Mycobacterium Avium Complex

Jud W. Gurney, MD, FACR

Key Facts

Terminology

NTM pulmonary infection, most commonly caused by MAC, causes 5 types of disease
Classic infection that mimics Mycobacterium tuberculosis
- Upper lobe fibrocavitary disease
- Solitary or multiple nodules
Bronchiectasis & nodules (Lady Windermere syndrome)
- Bronchiectasis, RML, and lingula
- Centrilobular nodules
Disseminated disease in immunosuppressed patients
- Normal to aggressive cavitary disease
- Lymphadenopathy or hepatosplenomegaly
Hypersensitivity pneumonia (“hot tub” lung)
- Subacute hypersensitivity pneumonitis pattern
Consolidative pattern associated with achalasia
- Pattern of aspiration pneumonia

Imaging Findings

Progression slow (average of 6 years to demonstrate progression)

Top Differential Diagnoses

Post-Primary Tuberculosis

Pathology

Pathophysiology of Lady Windermere syndrome
- Hypothesis: Fastidious suppression of cough leads to retained secretions in dependent portions of ventral lung
“Hot tub” lung: Either hypersensitivity reaction to MAC or true infection, preponderance of evidence suggests hypersensitivity reaction

Axial CECT shows clustered nodules in the right upper lobe

and the superior segment of the left lower lobe

Axial CECT shows a cavity

, feeding bronchus

, and clustered nodules

. MAC was cultured, and the findings were identical to those of Mycobacterium tuberculosis.

TERMINOLOGY

Abbreviations and Synonyms

Mycobacterium avium intracellulare complex (MAC), nontuberculous mycobacteria (NTM), atypical mycobacteria, chronic obstructive lung disease (COPD)

Definitions

NTM pulmonary infection, most commonly caused by MAC
- Other NTM: M. kansasii, M. xenopi, M. fortuitum, M. scrofulaceum, M. gordonae, M. abscessus, and M. chelonae
5 types of disease
- Classic infection that mimics Mycobacterium tuberculosis (MTb)
- Bronchiectasis & nodules (Lady Windermere syndrome)
- Disseminated disease in immunosuppressed patients
- Hypersensitivity pneumonia (“hot tub” lung)
- Consolidative pattern associated with achalasia and other esophageal motility disorders

IMAGING FINDINGS

CT Findings

Classic infection, 2 patterns, MTb or nodule(s)
- Indistinguishable from M. tuberculosis
  - Upper lobe predominance, apical posterior segments (anterior segment less common)
  - Thin-walled cavities (thick-walled cavities are less common) hallmark of infection, associated with airspace opacities, masses, and nodules
  - Mean diameter of cavity is 2.5 cm
  - Bronchogenic spread with 5-15 mm peripheral centrilobular nodules in dependent lung
  - Bronchial wall thickening → bronchiectasis
  - Feeding bronchus: Patent bronchus running into cavitary lesion
  - Lymphadenopathy and effusions, uncommon
- Nodule(s)
  - Solitary or multiple nodules; solitary nodules mimic bronchogenic carcinoma, range in size from 1 to 5 cm in diameter
  - Nodules of similar size often cluster together
- Associated findings from underlying predisposing factors such as emphysema and pulmonary fibrosis
Bronchiectasis & nodules (Lady Windermere syndrome)
- Bronchiectasis, cylindrical, distributed mainly in ventral lung, primarily right middle lobe (RML) and lingula
  - 33% of patients with bronchiectasis and nodules will have NTM infection
  - > 5 lobes with bronchiectasis highly associated with NTM
- Centrilobular nodules
  - 85% < 5 mm diameter
  - 80% within same lobe as bronchiectasis
- Bilateral multifocal bronchiolitis manifested by
  - Mosaic pattern of perfusion or hyperinflation with airway narrowing
  - Tree-in-bud pattern: Branching centrilobular nodules that occur within lobes affected with bronchiectasis but also scattered throughout uninvolved lung
- Consolidation, lobular
- Cavitation, mediastinal adenopathy, atelectasis rare
Disseminated disease in immunosuppressed patients
- Wide range of abnormalities
  - Often normal or subtle pulmonary findings such as a few scattered centrilobular nodules
  - Aggressive cavitary disease also possible
  - Generalized lymphadenopathy or hepatosplenomegaly common
  - Lymphadenopathy may have low-density necrotic centers
Hypersensitivity pneumonia (“hot tub” lung)
- Indistinguishable from other causes of subacute hypersensitivity pneumonitis
  - Diffuse centrilobular micronodules (85%) usually involving > 40% of lung
  - Centrilobular nodules either solid (50%) or ground-glass (50%)
  - Ground-glass opacities (75%), mosaic attenuation, and air-trapping at expiratory scanning (100%)
Achalasia and esophageal motility disorders
- Pattern of aspiration pneumonia
  - Large bilateral dependent areas of consolidation
  - Pleural effusions (20%)
  - Cavitary disease (15%)

Radiographic Findings

Radiography
- Classic infection
  - Should suspect in any patient with unexplained upper lung zone opacities
  - Tubular tram-track opacities suggest underlying bronchiectasis
  - Apical pleural thickening (50%) often striking; associated with cavitary disease even though cavities may not be radiographically apparent
  - Progression slow (average of 6 years to demonstrate progression)
- Bronchiectasis & nodules (Lady Windermere syndrome)
  - Should suspect in patients (especially elderly females) with tubular or ill-defined opacities in RML or lingula
  - Often hyperinflated (but not from emphysema)
- Disseminated disease in immunosuppressed patients
  - Normal radiograph with positive sputum cultures, common (20%)
  - Abnormal radiographs varied and nonspecific: Small scattered alveolar and nodular opacities, miliary nodules, & mass-like lesions
  - Cavitation more common in non-AIDS immunosuppressed patients
  - Hilar or mediastinal lymphadenopathy common and may be only finding
- Hypersensitivity pneumonia
  - Normal radiograph (20%)
  - Nonspecific diffuse interstitial or nodular ground-glass opacities
- Achalasia and esophageal motility disorder
  - Dilated esophagus, absent stomach bubble
  - Dependent airspace opacities