What are the causes of upper and lower lobe pulmonary fibrosis?

Causes of Upper and Lower Lobe Fibrosis with Mnemonics

The main causes of pulmonary fibrosis differ based on whether they predominantly affect the upper or lower lobes, with distinct etiologies and patterns for each distribution.

Upper Lobe Predominant Fibrosis - Mnemonic: "CHASTE"

• Chronic Hypersensitivity Pneumonitis - especially advanced stages with fibrotic changes
• History of tuberculosis (post-TB fibrosis)
• Asbestosis (rare variant with upper lobe involvement) 1
• Sarcoidosis - can progress to fibrotic stage
• Silicosis and other pneumoconioses - predominantly affects upper lobes 1
• Traction bronchiectasis with apical fibrosis
• Emphysema with fibrosis (combined pulmonary fibrosis and emphysema)

Lower Lobe Predominant Fibrosis - Mnemonic: "BRAID"

• Basement membrane disorders (collagen vascular diseases) - connective tissue diseases like rheumatoid arthritis, scleroderma, and polymyositis 1
• Radiotherapy-induced fibrosis
• Asbestosis (classic pattern) - predominantly peripheral, subpleural, bibasal reticular abnormalities 1
• Idiopathic pulmonary fibrosis (IPF) - characterized by UIP pattern with peripheral, subpleural lower lobe reticular opacities 1
• Drug-induced pulmonary fibrosis (amiodarone, bleomycin, methotrexate)

Clinical Pearls for Differentiation

Key Diagnostic Features for Upper Lobe Fibrosis:

• Often associated with occupational exposures (silica, coal dust)
• May show hilar lymphadenopathy (especially in sarcoidosis)
• Progressive massive fibrosis may develop in advanced pneumoconiosis 1
• Hypersensitivity pneumonitis often shows mosaic attenuation or "three-density sign" on HRCT 1

Key Diagnostic Features for Lower Lobe Fibrosis:

• IPF shows characteristic "honeycombing" on HRCT with subpleural, bibasal distribution 1
• Asbestosis often presents with concurrent pleural plaques or thickening 1
• Connective tissue disease-related fibrosis may show "exuberant honeycombing" or "straight-edge sign" 1
• Traction bronchiectasis and bronchiolectasis are common in advanced disease 1

Diagnostic Approach

1. HRCT Pattern Analysis: Essential for distinguishing upper vs. lower lobe predominance and specific patterns (UIP vs. non-UIP) 1

2. Exposure History: Critical for identifying occupational causes (asbestos, silica) or environmental exposures (hypersensitivity pneumonitis) 1

3. Serological Testing: To identify connective tissue diseases that can cause pulmonary fibrosis 1

4. Lung Biopsy: May be necessary when HRCT findings are indeterminate or inconsistent with clinical presentation 1

Common Pitfalls to Avoid

• Assuming all lower lobe fibrosis is IPF without excluding other causes like asbestosis or connective tissue diseases 1
• Failing to recognize combined patterns (e.g., combined pulmonary fibrosis and emphysema)
• Missing subtle pleural plaques that suggest asbestos exposure in patients with lower lobe fibrosis 1
• Overlooking rapidly progressive disease which may suggest alternative diagnoses rather than typical IPF 1
• Not considering rare presentations like the upper lobe fibrosis variant of asbestosis 1

Remember that the distribution of fibrosis provides important diagnostic clues, but overlap can occur, and some diseases can affect both upper and lower lobes at different stages of progression. The mnemonics "CHASTE" and "BRAID" can help recall the most common causes based on anatomical distribution.