Epidemiology

Etiology

Classification of idiopathic interstitial pneumonias

Idiopathic pulmonary fibrosis (IPF)

• Pathophysiology
  1. Repetitive lung microinjuries result in the loss of type 1 pneumocytes, which compose 95% of the gas-exchanging alveolar surface area.
  2. Type 1 pneumocytes are incapable of replication. Therefore, normal repair requires the proliferation of type 2 pneumocytes, with subsequent differentiation into type 1 pneumocytes to reestablish the alveolar epithelial lining.
  3. In IPF, type 2 pneumocytes undergo reactive hyperplasia but fail to differentiate into type 1 cells because of dysfunctional cell fate pathways (eg, Wnt/transforming growth factor-beta) and abnormalities of the underlying basement membrane.
  4. Impaired re-epithelialization causes lung fibroblasts to undergo focal proliferation and begin secreting excessive amounts of collagen, leading to interstitial fibrosis.
  5. The fibrosis exerts radial traction on the distal airways, leading to dilation of the terminal bronchioles and producing the honeycomb appearance characteristic of IPF.
• Microscopic findings
  • Patchy areas of interstitial fibrosis with chronic interstitial inflammation intermixed with normal lung
  • Early lesions consist of fibroblastic foci that become increasingly collagenous with time
  • Honeycomb pattern with fibrotic walls and cystic spaces lined by bronchiolar epithelium
  • Fibrosis most prominent in the subpleural and perilobular regions

Cryptogenic organizing pneumonia (COP)

• Definition: a rare, type of ILD characterized by inflammation of the bronchioles, alveolar ducts, and alveolar walls
• Epidemiology
  • Incidence: 1–3 per 100,000 hospital admissions
  • Affects mostly individuals 40–50 years of age
• Diagnostics: histologically characterized by the presence of Masson bodies (granulation tissue buds made of foamy macrophages, mononuclear cells, and fibrous tissue) and chronic patchy interstitial inflammation without fibrosis

Pathophysiology

• General
  • All types of ILDs share the same basic pathophysiology.
  • Repeated cycles of tissue injury in the lung parenchyma with aberrant wound healing → collagenous fibrosis → remodeling of the pulmonary interstitium
• Pneumoconiosis: inhalation of dust particles → phagocytosis by alveolar macrophages → destruction of alveolar macrophages, inflammatory reaction → scarring, granuloma formation

Clinical features

Diagnostics

High-resolution CT (HRCT) chest

• Typical UIP pattern findings
  • Honeycombing: multiple cystic lesions within the lung parenchyma due to fibrosis
  • Irregular thickening of intralobular septa
  • Reticular pattern and mild ground glass opacity (GGO)
  • Traction bronchiectasis (irreversible dilatation of the bronchi and bronchioles due to fibrosis)

Pulmonary function tests (PFTs)

• Restrictive lung disease pattern
  • ↓ Total lung capacity and ↓ vital capacity
  • Normal or ↓ FEV1
  • ↓ FVC
  • Normal or ↑ FEV1:FVC ratio
  • The increased elastic recoil results in increased radial traction (outward pulling) on the airways, leading to increased expiratory flow rates when corrected for the low lung volume.
• Decreased diffusing capacity for CO (DLCO): highly sensitive parameter

Treatment