What is the differential diagnosis of a tree-in-bud pattern?

Differential Diagnosis of Tree-in-Bud Pattern

The tree-in-bud pattern represents infectious bronchiolitis with mucoid impaction of small airways, with mycobacterial infections (particularly tuberculosis) being the most common cause at 39% of cases, followed by bacterial infections at 27%. 1

Radiologic Definition

The tree-in-bud (TIB) pattern consists of centrilobular nodules connected to branching linear structures resembling a budding tree, representing dilated and inflamed bronchioles with mucoid impaction visible on high-resolution CT (HRCT). 1 This pattern reflects inflammatory exudate, pus, or mucus plugging the terminal and respiratory bronchioles. 2

Primary Infectious Causes

Mycobacterial Infections (39% of cases)

• Mycobacterium tuberculosis is the classic cause, particularly with endobronchial dissemination, often associated with upper lobe cavitations. 1, 3
• Nontuberculous mycobacteria (especially M. avium complex) commonly present with this pattern in patients with bronchiectasis. 1, 2

Bacterial Infections (27% of cases)

• Pseudomonas aeruginosa in bronchiectasis patients is a frequent cause. 1, 2
• Other bacterial pathogens including Mycoplasma pneumoniae and community/hospital-acquired bacteria. 2
• The pattern appears in 17.6% of acute infectious bronchitis or pneumonia cases and 25.6% of bronchiectasis cases. 4

Fungal Infections

• Allergic bronchopulmonary aspergillosis (ABPA) presents with TIB pattern alongside centrilobular nodules and bronchiectasis. 1
• Other fungal pathogens can cause this pattern. 5

Viral and Parasitic Infections

• Viral bronchiolitis (including human parainfluenza) can produce TIB pattern. 5, 6
• Parasitic infections are rare causes. 5

Non-Infectious Causes

Inflammatory Disorders

• Diffuse panbronchiolitis is an important inflammatory cause. 1, 3
• Inflammatory bowel disease-related bronchiolitis can manifest this pattern. 1, 3

Aspiration

• Aspiration pneumonia accounts for 10.4% of TIB cases. 7
• Acute aspiration can present with TIB even without proximal airway abnormalities. 4

Malignancy (Less Common)

• Lung malignancy accounts for 4% of cases. 7
• Other malignancies (including chronic lymphatic leukemia) account for 9.5% of cases. 8, 7
• Neoplastic pulmonary emboli can rarely cause this pattern. 5

Other Causes

• Congenital disorders, idiopathic obliterative bronchiolitis, connective tissue disorders, and inhalation of toxic substances. 5

Critical Diagnostic Pitfall

In 25.6% of cases with TIB pattern, bronchiectasis or proximal airway wall thickening is present, which helps distinguish infectious causes from other etiologies. 4 Notably, emphysema, bronchiolitis obliterans, BOOP, extrinsic allergic alveolitis, and respiratory bronchiolitis do NOT produce TIB pattern. 4

Diagnostic Algorithm

Imaging

• Obtain HRCT without IV contrast as the preferred initial modality, with expiratory imaging to assess for air trapping and mosaic attenuation. 1

Microbiologic Workup

• Immediately collect sputum cultures for bacteria, mycobacteria, and fungi. 1, 2
• Obtain at least two expectorated sputum samples for acid-fast bacilli smears and cultures if mycobacterial infection is suspected. 2
• Proceed to bronchoscopy with bronchoalveolar lavage (BAL) if sputum studies are non-diagnostic. 1, 3

Clinical Context

• The microbiologic etiology reflects the overall incidence of organisms in community-acquired versus hospital-acquired populations, with no specific correlation between TIB distribution, immune status, and organism isolated. 7
• In immunocompromised patients (especially AIDS), tuberculosis may not follow classic patterns and can present with only mediastinal lymphadenopathy. 3