What are bronchoalveolar infiltrates?

What are Bronchoalveolar Infiltrates?

Bronchoalveolar infiltrates refer to abnormal accumulations of cells, fluid, or material within the bronchioles (small airways) and alveoli (air sacs), visible on imaging as opacities that follow a peribronchovascular distribution—meaning they center around airways and blood vessels. 1

Pathologic Basis

The term describes a pattern where disease processes affect both the conducting airways and adjacent alveolar spaces, creating a characteristic distribution on imaging studies. This can result from:

• Cellular accumulation: Macrophages, lymphocytes, neutrophils, or eosinophils filling the alveolar spaces and bronchiolar lumens 2
• Fluid accumulation: Edema, hemorrhage, or inflammatory exudate within these structures 3
• Inflammatory tissue: Granulation tissue plugs extending from bronchioles into alveolar ducts 3

Radiographic Appearance

On Chest X-ray

• Patchy, bilateral opacities that may appear as diffuse alveolar infiltrates 4
• Often peripheral or peribronchovascular in distribution 1
• May present as ground-glass opacities or consolidation 2

On High-Resolution CT (HRCT)

• Ground-glass opacities: Hazy increased attenuation with preserved bronchial and vascular margins 5
• Peribronchovascular distribution: Disease centered around airways and vessels 1
• Tree-in-bud pattern: Small nodular opacities representing bronchiolar inflammation 6
• Patchy consolidation: Often bilateral and non-segmental 4

Clinical Entities Associated with Bronchoalveolar Infiltrates

Smoking-Related Diseases

• Desquamative Interstitial Pneumonia (DIP): Uniform, diffuse intraalveolar macrophage accumulation that extends throughout lung parenchyma, accentuated around respiratory bronchioles 2
• Respiratory Bronchiolitis-ILD (RBILD): Pigmented macrophages within respiratory bronchiole lumens with bronchiolocentric distribution 2

Organizing Pneumonia

• Intraluminal plugs of granulation tissue in bronchioles extending into alveolar ducts 3
• Bilateral diffuse alveolar opacities with peripheral distribution 4
• Patchy consolidation following peribronchovascular pattern 4

Infectious Causes

• Viral pneumonias: Peribronchovascular distribution with ground-glass opacities 1, 3
• Atypical pneumonias: Mononuclear inflammatory infiltrate in alveolar septa and interstitial tissue 3
• Pneumocystis jirovecii: Diffuse bilateral perihilar infiltrates with ground-glass opacities 3

Drug-Induced Pneumonitis

• Bilateral non-segmental distribution with ground-glass opacities 3
• Can present as organizing pneumonia pattern 3

Diagnostic Approach

Temporal Pattern Recognition

• Acute onset (days to weeks) with fever and productive cough suggests infectious pneumonia 1
• Subacute onset (weeks to months) with dry cough suggests organizing pneumonia or drug-induced disease 1

Laboratory Evaluation

• Lymphopenia (<0.8 × 10⁹/L) suggests viral etiology and warrants repeat testing in 3 days 1
• Elevated procalcitonin (>0.10-0.45 ng/mL) and CRP (>50-170 mg/L) indicate bacterial infection 1

Bronchoalveolar Lavage (BAL) Cellular Analysis

When performed, BAL can reveal the specific cellular pattern:

• >15% lymphocytes: Sarcoidosis, hypersensitivity pneumonitis, drug-induced pneumonitis, organizing pneumonia 2, 3
• >3% neutrophils: Idiopathic pulmonary fibrosis, acute interstitial pneumonia 2, 3
• >1% eosinophils: Eosinophilic pneumonia, drug reactions 2, 3
• Macrophages with smoking-related inclusions: DIP, RBILD 2

A minimal volume of 5 mL pooled BAL sample is needed, with optimal volume of 10-20 mL 2

Critical Pitfalls to Avoid

• Do not confuse ground-glass opacity with consolidation: In ground-glass opacity, bronchovascular structures remain visible; in consolidation, they are obscured 5
• Do not assume infection without cultures: Infectious organisms must be excluded through BAL fluid cultures before attributing infiltrates to non-infectious causes 3
• Do not miss drug-induced disease: This requires drug cessation, not antimicrobials, while organizing pneumonia responds to corticosteroids, not antibiotics 1
• Reticular or nodular infiltrates on CT have lower BAL diagnostic yield (36.5%) compared to consolidated, ground-glass, or tree-in-bud patterns (61.2%) 6

Expected Evolution

Pneumonic consolidation should progress from patchy consolidation to strip-like opacity, then to grid-like interlobular septal thickening within 2-3 weeks 1. Persistent consolidation beyond 4 weeks warrants bronchoscopy with bronchoalveolar lavage 1.