Interstitial Lung Diseases

ILD Basics - The Lung's Labyrinth

• Definition: Diverse group of diffuse parenchymal lung diseases (DPLDs) primarily affecting the interstitium - the tissue space between alveolar epithelium and capillary endothelium, including basement membrane, perivascular, and perilymphatic tissues.
• Common Pathological Features: Varying degrees of inflammation and fibrosis of the pulmonary interstitium.
• Major Categories:
  • Idiopathic Interstitial Pneumonias (IIPs) (e.g., IPF)
  • ILD of known cause (e.g., CTD, drug, occupational pneumoconioses)
  • Granulomatous ILDs (e.g., Sarcoidosis, Hypersensitivity Pneumonitis)
  • Other rare ILDs

⭐ Idiopathic Pulmonary Fibrosis (IPF) is the most common and has the worst prognosis among IIPs.

IIPs Deep Dive - Mysterious Mists

⭐ Cryptogenic Organizing Pneumonia (COP) often presents with flu-like symptoms and shows excellent response to corticosteroids.

Secondary ILDs - Environmental & Autoimmune Invaders

• Pneumoconioses: Chronic mineral dust inhalation.

  Disease	Agent	Occupation	Key Pathology	CXR/CT Finding
  Silicosis	Silica	Mining, Sandblasting	Silicotic nodules, ↑TB risk	Apical "rock-like" nodules, Eggshell calcification (hilar)
  CWP	Coal	Coal mining	Coal macules, Progressive Massive Fibrosis (PMF)	-
  Asbestosis	Asbestos	Shipbuilding, Insulation	Asbestos bodies, Pleural plaques	Basal fibrosis, ↑Mesothelioma risk
  Berylliosis	Beryllium	Aerospace, Electronics	Non-caseating granulomas	Hilar lymphadenopathy

• Hypersensitivity Pneumonitis (HP): Immune reaction to inhaled organic antigens (Farmer's, Bird fancier's lung).
  • Acute: Neutrophils.
  • Chronic: Lymphocytes, granulomas, fibrosis.
• Drug-induced ILD: Amiodarone, Bleomycin, Methotrexate. Patterns: pneumonitis, fibrosis.
• ILD in Connective Tissue Diseases (CTD): RA, Scleroderma (NSIP common), SLE.

⭐ Asbestosis: ↑ risk of lung cancer (esp. smokers) & primary cause of mesothelioma.

Sarcoidosis Focus - Granuloma Galaxy

• Definition: Multisystem granulomatous disorder of unknown etiology, characterized by immune cell accumulation.
• Pathology: Hallmark is non-caseating epithelioid granulomas.
  • May contain Schaumann bodies (laminated calcium concretions) or Asteroid bodies (stellate inclusions).
• Pulmonary: Bilateral hilar lymphadenopathy (BHL) is classic. Also, pulmonary infiltrates, fibrosis.
  • Lofgren's syndrome: Acute BHL, erythema nodosum, fever, arthralgia.
• Extrapulmonary: Skin (erythema nodosum, lupus pernio), eyes (uveitis), heart, nervous system.
• Labs: Elevated serum Angiotensin-Converting Enzyme (ACE) levels (↑~60% cases), hypercalcemia.

⭐ Bilateral hilar lymphadenopathy (BHL) is the most common radiographic finding in pulmonary sarcoidosis.

ILD Diagnosis - Detective's Toolkit

• Clinical: Progressive dyspnea, non-productive cough, bibasilar crackles.
• PFTs: Restrictive pattern: ↓TLC, ↓FVC, ↓DLCO; FEV1/FVC normal or ↑.
• HRCT Chest: Key! Patterns: Ground-glass opacity (GGO), reticular, honeycombing (UIP), traction bronchiectasis.
• Biopsy: Transbronchial, cryobiopsy, or surgical (SLB) if diagnosis uncertain. Multidisciplinary discussion (MDD) vital.

⭐ Honeycombing on HRCT, often bibasilar and subpleural, is a hallmark of Usual Interstitial Pneumonia (UIP) pattern and indicates advanced fibrosis.

High‑Yield Points - ⚡ Biggest Takeaways

• Idiopathic Pulmonary Fibrosis (IPF) typically presents with a UIP pattern on histology and honeycomb lung on HRCT; prognosis is poor.
• Sarcoidosis is characterized by non-caseating granulomas, bilateral hilar lymphadenopathy, and often elevated serum ACE levels.
• Hypersensitivity Pneumonitis results from an immune reaction to inhaled organic antigens (e.g., Farmer's lung, Bird fancier's lung).
• Pneumoconioses are caused by inorganic dust inhalation; key examples include silicosis (eggshell calcification, ↑TB risk) and asbestosis (pleural plaques, ↑mesothelioma risk).
• Many drugs can induce ILD, notably amiodarone, bleomycin, and methotrexate.
• Connective tissue diseases such as rheumatoid arthritis and scleroderma are frequently associated with ILD development.
• Smoking-related ILDs include respiratory bronchiolitis-ILD (RB-ILD) and desquamative interstitial pneumonia (DIP), primarily affecting smokers.