April 2007, VOLUME 188
NUMBER 4

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April 2007, Volume 188, Number 4

Chest Imaging

Original Research

CT Findings of Granulomatous Pneumonitis Secondary to Mycobacterium avium-intracellulare Inhalation: “Hot Tub Lung”

+ Affiliations:
1Department of Radiology, Mayo Clinic, 200 First St. SW, Rochester, MN 55905.

2Department of Radiology, Mayo Clinic, Scottsdale, AZ.

3Department of Laboratory Medicine, Pathology, Mayo Clinic, Scottsdale, AZ.

4Department of Pulmonary & Critical Care Medicine, Mayo Clinic, Rochester, MN

Citation: American Journal of Roentgenology. 2007;188: 1050-1053. 10.2214/AJR.06.0546

ABSTRACT
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OBJECTIVE. The objective of our study was to describe the CT features of “hot tub lung” caused by exposure to Mycobacterium avium complex (MAC) organisms in contaminated water.

MATERIALS AND METHODS. Chart review was performed to identify all patients with a histologic diagnosis of granulomatous pneumonitis and positive cultures for MAC between January 1, 1995, and July 1, 2004. Individuals identified who also had a hot tub were included in the study. Twelve patients, seven females and five males with an average age of 50 years (range, 13-66 years), who had a CT scan were identified. The CT images were reviewed by two thoracic radiologists who assessed the images for the presence of any parenchymal abnormalities, including nodules, areas of ground-glass attenuation, reticular opacities, and air trapping, on expiratory images. When nodules, reticular opacities, areas of ground-glass attenuation, or a combination of these findings was present, the reviewers visually determined the extent of involvement of the lungs using a scale of < 10%, 10-40%, or > 40%. They also recorded the distribution of the involvement both cephalocaudal and transaxial. Decisions were reached by consensus of the reviewers.

RESULTS. Nodules were present in 10 (83%) of 12 patients. In eight (80%) of 10 patients, the nodules were diffuse with a centrilobular distribution. In the other two, the nodules were randomly distributed with an upper lung predominance. In five patients the nodules showed areas of ground-glass attenuation, whereas in the other five the nodules were solid. Areas of ground-glass attenuation were present in eight (75%) of 12 patients and were bilateral in all cases. The areas of ground-glass attenuation were diffuse in the cephalocaudal plane with a random distribution in the transaxial plane in seven (88%) of eight cases. In the remaining case, the areas of groundglass attenuation had a lower lung predominance with a random distribution in the transaxial plane. Expiratory images showed evidence of air trapping in all seven cases for which these images were available. In one patient, air trapping was the only abnormality identified.

CONCLUSION. The CT findings in patients with hot tub lung include areas of groundglass attenuation, centrilobular nodules, and air trapping on expiratory images. These findings are similar to previously published findings of subacute hypersensitivity pneumonitis. Therefore, in cases in which CT findings suggest hypersensitivity pneumonitis, hot tub lung should also be a diagnostic consideration.

Keywords: CT, granulomatous pneumonitis, highresolution CT, lung disease, Mycobacterium aviumintracellulare

Introduction
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Atypical mycobacteria cause a wide spectrum of disease in the chest. Mycobacterium avium complex (MAC) is the most common of the pathogens and can cause several different patterns of abnormalities [1, 2]. The classic MAC infection typically occurs in elderly men with underlying chronic obstructive pulmonary disease or pulmonary fibrosis. The radiologic manifestations are indistinguishable from reactivation tuberculosis [1-3]. The nonclassic MAC infection typically occurs in elderly women. The radiologic findings in nonclassic infection are characterized by bronchiectasis with multiple small nodules or nodul ar infiltrates [4-6].

Immunocompromised patients, including those with AIDS, often have findings in the chest that are part of a disseminated process, including nodules, masses, and miliary patterns [1, 2, 7, 8]. However, in some instances, adenopathy—mediastinal, hilar, or both mediastinal and hilar—may be the only finding [9]. Achalasia can also predispose individuals to atypical mycobacterial infections. Typically, the organism is Mycobacterium fortuitum or Mycobacterium chelonae. The radiologic findings resemble those seen in cases of aspiration pneumonia, although cavitation may also be present [1, 2, 10].

