How Bronchopneumonia Develops with Tree-in-Bud Appearance
Pathophysiologic Mechanism
The tree-in-bud pattern represents infectious bronchiolitis with mucoid impaction of small airways, where inflammatory exudate, pus, or mucus plugs the terminal and respiratory bronchioles, creating the characteristic branching nodular appearance on HRCT. 1, 2
The pathologic process occurs through these specific steps:
- Bronchiolar infection and inflammation causes the bronchiolar walls to thicken and dilate, with inflammatory cells infiltrating the airway walls 2
- Mucoid impaction develops as the infected bronchioles fill with mucous, pus, or inflammatory fluid that cannot be cleared effectively 3, 2
- Endobronchial spread occurs when infectious material disseminates through the bronchial tree to involve multiple secondary pulmonary lobules 4
- Peribronchiolar inflammation extends into adjacent alveolar spaces, creating the centrilobular nodules (2-4mm) connected to linear branching structures that resemble a budding tree 1, 5
Primary Infectious Causes
The most common etiologies causing this pattern include:
- Mycobacterial infections are the classic cause, particularly Mycobacterium tuberculosis and nontuberculous mycobacteria like M. avium complex 3, 4, 1
- Bacterial pathogens including Pseudomonas aeruginosa in bronchiectasis patients, Mycoplasma pneumoniae, and other community or hospital-acquired bacteria 1, 2, 6
- Fungal infections such as allergic bronchopulmonary aspergillosis can produce this pattern through bronchoinvasive disease 3, 5, 2
Diagnostic Approach
When tree-in-bud pattern is identified on HRCT, immediately obtain sputum cultures for bacteria, mycobacteria, and fungi to identify the causative pathogen. 1, 5
The diagnostic algorithm should proceed as follows:
- HRCT without IV contrast is the preferred imaging modality, showing centrilobular nodules with branching linear structures in peripheral lung regions 1, 5
- Look for associated findings: cavitations suggest mycobacterial infection, bronchiectasis indicates chronic infection, and mosaic attenuation on expiratory images indicates air trapping 3, 1
- Collect at least two expectorated sputum samples for acid-fast bacilli smears and cultures if mycobacterial infection is suspected 3
- Proceed to bronchoscopy with bronchoalveolar lavage if sputum studies are non-diagnostic or negative, as this significantly impacts treatment decisions 3, 5
Critical Pitfall to Avoid
Do not delay bronchoscopy when initial sputum studies are negative—the American College of Chest Physicians emphasizes that bronchoscopy may be required before excluding suppurative airways disease, particularly when purulent secretions or clinical suspicion remains high 3, 5
Management Based on Etiology
Treatment requires prolonged antibiotic therapy targeted at the identified pathogen, with macrolide-based multi-drug regimens for 12+ months when nontuberculous mycobacteria are confirmed per ATS/IDSA criteria. 1, 5
Specific treatment approaches:
- For NTM infections: Initiate macrolide-based combination therapy and monitor with serial sputum cultures every 4-12 weeks during treatment 3, 1, 5
- For bacterial infections: Use appropriate antibiotics based on culture and sensitivity results 1
- For invasive aspergillosis: Voriconazole is first-line therapy (with therapeutic drug monitoring), or liposomal amphotericin B as an alternative 5
- Monitor treatment response with follow-up CT after completing therapy to document radiological improvement 1, 5
Non-Infectious Considerations
While infection accounts for approximately 68% of tree-in-bud cases, consider these alternative etiologies when cultures are negative 6:
- Aspiration pneumonia (10% of cases) from chronic aspiration or toxic inhalation 1, 6
- Inflammatory conditions including diffuse panbronchiolitis and inflammatory bowel disease-related bronchiolitis 3, 1, 2
- Rare causes such as malignancy (lung cancer or lymphoproliferative disorders) require transbronchial biopsy for diagnosis 7, 6