In a patient with interstitial changes on imaging but preserved lung volumes on pulmonary function testing, which interstitial lung diseases are volume‑maintaining?

Volume-Maintaining Interstitial Lung Diseases

The volume-maintaining ILDs are primarily sarcoidosis, hypersensitivity pneumonitis, lymphangioleiomyomatosis (LAM), Langerhans cell histiocytosis, and smoking-related ILDs including desquamative interstitial pneumonia (DIP) and respiratory bronchiolitis-interstitial lung disease (RBILD). 1

Key Distinguishing Feature

• Volume preservation occurs when interstitial changes are present on imaging but pulmonary function testing shows normal or preserved lung volumes rather than the restrictive pattern typical of most ILDs 1, 2
• This contrasts with fibrotic ILDs like idiopathic pulmonary fibrosis (IPF), which characteristically show decreased lung volumes and restrictive physiology 1, 2

Specific Volume-Maintaining ILD Entities

Sarcoidosis

• Presents with bilateral infiltrates but often maintains normal lung volumes on PFT, particularly in early stages 1
• May show upper or mid-lung predominance with profuse micronodules and extensive ground-glass abnormality 3
• Lymphadenopathy is common and helps distinguish it from other ILDs 1

Hypersensitivity Pneumonitis (HP)

• Centrilobular nodules with mosaic attenuation pattern and mid or upper lung predominance are characteristic 3
• Shows peribronchovascular distribution with relative subpleural sparing, unlike typical IPF 3, 4
• The three-density pattern on CT (normal lung, ground-glass, and air-trapping) is highly suggestive 3
• Lung volumes may be preserved or even increased due to air-trapping 1

Smoking-Related ILDs

• Desquamative interstitial pneumonia (DIP) and respiratory bronchiolitis-interstitial lung disease (RBILD) show predominance of macrophages containing smoking-related inclusions 1
• Ground-glass opacities and centrilobular nodules are present, with markedly thick-walled bronchi 1
• These entities maintain relatively normal lung volumes compared to fibrotic ILDs 1

Langerhans Cell Histiocytosis

• Associated with smoking history and shows smoking-related inclusions on BAL 1
• Upper lobe predominance with cystic changes is characteristic
• Lung volumes may be preserved or increased due to cystic disease

Critical Diagnostic Pitfall

• Combined emphysema and IPF can present with preserved or increased lung volumes despite underlying fibrosis, creating a false impression of volume-maintaining ILD 1
• These patients show upper lobe oligemia (apparent reduction in blood vessels) and basal reticular opacities simultaneously 1
• This represents a distinct phenotype where emphysema masks the restrictive physiology of IPF, not a true volume-maintaining ILD 1

Diagnostic Approach

• When interstitial changes are present with preserved lung volumes, systematically evaluate for:

  • Detailed exposure history for hypersensitivity antigens and occupational exposures 3
  • Smoking history to identify DIP, RBILD, or Langerhans cell histiocytosis 1
  • Upper/mid-lung predominance suggesting sarcoidosis or HP rather than IPF 3, 4
  • Centrilobular nodules and mosaic attenuation indicating HP 3
  • Lymphadenopathy suggesting sarcoidosis 1

• Bronchoalveolar lavage with cell differential is particularly useful when diagnosis remains uncertain, as lymphocytosis (>15%) suggests HP or sarcoidosis, while pigmented macrophages indicate smoking-related ILD 1, 3

• Multidisciplinary discussion integrating clinical context, HRCT patterns, PFT results, and BAL findings is essential for accurate diagnosis 1, 5, 3