and methods Results Discussion References

 
In immunocompetent patients, MAC lung disease can cause lung cavities and consolidation. Characteristic pathological features of MAC lung infection include extensive granuloma affecting the airways. Peribronchial granuloma of the large airways and bronchioles can result in airway narrowing. Granuloma can disrupt the muscle layer of airways which may lead to bronchiectasis. Centrilobular nodules are owing to discrete granulomas or necrotic material in the lumen of bronchioles, possibly from transbronchial spread of necrotic tissue [2].

In this study there are two common patterns of MAC lung disease in immunocompetent patients. (1) Predominant upper lobe involvement, most often present in males with cavitary disease. In our opinion, this pattern of predominant upper lobe involvement, seen in patients with MAC lung disease, is indistinguishable from post primary Mycobacterium tuberculosis pulmonary infection [12–14]. (2) Predominant middle lobe and/or lingular imaging abnormality, almost exclusively seen in females [4, 8–11].

In evaluating abnormalities on chest radiography by correlating findings with CT, it is assumed that CT scans are superior in most aspects to chest radiographs in demonstrating abnormalities in patients with MAC lung disease. Comparatively, CT added significantly in the assessment of bronchiectasis (Figure 2) and cavities. This adds considerable confidence to the radiological diagnosis of MAC lung disease. The combination of multiple small nodules on CT with bronchiectasis, particularly in the middle lobe and/or lingula, should suggest the diagnosis, especially in older females [4, 10, 11].

One criticism of this study is that not all patients had a CT scan of the lungs with narrow collimation. In those patients scanned with a 7 mm or 10 mm collimation, the true rate of emphysema, bronchiectasis and small nodules may have been underestimated. In addition, this retrospective study had a small number of patients owing to the strict selection criteria of only using patients with active MAC lung disease.

A new finding demonstrated in this study is chest radiography demonstrating overinflation in patients with MAC lung disease, as seen in 18 of our cases. Overinflation may be a coincident finding or related to the cause or effect of the disease. 11 cases demonstrated pulmonary emphysema on CT. It has been suggested that this is a predisposing cause for MAC lung disease [4]. Air trapping has recently been described as an effect of MAC lung disease in patients without pre-existing lung disease [21]. Air trapping can lead to lung overinflation. It is postulated that in at least seven of our cases, overinflation seen on chest radiography is a result of MAC infection affecting the small airways.

Chest radiography is often used as the first medical imaging investigation of patients with suspected lung disease. In patients with pulmonary nodules the differential diagnosis includes neoplastic processes such as haematogenous metastases, lymphangitic carcinomatosis, bronchioalveolar carcinoma and lymphoma, sarcoidosis, silicosis and coal workers pneumoconiosis and fungal and mycobacterial infections. CT adds considerable information in the diagnostic work-up of these patients. It is able to demonstrate the number and distribution of nodules, characterize the opacities as ill defined, well defined or tree-in-bud (bronchiolar dilatation and filling by mucus, pus or fluid, resembling a budding tree), and detect associated abnormalities. In cases with MAC lung disease, nodules are ill defined or tree-in-bud, centrilobular in distribution and are associated with bronchiectasis (Figure 1, 2).

In conclusion, in this radiography–CT correlation study of MAC lung disease we found that chest radiography and CT complement each other. This study demonstrates a new finding; the importance of chest radiography in demonstrating overinflation in patients with MAC lung disease. In practice, all patients with new lung nodules on chest radiography require CT to demonstrate nodule distribution, characterize the nodules and detect associated abnormalities. We found two common patterns of MAC lung disease. 19% of cases had predominant upper lobe disease indistinguishable from post-primary Mycobacterium tuberculosis infection. 77% of cases demonstrated the major imaging criteria of MAC lung disease. These are ill defined pulmonary nodules, bronchiectasis, predominant middle lobe and/or lingular abnormalities, all with or without overinflation. We believe that these characteristic radiological signs will assist the physician in the diagnostic work-up of patients with MAC lung disease.

	Acknowledgments

 
The authors would like to thank Rose Baldwin for her assistance in the preparation of this manuscript.

Received for publication February 8, 2001. Revision received November 21, 2001. Accepted for publication January 7, 2002.

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