To this spectrum of pulmonary diseases caused by atypical mycobacteria, a new granulomatous disease process that has been referred to as “hot tub lung” has been described in the literature [11-13]. It is hypothesized that contaminated water in a hot tub is aerosolized and inhaled, thereby causing a hypersensitivity-type reaction [11-14]. We have previously reported a clinical series of patients with MAC-induced hot tub lung [15], and there are case reports of the CT findings associated with this disease process [12, 16, 17]. However, to our knowledge, no series focusing on the CT findings of pathologically proven MAC-induced hot tub lung has been reported in the literature. This series focuses on those findings in a subset of patients from our previous report [15].

Materials and Methods
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Approval for this study was obtained from our institutional review board. A chart review was performed of all patients who had a histologic diagnosis of granulomatous pneumonitis and positive cultures for MAC between January 1, 1995, and July 1, 2004. From this group, 27 patients who had exposure to a hot tub were identified. Twelve of these patients had undergone CT of the chest: five males and seven females with an average age of 50 years (range, 13-66 years).

The duration of exposure to a hot tub ranged from 20 years to less than 1 month before the onset of symptoms. In eight (75%) of 12 patients, samples of the hot tub water were obtained and were shown to be culture-positive for MAC in all cases. All 12 patients presented with dyspnea, and nine presented with a cough. Symptoms had been present for an average of 8 months (range, 2-32 months).

Pulmonary function tests were performed in all 12 patients. The results showed an obstructive pattern in six patients and a restrictive pattern in two patients; in the remaining four patients, the results were normal. None of the patients was immunocompromised by disease or drugs.

Four patients underwent surgical biopsy and eight, transbronchial biopsy. In all of the patients, CT scans were obtained within 1 month of biopsy. Histologically, the biopsies showed bronchiolocentric granulomatous inflammation with well-formed predominately nonnecrotizing granulomas. The granulomas were randomly distributed within the air spaces and interstitium, but were frequently adjacent to airways in the bronchiolar walls or in the bronchiolar lumens.

The CT scans were obtained on a variety of scanners. In 11 of the 12 patients, high-resolution images were obtained (1- to 2-mm collimation with 10- to 20-mm spacing and a high-spatial-frequency reconstruction algorithm). In the remaining patient, a conventional scan was obtained (7-mm collimation with 7-mm spacing and a standard reconstruction algorithm). In seven patients, additional expiratory high-resolution images were obtained.

The CT scans were evaluated by two thoracic radiologists who reached consensus on the findings present and the extent of findings when present. The CT scans were evaluated for the presence or absence of nodules, reticular opacities, areas of ground-glass attenuation, consolidation, honeycombing, and cysts. When any of these findings was present, the distribution of the abnormality was noted both in the cephalocaudal plane (upper, mid, lower, or diffuse predominance) and in the transaxial plane (central, peripheral, diffuse, or random).

Nodules were further evaluated by their centrilobular, perilymphatic, or random distribution. Perilymphatic distribution indicated the presence of the abnormality along the lymphatic distribution and included the peribronchovascular and interlobular septal regions of the interstitium. The extent of the abnormalities was assessed visually and recorded as involving < 10%, 10-40%, or > 40% of the lung.

Air trapping was deemed present if a mosaic pattern of attenuation was detected on inspiratory images and if the mosaic pattern became more conspicuous on expiratory images, with decreasing volume of the higher attenuation regions and no or minimal change in volume of the lower attenuation regions. If only a few scattered lobules showed these changes, this was thought to be physiologic and was not recorded as positive for air trapping.

The presence or absence of bronchiectasis was also evaluated and when present was classified as cylindric, varicose, or cystic. Additional findings, such as adenopathy, effusions, and emphysema, were recorded when present.

Results
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Nodular opacities were present in 10 (83%) of the 12 patients. They were bilateral and symmetric in eight patients and bilateral and asymmetric in the other two. In eight patients, there was a diffuse distribution of the nodules in the cephalocaudal plane with a centrilobular distribution of the nodular opacities. In two of the 10 patients, there was an upper lung predominance to the nodules in the cephalocaudal plane with a random distribution in the transaxial plane. In five patients the nodules were well defined, and in the other five patients, the nodules were poorly defined ground-glass attenuation nodules (Fig. 1). In all cases, the nodules involved more than 40% of the lungs.

