What are the causes of a tree-in-bud pattern on a CT (computed tomography) thorax?

Causes of Tree-in-Bud Pattern on CT Thorax

The tree-in-bud (TIB) pattern on CT thorax is primarily caused by infectious etiologies, particularly bacterial and mycobacterial infections, accounting for approximately 67.5% of all cases. 1

Definition and Appearance

• Tree-in-bud pattern consists of small centrilobular nodules with branching opacities resembling a budding tree
• Represents direct visualization of bronchiolar inflammation and mucoid impaction
• Usually appears as centrilobular nodules (<5mm) connected to branching linear structures of similar caliber 2, 3

Major Causes

1. Infectious Causes (Most Common)

• Bacterial infections:

  • Common respiratory pathogens (Haemophilus influenzae, Pseudomonas aeruginosa)
  • Present in 17.6% of cases with acute bronchitis or pneumonia 4

• Mycobacterial infections:

  • Tuberculosis (originally described with this pattern)
  • Nontuberculous mycobacteria (MAC, M. kansasii)

• Fungal infections:

  • Aspergillus species
  • Candida
  • Endemic fungi

• Viral infections:

  • Cytomegalovirus
  • Respiratory syncytial virus

• Parasitic infections:

  • Parasitic bronchitis

2. Non-Infectious Causes

• Aspiration/Inhalation:

  • Aspiration pneumonia (10.4% of TIB cases) 1
  • Toxic inhalation (including recreational drugs like cocaine) 5
  • Foreign substance aspiration

• Congenital Disorders:

  • Cystic fibrosis
  • Primary ciliary dyskinesia

• Idiopathic Inflammatory Disorders:

  • Diffuse panbronchiolitis (DPB) - characterized by bilateral basal-predominant centrilobular nodules 2
  • Obliterative bronchiolitis
  • Inflammatory bowel disease-related bronchiolitis 2

• Connective Tissue Disorders:

  • Rheumatoid arthritis
  • Sjögren's syndrome

• Vascular Disorders:

  • Neoplastic pulmonary emboli
  • Pulmonary tumor thrombotic microangiopathy

• Malignancies:

  • Lung malignancy (4% of TIB cases) 1
  • Other malignancies (9.5% of TIB cases) including lymphoma and leukemic infiltrates 1, 6

Diagnostic Approach

1. Clinical correlation is essential - assess for:

   • Cough, sputum production, fever
   • Immunocompromised status
   • History of tuberculosis exposure
   • Smoking history
   • Occupational exposures 7

2. Bronchoscopy with bronchoalveolar lavage (BAL) is recommended for:

   • Identifying infectious etiology
   • Collecting samples for microbiologic studies 7

3. Microbiologic studies should include:

   • Bacterial cultures
   • Fungal cultures
   • Mycobacterial cultures
   • Viral PCR testing 7

Important Clinical Considerations

• TIB pattern is frequently associated with bronchiectasis or proximal airway wall thickening (26/27 cases in one study) 4
• Incidence of TIB pattern is approximately 1.8% of all chest CTs 1
• The distribution of TIB pattern does not reliably predict the specific organism 1
• In cases where infection is not identified, consider non-infectious causes including malignancy and inflammatory conditions
• Surgical lung biopsy may be necessary when non-infectious etiologies are strongly suspected 7

Treatment Approach

Treatment should be directed at the underlying cause:

• Prolonged antibiotic therapy for bacterial causes
• Multiple drug regimen for mycobacterial infections
• Cessation of exposure plus corticosteroids for toxic/antigenic exposure 7
• For diffuse panbronchiolitis, macrolide antibiotics (particularly low-dose erythromycin) have shown significant improvement 2

Remember that while TIB pattern was originally described in tuberculosis, the differential diagnosis is broad, and a systematic approach to diagnosis is essential for appropriate management.