Most Common Diagnosis with Bronchiectasis and Tree-in-Bud Pattern
Nontuberculous mycobacterial (NTM) infection, particularly Mycobacterium avium complex (MAC), is the most common diagnosis when bronchiectasis and tree-in-bud pattern appear together on imaging. 1
Primary Diagnostic Consideration
MAC infection classically presents with bronchiectasis and nodular densities distributed around peripheral vessels and airways, frequently with a tree-in-bud configuration. 1 This pattern occurs predominantly in white women in their seventh or eighth decade of life with no underlying immune compromise (Lady Windermere syndrome). 1
Key Imaging Features of MAC
- The nodules are typically distributed around peripheral vessels and airways with tree-in-bud configuration, most commonly affecting the middle lobe and lingula. 1
- Bronchiectasis in MAC is more severe and extensive compared to other NTM species, with significantly higher bronchiectasis scores and higher prevalence of nodules. 2
- The tree-in-bud pattern represents infectious bronchiolitis with mucoid impaction of small airways, where inflammatory exudate fills the terminal and respiratory bronchioles. 3, 4
Differential Considerations
While MAC is most common, other diagnoses must be considered:
Mycobacterium abscessus
- Bilateral micronodules, tree-in-bud sign, and multiple bronchiectasis are more frequently seen with M. abscessus than MAC. 5
- M. abscessus accounts for approximately 80% of rapidly growing mycobacteria (RGM) respiratory disease isolates and is the third most frequently recovered NTM respiratory pathogen. 1
- The radiographic pattern shows multilobar, patchy, reticulonodular opacities with cylindrical bronchiectasis and multiple small (≤5 mm) nodules. 1
Primary Ciliary Dyskinesia (PCD)
- PCD shows predominance of bronchiectasis in the middle and lower lobes with severe tree-in-bud pattern, mucous plugging, and atelectasis. 6
- Situs inversus (Kartagener syndrome) occurs in 55% of PCD patients and is pathognomonic when present. 1, 6
- All PCD patients have chronic rhinitis/sinusitis, 95% have recurrent otitis media, and 73% have neonatal respiratory disease. 1
Other Bacterial Infections
- Pseudomonas aeruginosa in bronchiectasis patients commonly causes tree-in-bud pattern. 3, 4
- Mycoplasma pneumoniae and other community-acquired bacteria can produce this pattern but are less commonly associated with established bronchiectasis. 3
Diagnostic Algorithm
Immediate Steps
- Obtain at least two expectorated sputum samples for acid-fast bacilli smears and mycobacterial cultures. 3, 4
- Collect sputum cultures for bacteria and fungi to identify alternative pathogens. 3, 4
- Proceed to bronchoscopy with bronchoalveolar lavage if sputum studies are non-diagnostic. 4
Clinical Context Assessment
- Evaluate for typical MAC demographics: white women, seventh or eighth decade, no immune compromise. 1
- Assess for PCD features: chronic sinusitis from childhood, recurrent otitis media, male infertility, situs inversus. 1
- Review for rapid progression suggesting M. abscessus, particularly in younger patients (<50 years) with underlying lung disease. 1
Important Caveats
MAC infection was historically mischaracterized as "colonization" of preexisting bronchiectasis, but the presence of granulomas in airways and response to antimycobacterial therapy indicates MAC is the primary disorder causing progressive airway damage. 1 This distinction is critical because it changes management from observation to active treatment.
Approximately 15% of patients with M. abscessus lung disease also have MAC isolated from sputum, suggesting overlap and the possibility of dual infection. 1 This requires careful microbiological assessment and may necessitate treatment targeting both organisms.
Sputum cultures are not sufficiently sensitive to establish NTM diagnosis in many patients, with diagnosis sometimes requiring bronchoscopic biopsies or empiric treatment based on typical clinical and radiographic features. 1