Areas of ground-glass attenuation were seen in eight (75%) of the 12 patients. These areas were bilateral and symmetric in all of the cases (Fig. 2). The areas of ground-glass attenuation were diffusely distributed in the cephalocaudal plane and randomly distributed in the transaxial plane in seven of the eight patients. In the remaining case, a lower lung predominance of the areas of groundglass attenuation with a random distribution in the transaxial plane was seen. In five of the patients, the areas of ground-glass attenuation involved more than 40% of the lung. In the other three cases, the areas of ground-glass attenuation involved 10-40% of the lung.

Reticular opacities were present in only one patient. In this case, they were bilateral and asymmetric with a subpleural and lower lung predominance and involved less than 10% of the lung.

Consolidation, cysts, honeycombing, emphysema, bronchiectasis, pleural effusions, and adenopathy were not present in any of the cases.

In all seven of the patients with expiratory images, evidence of air trapping was seen (Fig. 3A, 3B). In one of the patients, air trapping was the only abnormality shown on highresolution CT.

Discussion
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Recently, a new pattern of pulmonary disease related to MAC has been recognized in the literature. For this study, we describe the CT findings associated with this granulomatous pneumonitis, which has been termed “hot tub lung.” Centrilobular nodules and areas of ground-glass attenuation are the most common findings and typically have diffuse but patchy distribution throughout both lungs. In addition, in the seven cases in which expiratory images were obtained, there was evidence of air trapping.

Although this combination of findings is relatively characteristic of MAC-induced granulomatous pneumonitis, the findings are not specific. Most significantly, they overlap with the previously reported findings of subacute hypersensitivity pneumonitis [18-20]. In fact, review of the initial interpretations of the CT images included in this study indicates that hypersensitivity pneumonitis was a primary diagnostic consideration in each case.

The finding of air trapping on expiratory imaging is likely explained by the bronchiolocentric location of some of the granulomas seen histologically. The presence of the granulomas in the walls and lumens of the bronchioles could be expected to cause compromise of the bronchiolar lumen, thereby leading to air trapping.

There was a wide variation in the duration of hot tub exposure before symptoms developed in this group of patients. This finding is likely explained by the fact that it is not the hot tub itself, but the MAC-contaminated water that is responsible for symptoms. Until the patients came in contact with contaminated water, they had no problems with the use of hot tubs.

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Fig. 1 —High-resolution CT image through lungs at level of bronchus intermedius in 54-year-old woman shows nodular areas of ground-glass attenuation with centrilobular distribution bilaterally.

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Fig. 2 —High-resolution CT image of lower lungs in 52-year-old man shows patchy bilateral areas of ground-glass attenuation.

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Fig. 3A —High-resolution CT images through upper lungs in 53-year-old woman. Inspiratory image shows patchy areas of ground-glass attenuation bilaterally with few areas of uninvolved lung.

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Fig. 3B —High-resolution CT images through upper lungs in 53-year-old woman. Expiratory image shows air trapping in uninvolved areas of upper lungs that is more prominent on right.

Whether this new disease process should be treated as a granulomatous infection [12] or as a hypersensitivity reaction is currently a topic of debate [11]; however, the clinical course is more typical of a hypersensitivity-type reaction [16]. Often resolution of the symptoms occurs with removal of the patient from the vicinity of the contaminated water in the hot tub, and treatment with antituberculosis medication is usually not necessary [16]. In some instances in which the disease process failed to resolve, antituberculosis medications were given with success, whereas in other cases steroids were given with equal success [13].

Regardless of whether this entity is ultimately shown to be a granulomatous pneumonitis or a hypersensitivity reaction, radiologists must be aware of this new manifestation of MAC pulmonary disease. When CT findings of patchy areas of ground-glass attenuation, centrilobular nodules, or both are present that might suggest a diagnosis of hypersensitivity pneumonitis, the radiologist should also consider the possibility of MAC-induced granulomatous pneumonitis or hot tub lung.

Address correspondence to T. E. Hartman.

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