What are bilateral lower lobe tree-in-bud opacities?

From the Guidelines

Bilateral lower lobe tree-in-bud opacities are a radiological finding commonly associated with infectious bronchiolitis, particularly from mycobacterial infections, as seen in MAC lung disease, which can present with nodular and interstitial nodular infiltrates 1. This pattern appears as small, branching nodular opacities that resemble a budding tree in the lower lobes of both lungs, representing filled bronchioles. The tree-in-bud pattern occurs when small airways become filled with fluid, mucus, pus, or cells, causing them to become visible on imaging. Some key points to consider include:

• The natural history of MAC lung disease depends on the type of clinical disease present, with two main forms: apical fibrocavitary lung disease and nodular bronchiectatic disease 1.
• Nodular bronchiectatic disease, sometimes labeled the “Lady Windomere syndrome,” tends to have a slower progression than cavitary disease and is characterized by HRCT findings that include multiple, small peripheral pulmonary nodules centered on the bronchovascular tree and cylindrical bronchiectasis 1.
• The HRCT pattern of these predominantly peripheral, small nodular densities has been termed “tree-in-bud,” and reflects inflammatory changes including bronchiolitis 1.
• Patients with nodular/bronchiectatic MAC lung disease often have additional microbiologic findings associated with bronchiectasis, including respiratory cultures positive for P. aeruginosa and occasionally for other NTM such as M. abscessus 1.
• Treatment depends on identifying and addressing the specific underlying cause, which often involves antimicrobial therapy for infectious etiologies or management of the underlying condition for non-infectious causes.

From the Research

Definition and Causes of Tree-in-Bud Opacities

• Tree-in-bud (TIB) opacities are a radiologic pattern seen on high-resolution chest CT, reflecting bronchiolar mucoid impaction occasionally with additional involvement of adjacent alveoli 2.
• The causes of TIB opacities are diverse, including respiratory infections (72%) with mycobacteria (39%), bacteria (27%), viruses (3%), or multiple organisms (4%) 3.
• Aspiration is also a common cause of TIB opacities, accounting for 25% of cases 3.

Patterns of Disease Associated with Tree-in-Bud Opacities

• Alternating areas of normal lung with regions of small airways disease (TIB opacities, bronchiectasis) is specific for Mycobacterium avium complex infection 3.
• A nearly uniform distribution of bronchiectasis is specific for diseases predisposing to airway infection, such as cystic fibrosis, primary ciliary dyskinesia, allergic bronchopulmonary aspergillosis, and immunodeficiency states 3.
• Consolidation and TIB opacities are usually due to bacterial infection or aspiration 3.

Bilateral Lower Lobe Tree-in-Bud Opacities

• There is no specific information available on bilateral lower lobe tree-in-bud opacities in the provided studies.
• However, the studies suggest that TIB opacities can be caused by various factors, including infections and aspiration, and can be seen in different patterns and distributions 3, 2.

Treatment and Management

• The treatment of TIB opacities depends on the underlying cause, and may include antibiotics such as amoxicillin-clavulanate or azithromycin 4, 5.
• Azithromycin has been shown to be non-inferior to amoxicillin-clavulanate for resolving exacerbations in children with non-severe bronchiectasis 4.